STOC 2016 – Author Index 
Contents 
Abstracts 
Authors

A B C D E F G H I J K L M N O P R S T U V W X Y Z
Aaronson, Scott 
STOC'16: "Separations in Query ..."
Separations in Query Complexity using Cheat Sheets
Scott Aaronson, Shalev BenDavid, and Robin Kothari (Massachusetts Institute of Technology, USA)
We show a power 2.5 separation between boundederror randomized and quantum query complexity for a total Boolean function, refuting the widely believed conjecture that the best such separation could only be quadratic (from Grover's algorithm). We also present a total function with a power 4 separation between quantum query complexity and approximate polynomial degree, showing severe limitations on the power of the polynomial method. Finally, we exhibit a total function with a quadratic gap between quantum query complexity and certificate complexity, which is optimal (up to log factors). These separations are shown using a new, general technique that we call the cheat sheet technique, which builds upon the techniques of Ambainis et al. [STOC 2016]. The technique is based on a generic transformation that converts any (possibly partial) function into a new total function with desirable properties for showing separations. The framework also allows many known separations, including some recent breakthrough results of Ambainis et al. [STOC 2016], to be shown in a unified manner.
@InProceedings{STOC16p863,
author = {Scott Aaronson and Shalev BenDavid and Robin Kothari},
title = {Separations in Query Complexity using Cheat Sheets},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {863876},
doi = {10.1145/2897518.2897644},
year = {2016},
}
Publisher's Version
Article Search


Abboud, Amir 
STOC'16: "The 4/3 Additive ..."
The 4/3 Additive Spanner Exponent Is Tight
Amir Abboud and Greg Bodwin (Stanford University, USA) A spanner is a sparse subgraph that approximately preserves the pairwise distances of the original graph. It is well known that there is a smooth tradeoff between the sparsity of a spanner and the quality of its approximation, so long as distance error is measured multiplicatively. A central open question in the field is to prove or disprove whether such a tradeoff exists also in the regime of additive error. That is, is it true that for all ε>0, there is a constant k_{ε} such that every graph has a spanner on O(n^{1+ε}) edges that preserves its pairwise distances up to +k_{ε}? Previous lower bounds are consistent with a positive resolution to this question, while previous upper bounds exhibit the beginning of a tradeoff curve: all graphs have +2 spanners on O(n^{3/2}) edges, +4 spanners on Õ(n^{7/5}) edges, and +6 spanners on O(n^{4/3}) edges. However, progress has mysteriously halted at the n^{4/3} bound, and despite significant effort from the community, the question has remained open for all 0 < ε < 1/3. Our main result is a surprising negative resolution of the open question, even in a highly generalized setting. We show a new information theoretic incompressibility bound: there is no function that compresses graphs into O(n^{4/3 − ε}) bits so that distance information can be recovered within +n^{o(1)} error. As a special case of our theorem, we get a tight lower bound on the sparsity of additive spanners: the +6 spanner on O(n^{4/3}) edges cannot be improved in the exponent, even if any subpolynomial amount of additive error is allowed. Our theorem implies new lower bounds for related objects as well; for example, the twentyyearold +4 emulator on O(n^{4/3}) edges also cannot be improved in the exponent unless the error allowance is polynomial. Central to our construction is a new type of graph product, which we call the Obstacle Product. Intuitively, it takes two graphs G,H and produces a new graph G H whose shortest paths structure looks locally like H but globally like G. Amir Abboud, Thomas Dueholm Hansen, Virginia Vassilevska Williams, and Ryan Williams (Stanford University, USA; Aarhus University, Denmark) A recent, active line of work achieves tight lower bounds for fundamental problems under the Strong Exponential Time Hypothesis (SETH). A celebrated result of Backurs and Indyk (STOC’15) proves that computing the Edit Distance of two sequences of length n in truly subquadratic O(n^{2−ε}) time, for some ε>0, is impossible under SETH. The result was extended by followup works to simpler looking problems like finding the Longest Common Subsequence (LCS). SETH is a very strong assumption, asserting that even linear size CNF formulas cannot be analyzed for satisfiability with an exponential speedup over exhaustive search. We consider much safer assumptions, e.g. that such a speedup is impossible for SAT on more expressive representations, like subexponentialsize NC circuits. Intuitively, this assumption is much more plausible: NC circuits can implement linear algebra and complex cryptographic primitives, while CNFs cannot even approximately compute an XOR of bits. Our main result is a surprising reduction from SAT on Branching Programs to fundamental problems in P like Edit Distance, LCS, and many others. Truly subquadratic algorithms for these problems therefore have far more remarkable consequences than merely faster CNFSAT algorithms. For example, SAT on arbitrary o(n)depth bounded fanin circuits (and therefore also NCCircuitSAT) can be solved in (2−ε)^{n} time. An interesting feature of our work is that we get major consequences even from mildly subquadratic algorithms for Edit Distance or LCS. For example, we show that if an arbitrarily large polylog factor is shaved from n^{2} for Edit Distance then NEXP does not have nonuniform NC^{1} circuits. 

Ambainis, Andris 
STOC'16: "Separations in Query ..."
Separations in Query Complexity Based on Pointer Functions
Andris Ambainis, Kaspars Balodis, Aleksandrs Belovs, Troy Lee, Miklos Santha, and Juris Smotrovs (University of Latvia, Latvia; CWI, Netherlands; Nanyang Technological University, Singapore; National University of Singapore, Singapore; CNRS, France) In 1986, Saks and Wigderson conjectured that the largest separation between deterministic and zeroerror randomized query complexity for a total boolean function is given by the function f on n=2^{k} bits defined by a complete binary tree of NAND gates of depth k, which achieves R_{0}(f) = O(D(f)^{0.7537…}). We show this is false by giving an example of a total boolean function f on n bits whose deterministic query complexity is Ω(n/log(n)) while its zeroerror randomized query complexity is Õ(√n). We further show that the quantum query complexity of the same function is Õ(n^{1/4}), giving the first example of a total function with a superquadratic gap between its quantum and deterministic query complexities. We also construct a total boolean function g on n variables that has zeroerror randomized query complexity Ω(n/log(n)) and boundederror randomized query complexity R(g) = Õ(√n). This is the first superlinear separation between these two complexity measures. The exact quantum query complexity of the same function is Q_{E}(g) = Õ(√n). These functions show that the relations D(f) = O(R_{1}(f)^{2}) and R_{0}(f) = Õ(R(f)^{2}) are optimal, up to polylogarithmic factors. Further variations of these functions give additional separations between other query complexity measures: a cubic separation between Q and R_{0}, a 3/2power separation between Q_{E} and R, and a 4th power separation between approximate degree and boundederror randomized query complexity. All of these examples are variants of a function recently introduced by Goos, Pitassi, and Watson which they used to separate the unambiguous 1certificate complexity from deterministic query complexity and to resolve the famous Clique versus Independent Set problem in communication complexity. 

Applebaum, Benny 
STOC'16: "Algebraic Attacks ..."
Algebraic Attacks against Random Local Functions and Their Countermeasures
Benny Applebaum and Shachar Lovett (Tel Aviv University, Israel; University of California at San Diego, USA) Suppose that you have n truly random bits x=(x_{1},…,x_{n}) and you wish to use them to generate m≫ n pseudorandom bits y=(y_{1},…, y_{m}) using a local mapping, i.e., each y_{i} should depend on at most d=O(1) bits of x. In the polynomial regime of m=n^{s}, s>1, the only known solution, originates from (Goldreich, ECCC 2000), is based on Random Local Functions: Compute y_{i} by applying some fixed (public) dary predicate P to a random (public) tuple of distinct inputs (x_{i1},…,x_{id}). Our goal in this paper is to understand, for any value of s, how the pseudorandomness of the resulting sequence depends on the choice of the underlying predicate. We derive the following results: (1) We show that pseudorandomness against F_{2}linear adversaries (i.e., the distribution y has lowbias) is achieved if the predicate is (a) k=Ω(s)resilience, i.e., uncorrelated with any ksubset of its inputs, and (b) has algebraic degree of Ω(s) even after fixing Ω(s) of its inputs. We also show that these requirements are necessary, and so they form a tight characterization (up to constants) of security against linear attacks. Our positive result shows that a dlocal lowbias generator can have output length of n^{Ω(d)}, answering an open question of Mossel, Shpilka and Trevisan (FOCS, 2003). Our negative result shows that a candidate for pseudorandom generator proposed by the first author (computational complexity, 2015) and by O’Donnell and Witmer (CCC 2014) is insecure. We use similar techniques to refute a conjecture of Feldman, Perkins and Vempala (STOC 2015) regarding the hardness of planted constraint satisfaction problems. (2) Motivated by the cryptanalysis literature, we consider security against algebraic attacks. We provide the first theoretical treatment of such attacks by formalizing a general notion of algebraic inversion and distinguishing attacks based on the Polynomial Calculus proof system. We show that algebraic attacks succeed if and only if there exist a degree e=O(s) nonzero polynomial Q whose roots cover the roots of P or cover the roots of P’s complement. As a corollary, we obtain the first example of a predicate P for which the generated sequence y passes all linear tests but fails to pass some polynomialtime computable test, answering an open question posed by the first author (Question 4.9, computational complexity 2015). 

Aronov, Boris 
STOC'16: "Almost Tight Bounds ..."
Almost Tight Bounds for Eliminating Depth Cycles in Three Dimensions
Boris Aronov and Micha Sharir (New York University, USA; Tel Aviv University, Israel) Given n nonvertical lines in 3space, their vertical depth (above/below) relation can contain cycles. We show that the lines can be cut into O(n^{3/2}polylog n) pieces, such that the depth relation among these pieces is now a proper partial order. This bound is nearly tight in the worst case. As a consequence, we deduce that the number of pairwise nonoverlapping cycles, namely, cycles whose xyprojections do not overlap, is O(n^{3/2}polylog n); this bound too is almost tight in the worst case. Previous results on this topic could only handle restricted cases of the problem (such as handling only triangular cycles, by Aronov, Koltun, and Sharir, or only cycles in gridlike patterns, by Chazelle et al.), and the bounds were considerably weaker—much closer to the trivial quadratic bound. Our proof uses a recent variant of the polynomial partitioning technique, due to Guth, and some simple tools from algebraic geometry. It is much more straightforward than the previous “purely combinatorial” methods. Our approach extends to eliminating all cycles in the depth relation among segments, and among constantdegree algebraic arcs. We hope that a suitable extension of this technique could be used to handle the much more difficult case of pairwisedisjoint triangles as well. Our results almost completely settle a longstanding (35 years old) open problem in computational geometry, motivated by hiddensurface removal in computer graphics. 

Asharov, Gilad 
STOC'16: "Searchable Symmetric ..."
Searchable Symmetric Encryption: Optimal Locality in Linear Space via TwoDimensional Balanced Allocations
Gilad Asharov, Moni Naor, Gil Segev, and Ido Shahaf (IBM Research, USA; Weizmann Institute of Science, Israel; Hebrew University of Jerusalem, Israel) Searchable symmetric encryption (SSE) enables a client to store a database on an untrusted server while supporting keyword search in a secure manner. Despite the rapidly increasing interest in SSE technology, experiments indicate that the performance of the known schemes scales badly to large databases. Somewhat surprisingly, this is not due to their usage of cryptographic tools, but rather due to their poor locality (where locality is defined as the number of noncontiguous memory locations the server accesses with each query). The only known schemes that do not suffer from poor locality suffer either from an impractical space overhead or from an impractical read efficiency (where read efficiency is defined as the ratio between the number of bits the server reads with each query and the actual size of the answer). We construct the first SSE schemes that simultaneously enjoy optimal locality, optimal space overhead, and nearlyoptimal read efficiency. Specifically, for a database of size N, under the modest assumption that no keyword appears in more than N^{1 − 1/loglogN} documents, we construct a scheme with read efficiency Õ(loglogN). This essentially matches the lower bound of Cash and Tessaro (EUROCRYPT ’14) showing that any SSE scheme must be suboptimal in either its locality, its space overhead, or its read efficiency. In addition, even without making any assumptions on the structure of the database, we construct a scheme with read efficiency Õ(logN). Our schemes are obtained via a twodimensional generalization of the classic balanced allocations (“balls and bins”) problem that we put forward. We construct nearlyoptimal twodimensional balanced allocation schemes, and then combine their algorithmic structure with subtle cryptographic techniques. 

Assadi, Sepehr 
STOC'16: "Tight Bounds for ..."
Tight Bounds for SinglePass Streaming Complexity of the Set Cover Problem
Sepehr Assadi, Sanjeev Khanna, and Yang Li (University of Pennsylvania, USA) We resolve the space complexity of singlepass streaming algorithms for approximating the classic set cover problem. For finding an αapproximate set cover (for α= o(√n)) via a singlepass streaming algorithm, we show that Θ(mn/α) space is both sufficient and necessary (up to an O(logn) factor); here m denotes number of the sets and n denotes size of the universe. This provides a strong negative answer to the open question posed by Indyk (2015) regarding the possibility of having a singlepass algorithm with a small approximation factor that uses sublinear space. We further study the problem of estimating the size of a minimum set cover (as opposed to finding the actual sets), and establish that an additional factor of α saving in the space is achievable in this case and that this is the best possible. In other words, we show that Θ(mn/α^{2}) space is both sufficient and necessary (up to logarithmic factors) for estimating the size of a minimum set cover to within a factor of α. Our algorithm in fact works for the more general problem of estimating the optimal value of a covering integer program. On the other hand, our lower bound holds even for set cover instances where the sets are presented in a random order. 

Aziz, Haris 
STOC'16: "A Discrete and Bounded ..."
A Discrete and Bounded EnvyFree Cake Cutting Protocol for Four Agents
Haris Aziz and Simon Mackenzie (Data61, Australia; UNSW, Australia)
We consider the wellstudied cake cutting problem in which the goal is to identify an envyfree allocation based on a minimal number of queries from the agents. The problem has attracted considerable attention within various branches of computer science, mathematics, and economics. Although, the elegant SelfridgeConway envyfree protocol for three agents has been known since 1960, it has been a major open problem to obtain a bounded envyfree protocol for more than three agents. The problem has been termed the central open problem in cake cutting. We solve this problem by proposing a discrete and bounded envyfree protocol for four agents.
@InProceedings{STOC16p454,
author = {Haris Aziz and Simon Mackenzie},
title = {A Discrete and Bounded EnvyFree Cake Cutting Protocol for Four Agents},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {454464},
doi = {10.1145/2897518.2897522},
year = {2016},
}
Publisher's Version
Article Search


Babai, László 
STOC'16: "Graph Isomorphism ..."
Graph Isomorphism in Quasipolynomial Time [Extended Abstract]
László Babai (University of Chicago, USA)
We show that the Graph Isomorphism (GI) problem and the more general problems of String Isomorphism (SI) andCoset Intersection (CI) can be solved in quasipolynomial(exp((logn)O(1))) time. The best previous bound for GI was exp(O( √n log n)), where n is the number of vertices (Luks, 1983); for the other two problems, the bound was similar, exp(O~(√ n)), where n is the size of the permutation domain (Babai, 1983). Following the approach of Luks’s seminal 1980/82 paper, the problem we actually address is SI. This problem takes two strings of length n and a permutation group G of degree n (the “ambient group”) as input (G is given by a list of generators) and asks whether or not one of the strings can be transformed into the other by some element of G. Luks’s divideandconquer algorithm for SI proceeds by recursion on the ambient group. We build on Luks’s framework and attack the obstructions to efficient Luks recurrence via an interplay between local and global symmetry. We construct group theoretic “local certificates” to certify the presence or absence of local symmetry, aggregate the negative certificates to canonical kary relations where k = O(log n), and employ combinatorial canonical partitioning techniques to split the kary relational structure for efficient divideand conquer. We show that in a well–defined sense, Johnson graphs are the only obstructions to effective canonical partitioning. The central element of the algorithm is the “local certificates” routine which is based on a new group theoretic result, the “Unaffected stabilizers lemma,” that allows us to construct global automorphisms out of local information.
@InProceedings{STOC16p684,
author = {László Babai},
title = {Graph Isomorphism in Quasipolynomial Time [Extended Abstract]},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {684697},
doi = {10.1145/2897518.2897542},
year = {2016},
}
Publisher's Version
Article Search


Balodis, Kaspars 
STOC'16: "Separations in Query ..."
Separations in Query Complexity Based on Pointer Functions
Andris Ambainis, Kaspars Balodis, Aleksandrs Belovs, Troy Lee, Miklos Santha, and Juris Smotrovs (University of Latvia, Latvia; CWI, Netherlands; Nanyang Technological University, Singapore; National University of Singapore, Singapore; CNRS, France) In 1986, Saks and Wigderson conjectured that the largest separation between deterministic and zeroerror randomized query complexity for a total boolean function is given by the function f on n=2^{k} bits defined by a complete binary tree of NAND gates of depth k, which achieves R_{0}(f) = O(D(f)^{0.7537…}). We show this is false by giving an example of a total boolean function f on n bits whose deterministic query complexity is Ω(n/log(n)) while its zeroerror randomized query complexity is Õ(√n). We further show that the quantum query complexity of the same function is Õ(n^{1/4}), giving the first example of a total function with a superquadratic gap between its quantum and deterministic query complexities. We also construct a total boolean function g on n variables that has zeroerror randomized query complexity Ω(n/log(n)) and boundederror randomized query complexity R(g) = Õ(√n). This is the first superlinear separation between these two complexity measures. The exact quantum query complexity of the same function is Q_{E}(g) = Õ(√n). These functions show that the relations D(f) = O(R_{1}(f)^{2}) and R_{0}(f) = Õ(R(f)^{2}) are optimal, up to polylogarithmic factors. Further variations of these functions give additional separations between other query complexity measures: a cubic separation between Q and R_{0}, a 3/2power separation between Q_{E} and R, and a 4th power separation between approximate degree and boundederror randomized query complexity. All of these examples are variants of a function recently introduced by Goos, Pitassi, and Watson which they used to separate the unambiguous 1certificate complexity from deterministic query complexity and to resolve the famous Clique versus Independent Set problem in communication complexity. 

Bansal, Nikhil 
STOC'16: "LiftandRound to ..."
LiftandRound to Improve Weighted Completion Time on Unrelated Machines
Nikhil Bansal, Aravind Srinivasan, and Ola Svensson (Eindhoven University of Technology, Netherlands; University of Maryland, USA; EPFL, Switzerland)
We consider the problem of scheduling jobs on unrelated machines so as to minimize the sum of weighted completion times. Our main result is a (3/2c)approximation algorithm for some fixed c>0, improving upon the longstanding bound of 3/2. To do this, we first introduce a new liftandproject based SDP relaxation for the problem. This is necessary as the previous convex programming relaxations have an integrality gap of 3/2. Second, we give a new general bipartiterounding procedure that produces an assignment with certain strong negative correlation properties.
@InProceedings{STOC16p156,
author = {Nikhil Bansal and Aravind Srinivasan and Ola Svensson},
title = {LiftandRound to Improve Weighted Completion Time on Unrelated Machines},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {156167},
doi = {10.1145/2897518.2897572},
year = {2016},
}
Publisher's Version
Article Search


Bassily, Raef 
STOC'16: "Algorithmic Stability ..."
Algorithmic Stability for Adaptive Data Analysis
Raef Bassily, Kobbi Nissim, Adam Smith, Thomas Steinke, Uri Stemmer, and Jonathan Ullman (University of California at San Diego, USA; BenGurion University of the Negev, Israel; Harvard University, USA; Pennsylvania State University, USA; Northeastern University, USA)
Adaptivity is an important feature of data analysis  the choice of questions to ask about a dataset often depends on previous interactions with the same dataset. However, statistical validity is typically studied in a nonadaptive model, where all questions are specified before the dataset is drawn. Recent work by Dwork et al. (STOC, 2015) and Hardt and Ullman (FOCS, 2014) initiated a general formal study of this problem, and gave the first upper and lower bounds on the achievable generalization error for adaptive data analysis.
Specifically, suppose there is an unknown distribution P and a set of n independent samples x is drawn from P. We seek an algorithm that, given x as input, accurately answers a sequence of adaptively chosen ``queries'' about the unknown distribution P. How many samples n must we draw from the distribution, as a function of the type of queries, the number of queries, and the desired level of accuracy?
In this work we make two new contributions towards resolving this question:
@InProceedings{STOC16p1046,
author = {Raef Bassily and Kobbi Nissim and Adam Smith and Thomas Steinke and Uri Stemmer and Jonathan Ullman},
title = {Algorithmic Stability for Adaptive Data Analysis},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {10461059},
doi = {10.1145/2897518.2897566},
year = {2016},
}
Publisher's Version
Article Search


Baswana, Surender 
STOC'16: "Fault Tolerant Subgraph ..."
Fault Tolerant Subgraph for Single Source Reachability: Generic and Optimal
Surender Baswana, Keerti Choudhary, and Liam Roditty (IIT Kanpur, India; BarIlan University, Israel) Let G=(V,E) be an nvertices medges directed graph. Let s∈ V be any designated source vertex. We address the problem of single source reachability (SSR) from s in presence of failures of vertices/edges. We show that for every k≥ 1, there is a subgraph H of G with at most 2^{k} n edges that preserves the reachability from s even after the failure of any k edges. Formally, given a set F of k edges, a vertex u∈ V is reachable from s in G∖ F if and only if u is reachable from s in H∖ F. We call H a kFault Tolerant Reachability Subgraph (kFTRS). We prove also a matching lower bound of Ω(2^{k}n) for such subgraphs. Our results extend to vertex failures without any extra overhead. The general construction of kFTRS is interesting from several different perspectives. From the Graph theory perspective it reveals a separation between SSR and single source shortest paths (SSSP) in directed graphs. More specifically, in the case of SSSP in weighted directed graphs, there is a lower bound of Ω(m) even for a single edge failure. In the case of unweighted graphs there is a lower bound of Ω(n^{3/2}) edges, again, even for a single edge failure. There is also a matching upper bound but nothing is known for two or more failures in the directed graphs. From the Algorithms perspective it implies fault tolerant solutions to other interesting problems, namely, (i) verifying if the strong connectivity of a graph is preserved after k edge or vertex failures, (ii) computing a dominator tree of a graph after kfailures. From the perspective of Techniques it makes an interesting usage of the concept of farthest mincut which was already introduced by Ford and Fulkerson in their pioneering work on flows and cuts. We show that there is a close relationship between the farthest mincut and the kFTRS. We believe that our new technique is of independent interest. 

Bateni, MohammadHossein 
STOC'16: "A PTAS for Planar ..."
A PTAS for Planar Group Steiner Tree via Spanner Bootstrapping and Prize Collecting
MohammadHossein Bateni, Erik D. Demaine, MohammadTaghi Hajiaghayi, and Dániel Marx (Google, USA; Massachusetts Institute of Technology, USA; University of Maryland at College Park, USA; Hungarian Academy of Sciences, Hungary) We present the first polynomialtime approximation scheme (PTAS), i.e., (1+ε)approximation algorithm for any constant ε> 0, for the planar group Steiner tree problem (in which each group lies on a boundary of a face). This result improves on the best previous approximation factor of O(logn (loglogn)^{O(1)}). We achieve this result via a novel and powerful technique called spanner bootstrapping, which allows one to bootstrap from a superconstant approximation factor (even superpolynomial in the input size) all the way down to a PTAS. This is in contrast with the popular existing approach for planar PTASs of constructing lightweight spanners in one iteration, which notably requires a constantfactor approximate solution to start from. Spanner bootstrapping removes one of the main barriers for designing PTASs for problems which have no known constantfactor approximation (even on planar graphs), and thus can be used to obtain PTASs for several difficulttoapproximate problems. Our second major contribution required for the planar group Steiner tree PTAS is a spanner construction, which reduces the graph to have total weight within a factor of the optimal solution while approximately preserving the optimal solution. This is particularly challenging because group Steiner tree requires deciding which terminal in each group to connect by the tree, making it much harder than recent previous approaches to construct spanners for planar TSP by Klein [SIAM J. Computing 2008], subset TSP by Klein [STOC 2006], Steiner tree by Borradaile, Klein, and Mathieu [ACM Trans. Algorithms 2009], and Steiner forest by Bateni, Hajiaghayi, and Marx [J. ACM 2011] (and its improvement to an efficient PTAS by Eisenstat, Klein, and Mathieu [SODA 2012]. The main conceptual contribution here is realizing that selecting which terminals may be relevant is essentially a complicated prizecollecting process: we have to carefully weigh the cost and benefits of reaching or avoiding certain terminals in the spanner. Via a sequence of involved prizecollecting procedures, we can construct a spanner that reaches a set of terminals that is sufficient for an almostoptimal solution. Our PTAS for planar group Steiner tree implies the first PTAS for geometric Euclidean group Steiner tree with obstacles, as well as a (2+)approximation algorithm for group TSP with obstacles, improving over the best previous constantfactor approximation algorithms. By contrast, we show that planar group Steiner forest, a slight generalization of planar group Steiner tree, is APXhard on planar graphs of treewidth 3, even if the groups are pairwise disjoint and every group is a vertex or an edge. 

Belovs, Aleksandrs 
STOC'16: "A Polynomial Lower ..."
A Polynomial Lower Bound for Testing Monotonicity
Aleksandrs Belovs and Eric Blais (CWI, Netherlands; University of Waterloo, Canada) We show that every algorithm for testing nvariate Boolean functions for monotonicity has query complexity Ω(n^{1/4}). All previous lower bounds for this problem were designed for nonadaptive algorithms and, as a result, the best previous lower bound for general (possibly adaptive) monotonicity testers was only Ω(logn). Combined with the query complexity of the nonadaptive monotonicity tester of Khot, Minzer, and Safra (FOCS 2015), our lower bound shows that adaptivity can result in at most a quadratic reduction in the query complexity for testing monotonicity. By contrast, we show that there is an exponential gap between the query complexity of adaptive and nonadaptive algorithms for testing regular linear threshold functions (LTFs) for monotonicity. Chen, De, Servedio, and Tan (STOC 2015) recently showed that nonadaptive algorithms require almost Ω(n^{1/2}) queries for this task. We introduce a new adaptive monotonicity testing algorithm which has query complexity O(logn) when the input is a regular LTF. Andris Ambainis, Kaspars Balodis, Aleksandrs Belovs, Troy Lee, Miklos Santha, and Juris Smotrovs (University of Latvia, Latvia; CWI, Netherlands; Nanyang Technological University, Singapore; National University of Singapore, Singapore; CNRS, France) In 1986, Saks and Wigderson conjectured that the largest separation between deterministic and zeroerror randomized query complexity for a total boolean function is given by the function f on n=2^{k} bits defined by a complete binary tree of NAND gates of depth k, which achieves R_{0}(f) = O(D(f)^{0.7537…}). We show this is false by giving an example of a total boolean function f on n bits whose deterministic query complexity is Ω(n/log(n)) while its zeroerror randomized query complexity is Õ(√n). We further show that the quantum query complexity of the same function is Õ(n^{1/4}), giving the first example of a total function with a superquadratic gap between its quantum and deterministic query complexities. We also construct a total boolean function g on n variables that has zeroerror randomized query complexity Ω(n/log(n)) and boundederror randomized query complexity R(g) = Õ(√n). This is the first superlinear separation between these two complexity measures. The exact quantum query complexity of the same function is Q_{E}(g) = Õ(√n). These functions show that the relations D(f) = O(R_{1}(f)^{2}) and R_{0}(f) = Õ(R(f)^{2}) are optimal, up to polylogarithmic factors. Further variations of these functions give additional separations between other query complexity measures: a cubic separation between Q and R_{0}, a 3/2power separation between Q_{E} and R, and a 4th power separation between approximate degree and boundederror randomized query complexity. All of these examples are variants of a function recently introduced by Goos, Pitassi, and Watson which they used to separate the unambiguous 1certificate complexity from deterministic query complexity and to resolve the famous Clique versus Independent Set problem in communication complexity. 

BenDavid, Shalev 
STOC'16: "Separations in Query ..."
Separations in Query Complexity using Cheat Sheets
Scott Aaronson, Shalev BenDavid, and Robin Kothari (Massachusetts Institute of Technology, USA)
We show a power 2.5 separation between boundederror randomized and quantum query complexity for a total Boolean function, refuting the widely believed conjecture that the best such separation could only be quadratic (from Grover's algorithm). We also present a total function with a power 4 separation between quantum query complexity and approximate polynomial degree, showing severe limitations on the power of the polynomial method. Finally, we exhibit a total function with a quadratic gap between quantum query complexity and certificate complexity, which is optimal (up to log factors). These separations are shown using a new, general technique that we call the cheat sheet technique, which builds upon the techniques of Ambainis et al. [STOC 2016]. The technique is based on a generic transformation that converts any (possibly partial) function into a new total function with desirable properties for showing separations. The framework also allows many known separations, including some recent breakthrough results of Ambainis et al. [STOC 2016], to be shown in a unified manner.
@InProceedings{STOC16p863,
author = {Scott Aaronson and Shalev BenDavid and Robin Kothari},
title = {Separations in Query Complexity using Cheat Sheets},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {863876},
doi = {10.1145/2897518.2897644},
year = {2016},
}
Publisher's Version
Article Search


Bender, Michael A. 
STOC'16: "Contention Resolution ..."
Contention Resolution with LogLogstar Channel Accesses
Michael A. Bender, Tsvi Kopelowitz, Seth Pettie, and Maxwell Young (Stony Brook University, USA; University of Michigan, USA; Mississippi State University, USA)
For decades, randomized exponential backoff has provided a critical
algorithmic building block in situations where multiple devices seek
access to a shared resource. Surprisingly, despite this history,
the performance of standard backoff is poor under worstcase
scheduling of demands on the resource: (i) subconstant throughput
can occur under plausible scenarios, and (ii) each of N devices
requires Omega(log N) access attempts before obtaining the
resource.
In this paper, we address these shortcomings by offering a new
backoff protocol for a shared communications channel that guarantees
expected constant throughput with only O(log(log* N)) access
attempts in expectation. Central to this result are new algorithms
for approximate counting and leader election with the
same performance guarantees.
@InProceedings{STOC16p499,
author = {Michael A. Bender and Tsvi Kopelowitz and Seth Pettie and Maxwell Young},
title = {Contention Resolution with LogLogstar Channel Accesses},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {499508},
doi = {10.1145/2897518.2897655},
year = {2016},
}
Publisher's Version
Article Search


Bernstein, Aaron 
STOC'16: "Deterministic Decremental ..."
Deterministic Decremental Single Source Shortest Paths: Beyond the O(mn) Bound
Aaron Bernstein and Shiri Chechik (Columbia University, USA; Tel Aviv University, Israel) In this paper we consider the decremental singlesource shortest paths (SSSP) problem, where given a graph G and a source node s the goal is to maintain shortest paths between s and all other nodes in G under a sequence of online adversarial edge deletions. In their seminal work, Even and Shiloach [JACM 1981] presented an exact solution to the problem with only O(mn) total update time over all edge deletions. Their classic algorithm was the best known result for the decremental SSSP problem for three decades, even when approximate shortest paths are allowed. The first improvement over the EvenShiloach algorithm was given by Bernstein and Roditty [SODA 2011], who for the case of an unweighted and undirected graph presented an approximate (1+) algorithm with constant query time and a total update time of O(n^{2+O(1/√logn)}). This work triggered a series of new results, culminating in a recent breakthrough of Henzinger, Krinninger and Nanongkai [FOCS 14], who presented a approximate algorithm whose total update time is near linear O(m^{1+ O(1/√logn)}). In this paper they posed as a major open problem the question of derandomizing their result. In fact, all known improvements over the EvenShiloach algorithm are randomized. All these algorithms maintain some truncated shortest path trees from a small subset of nodes. While in the randomized setting it is possible to “hide” these nodes from the adversary, in the deterministic setting this is impossible: the adversary can delete all edges touching these nodes, thus forcing the algorithm to choose a new set of nodes and incur a new computation of shortest paths. In this paper we present the first deterministic decremental SSSP algorithm that breaks the EvenShiloach bound of O(mn) total update time, for unweighted and undirected graphs. Our algorithm is (1 + T) approximate and achieves a total update time of Õ(n^{2}). Our algorithm can also achieve the same bounds in the incremental setting. It is worth mentioning that for dense instances where 

Bhattacharya, Sayan 
STOC'16: "New Deterministic ..."
New Deterministic Approximation Algorithms for Fully Dynamic Matching
Sayan Bhattacharya, Monika Henzinger, and Danupon Nanongkai (Institute of Mathematical Sciences at Chennai, India; University of Vienna, Austria; KTH, Sweden) We present two deterministic dynamic algorithms for the maximum matching problem. (1) An algorithm that maintains a (2+T)approximate maximum matching in general graphs with O(poly(logn, 1/T)) update time. (2) An algorithm that maintains an α_{K} approximation of the value of the maximum matching with O(n^{2/K}) update time in bipartite graphs, for every sufficiently large constant positive integer K. Here, 1≤ α_{K} < 2 is a constant determined by the value of K. Result (1) is the first deterministic algorithm that can maintain an o(logn)approximate maximum matching with polylogarithmic update time, improving the seminal result of Onak et al. [STOC 2010]. Its approximation guarantee almost matches the guarantee of the best randomized polylogarithmic update time algorithm [Baswana et al. FOCS 2011]. Result (2) achieves a betterthantwo approximation with arbitrarily small polynomial update time on bipartite graphs. Previously the best update time for this problem was O(m^{1/4}) [Bernstein et al. ICALP 2015], where m is the current number of edges in the graph. 

Blais, Eric 
STOC'16: "A Polynomial Lower ..."
A Polynomial Lower Bound for Testing Monotonicity
Aleksandrs Belovs and Eric Blais (CWI, Netherlands; University of Waterloo, Canada) We show that every algorithm for testing nvariate Boolean functions for monotonicity has query complexity Ω(n^{1/4}). All previous lower bounds for this problem were designed for nonadaptive algorithms and, as a result, the best previous lower bound for general (possibly adaptive) monotonicity testers was only Ω(logn). Combined with the query complexity of the nonadaptive monotonicity tester of Khot, Minzer, and Safra (FOCS 2015), our lower bound shows that adaptivity can result in at most a quadratic reduction in the query complexity for testing monotonicity. By contrast, we show that there is an exponential gap between the query complexity of adaptive and nonadaptive algorithms for testing regular linear threshold functions (LTFs) for monotonicity. Chen, De, Servedio, and Tan (STOC 2015) recently showed that nonadaptive algorithms require almost Ω(n^{1/2}) queries for this task. We introduce a new adaptive monotonicity testing algorithm which has query complexity O(logn) when the input is a regular LTF. 

Bodwin, Greg 
STOC'16: "The 4/3 Additive ..."
The 4/3 Additive Spanner Exponent Is Tight
Amir Abboud and Greg Bodwin (Stanford University, USA) A spanner is a sparse subgraph that approximately preserves the pairwise distances of the original graph. It is well known that there is a smooth tradeoff between the sparsity of a spanner and the quality of its approximation, so long as distance error is measured multiplicatively. A central open question in the field is to prove or disprove whether such a tradeoff exists also in the regime of additive error. That is, is it true that for all ε>0, there is a constant k_{ε} such that every graph has a spanner on O(n^{1+ε}) edges that preserves its pairwise distances up to +k_{ε}? Previous lower bounds are consistent with a positive resolution to this question, while previous upper bounds exhibit the beginning of a tradeoff curve: all graphs have +2 spanners on O(n^{3/2}) edges, +4 spanners on Õ(n^{7/5}) edges, and +6 spanners on O(n^{4/3}) edges. However, progress has mysteriously halted at the n^{4/3} bound, and despite significant effort from the community, the question has remained open for all 0 < ε < 1/3. Our main result is a surprising negative resolution of the open question, even in a highly generalized setting. We show a new information theoretic incompressibility bound: there is no function that compresses graphs into O(n^{4/3 − ε}) bits so that distance information can be recovered within +n^{o(1)} error. As a special case of our theorem, we get a tight lower bound on the sparsity of additive spanners: the +6 spanner on O(n^{4/3}) edges cannot be improved in the exponent, even if any subpolynomial amount of additive error is allowed. Our theorem implies new lower bounds for related objects as well; for example, the twentyyearold +4 emulator on O(n^{4/3}) edges also cannot be improved in the exponent unless the error allowance is polynomial. Central to our construction is a new type of graph product, which we call the Obstacle Product. Intuitively, it takes two graphs G,H and produces a new graph G H whose shortest paths structure looks locally like H but globally like G. 

Boutsidis, Christos 
STOC'16: "Optimal Principal ..."
Optimal Principal Component Analysis in Distributed and Streaming Models
Christos Boutsidis, David P. Woodruff, and Peilin Zhong (Goldman Sachs, USA; IBM Research, USA; Tsinghua University, China)
This paper studies the Principal Component Analysis (PCA) problem
in the distributed and streaming models of computation. Given a matrix
A ∈ R^{m×n}, a rank parameter k<rank(A),
and an accuracy parameter 0<ε<1, we want to output an m×k orthonormal matrix U for which
AUU^{T}A^{2}_{F}≤(1+ε)AA_{k}^{2}_{F}
where A_{k}∈R^{m×n} is the best rankk approximation to A.
Our contributions are summarized as follows:
1. In the arbitrary partition distributed model of Kannan et al. (COLT 2014), each
of s machines holds a matrix A^{i} and A=ΣA^{i}. Each machine
should output U. Kannan et al. achieve O(skm/ε)+poly(sk/ε) words
(of O(log(nm)) bits)
communication.
We obtain the improved bound of O(skm)+poly(sk/ε) words,
and show an optimal (up to low order terms) Ω(skm) lower bound.
This resolves an open question in the literature.
A poly(ε^{1}) dependence is known to be required, but we separate this
dependence from m.
2. In a more specific distributed model where each server receives a subset of columns of A,
we bypass the above lower bound when A is φsparse in each column. Here we obtain
an O(skφ/ε)+poly(sk/ε) word protocol.
Our communication is independent of the matrix dimensions, and achieves the guarantee
that each server, in addition to outputting U,
outputs a subset of O(k/ε) columns of A containing a U in its span
(that is, for the first time, we solve distributed column subset selection).
Additionally, we show a matching Ω(skφ/ε) lower bound for
distributed column subset selection.
Achieving our communication bound when A is sparse in general but not
sparse in each column, is impossible.
3.
In the streaming model of computation, in which the columns of the matrix A
arrive one at a time, an algorithm of Liberty (KDD, 2013) with
an improved analysis by Ghashami and Phillips (SODA, 2014) achieves
O(km/ε) "real numbers" space complexity.
We improve this result, since our onepass streaming PCA algorithm achieves
an O(km/ε)+poly(k/ε) word space upper bound. This
almost matches a known Ω(km/ε) bit lower bound of Woodruff (NIPS, 2014).
We show that with two passes over the columns of A one can achieve an O(km)+poly(k/ε) word space upper bound;
another lower bound of Woodruff (NIPS, 2014) shows that this is optimal for any constant number of
passes (up to the poly(k/ε) term and the distinction between words versus bits).
4. Finally, in turnstile streams, in which we receive entries of A one at a time in an
arbitrary order, we describe an algorithm with O((m+n)kε^{1}) words of space.
This improves the O((m+n)ε^{2})kε^{2}) upper bound of Clarkson and Woodruff (STOC 2009), and
matches their Ω((m+n)kε^{1}) word lower bound.
Notably, our results do not depend on the condition number or any singular value gaps of A.
@InProceedings{STOC16p236,
author = {Christos Boutsidis and David P. Woodruff and Peilin Zhong},
title = {Optimal Principal Component Analysis in Distributed and Streaming Models},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {236249},
doi = {10.1145/2897518.2897646},
year = {2016},
}
Publisher's Version
Article Search


Brandt, Sebastian 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Braverman, Mark 
STOC'16: "ConstantRate Coding ..."
ConstantRate Coding for Multiparty Interactive Communication Is Impossible
Mark Braverman, Klim Efremenko, Ran Gelles, and Bernhard Haeupler (Princeton University, USA; Tel Aviv University, Israel; Carnegie Mellon University, USA)
We study coding schemes for multiparty interactive communication over synchronous networks that suffer from stochastic noise, where each bit is independently flipped with probability ε. We analyze the minimal overhead that must be added by the coding scheme in order to succeed in performing the computation despite the noise. Our main result is a lower bound on the communication of any noiseresilient protocol over a synchronous star network with nparties (where all parties communicate in every round). Specifically, we show a task that can be solved by communicating T bits over the noisefree network, but for which any protocol with success probability of 1o(1) must communicate at least Ω(T log n / log log n) bits when the channels are noisy. By a 1994 result of Rajagopalan and Schulman, the slowdown we prove is the highest one can obtain on any topology, up to a log log n factor. We complete our lower bound with a matching coding scheme that achieves the same overhead; thus, the capacity of (synchronous) star networks is Θ(log log n / log n). Our bounds prove that, despite several previous coding schemes with rate Ω(1) for certain topologies, no coding scheme with constant rate Ω(1) exists for arbitrary nparty noisy networks.
@InProceedings{STOC16p999,
author = {Mark Braverman and Klim Efremenko and Ran Gelles and Bernhard Haeupler},
title = {ConstantRate Coding for Multiparty Interactive Communication Is Impossible},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {9991010},
doi = {10.1145/2897518.2897563},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "Parallel Algorithms ..."
Parallel Algorithms for Select and Partition with Noisy Comparisons
Mark Braverman, Jieming Mao, and S. Matthew Weinberg (Princeton University, USA)
We consider the problem of finding the kth highest element in a totally ordered set of n elements (Select), and partitioning a totally ordered set into the top k and bottom n − k elements (Partition) using pairwise comparisons. Motivated by settings like peer grading or crowdsourcing, where multiple rounds of interaction are costly and queried comparisons may be inconsistent with the ground truth, we evaluate algorithms based both on their total runtime and the number of interactive rounds in three comparison models: noiseless (where the comparisons are correct), erasure (where comparisons are erased with probability 1 − γ), and noisy (where comparisons are correct with probability 1/2 + γ/2 and incorrect otherwise). We provide numerous matching upper and lower bounds in all three models. Even our results in the noiseless model, which is quite wellstudied in the TCS literature on parallel algorithms, are novel.
@InProceedings{STOC16p851,
author = {Mark Braverman and Jieming Mao and S. Matthew Weinberg},
title = {Parallel Algorithms for Select and Partition with Noisy Comparisons},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {851862},
doi = {10.1145/2897518.2897642},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "Communication Lower ..."
Communication Lower Bounds for Statistical Estimation Problems via a Distributed Data Processing Inequality
Mark Braverman, Ankit Garg, Tengyu Ma, Huy L. Nguyen, and David P. Woodruff (Princeton University, USA; Toyota Technological Institute at Chicago, USA; IBM Research, USA) We study the tradeoff between the statistical error and communication cost of distributed statistical estimation problems in high dimensions. In the distributed sparse Gaussian mean estimation problem, each of the m machines receives n data points from a ddimensional Gaussian distribution with unknown mean θ which is promised to be ksparse. The machines communicate by message passing and aim to estimate the mean θ. We provide a tight (up to logarithmic factors) tradeoff between the estimation error and the number of bits communicated between the machines. This directly leads to a lower bound for the distributed sparse linear regression problem: to achieve the statistical minimax error, the total communication is at least Ω(minn,dm), where n is the number of observations that each machine receives and d is the ambient dimension. These lower results improve upon Shamir (NIPS’14) and SteinhardtDuchi (COLT’15) by allowing multiround iterative communication model. We also give the first optimal simultaneous protocol in the dense case for mean estimation. As our main technique, we prove a distributed data processing inequality, as a generalization of usual data processing inequalities, which might be of independent interest and useful for other problems. 

Braverman, Vladimir 
STOC'16: "Beating CountSketch ..."
Beating CountSketch for Heavy Hitters in Insertion Streams
Vladimir Braverman, Stephen R. Chestnut, Nikita Ivkin, and David P. Woodruff (Johns Hopkins University, USA; ETH Zurich, Switzerland; IBM Research, USA) Given a stream p_{1}, …, p_{m} of items from a universe U, which, without loss of generality we identify with the set of integers {1, 2, …, n}, we consider the problem of returning all ℓ_{2}heavy hitters, i.e., those items j for which f_{j} ≥ T √F_{2}, where f_{j} is the number of occurrences of item j in the stream, and F_{2} = ∑_{i ∈ [n]} f_{i}^{2}. Such a guarantee is considerably stronger than the ℓ_{1}guarantee, which finds those j for which f_{j} ≥ T m. In 2002, Charikar, Chen, and FarachColton suggested the CountSketch data structure, which finds all such j using Θ(log^{2} n) bits of space (for constant T > 0). The only known lower bound is Ω(logn) bits of space, which comes from the need to specify the identities of the items found. In this paper we show one can achieve O(logn loglogn) bits of space for this problem. Our techniques, based on Gaussian processes, lead to a number of other new results for data streams, including: (1) The first algorithm for estimating F_{2} simultaneously at all points in a stream using only O(lognloglogn) bits of space, improving a natural union bound. (2) A way to estimate the ℓ_{∞} norm of a stream up to additive error T √F_{2} with O(lognloglogn) bits of space, resolving Open Question 3 from the IITK 2006 list for insertion only streams. 

Cai, Yang 
STOC'16: "A Duality Based ..."
A Duality Based Unified Approach to Bayesian Mechanism Design
Yang Cai, Nikhil R. Devanur, and S. Matthew Weinberg (McGill University, Canada; Microsoft Research, USA; Princeton University, USA)
We provide a unified view of many recent developments in Bayesian mechanism design, including the blackbox reductions of Cai et. al., simple auctions for additive buyers, and postedprice mechanisms for unitdemand buyers. Additionally, we show that viewing these three previously disjoint lines of work through the same lens leads to new developments as well. First, we provide a duality framework for Bayesian mechanism design, which naturally accommodates multiple agents and arbitrary objectives/feasibility constraints. Using this, we prove that either a postedprice mechanism or the VCG auction with perbidder entry fees achieves a constantfactor of the optimal Bayesian IC revenue whenever buyers are unitdemand or additive, unifying previous breakthroughs of Chawla et. al. and Yao, and improving both approximation ratios (from 33.75 to 24 and 69 to 8). Finally, we show that this view also leads to improved structural characterizations in the Cai et. al. framework.
@InProceedings{STOC16p926,
author = {Yang Cai and Nikhil R. Devanur and S. Matthew Weinberg},
title = {A Duality Based Unified Approach to Bayesian Mechanism Design},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {926939},
doi = {10.1145/2897518.2897645},
year = {2016},
}
Publisher's Version
Article Search


Chakraborty, Diptarka 
STOC'16: "Streaming Algorithms ..."
Streaming Algorithms for Embedding and Computing Edit Distance in the Low Distance Regime
Diptarka Chakraborty, Elazar Goldenberg, and Michal Koucký (IIT Kanpur, India; Charles University in Prague, Czechia) The Hamming and the edit metrics are two common notions of measuring distances between pairs of strings x,y lying in the Boolean hypercube. The edit distance between x and y is defined as the minimum number of character insertion, deletion, and bit flips needed for converting x into y. Whereas, the Hamming distance between x and y is the number of bit flips needed for converting x to y. In this paper we study a randomized injective embedding of the edit distance into the Hamming distance with a small distortion. We show a randomized embedding with quadratic distortion. Namely, for any x,y satisfying that their edit distance equals k, the Hamming distance between the embedding of x and y is O(k^{2}) with high probability. This improves over the distortion ratio of O( n ^{*} n) obtained by Jowhari (2012) for small values of k. Moreover, the embedding output size is linear in the input size and the embedding can be computed using a single pass over the input. We provide several applications for this embedding. Among our results we provide a onepass (streaming) algorithm for edit distance running in space O(s) and computing edit distance exactly upto distance s^{1}/6. This algorithm is based on kernelization for edit distance that is of independent interest. 

Chattopadhyay, Eshan 
STOC'16: "Explicit TwoSource ..."
Explicit TwoSource Extractors and Resilient Functions
Eshan Chattopadhyay and David Zuckerman (University of Texas at Austin, USA)
We explicitly construct an extractor for two independent sources on n bits, each with polylogarithmic minentropy. Our extractor outputs one bit and has polynomially small error.
The best previous extractor, by Bourgain, required each source to have minentropy .499n.
A key ingredient in our construction is an explicit construction of a monotone, almostbalanced Boolean functions that are resilient to coalitions.
In fact, our construction is stronger in that it gives an explicit extractor for a generalization of nonoblivious bitfixing sources on n bits, where some unknown
nq bits are chosen almost polylogarithmicwise independently, and the remaining q bits are chosen by an adversary as an arbitrary function of the nq bits.
The best previous construction, by Viola, achieved q quadratically smaller than our result.
Our explicit twosource extractor directly implies improved constructions of a KRamsey graph over N vertices, improving bounds obtained by Barak et al. and matching independent work by Cohen.
@InProceedings{STOC16p670,
author = {Eshan Chattopadhyay and David Zuckerman},
title = {Explicit TwoSource Extractors and Resilient Functions},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {670683},
doi = {10.1145/2897518.2897528},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "Extractors for Sumset ..."
Extractors for Sumset Sources
Eshan Chattopadhyay and Xin Li (University of Texas at Austin, USA; Johns Hopkins University, USA)
We propose a new model of weak random sources which we call sumset sources. A sumset source X is the sum of C independent sources, with each source on n bits source having minentropy k. We show that extractors for this class of sources can be used to give extractors for most classes of weak sources that have been studied previously, including independent sources, affine sources (which generalizes oblivious bitfixing sources), small space sources, total entropy independent sources, and interleaved sources. This provides a unified approach for randomness extraction.
A known extractor for this class of sources, prior to our work, is the Paley graph function introduced by Chor and Goldreich, which works for the sum of 2 independent sources, where one has minentropy at least 0.51n and the other has logarithmic minentropy. To the best of our knowledge, the only other known construction is from the work of Kamp, Rao, Vadhan and Zuckerman, which can extract from the sum of exponentially many independent sources.
Our main result is an explicit extractor for the sum of C independent sources for some large enough constant C, where each source has polylogarithmic minentropy. We then use this extractor to obtain improved extractors for other well studied classes of sources including smallspace sources, affine sources and interleaved sources.
@InProceedings{STOC16p299,
author = {Eshan Chattopadhyay and Xin Li},
title = {Extractors for Sumset Sources},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {299311},
doi = {10.1145/2897518.2897643},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "Nonmalleable Extractors ..."
Nonmalleable Extractors and Codes, with Their Many Tampered Extensions
Eshan Chattopadhyay, Vipul Goyal, and Xin Li (University of Texas at Austin, USA; Microsoft Research, India; Johns Hopkins University, USA) Randomness extractors and error correcting codes are fundamental objects in computer science. Recently, there have been several natural generalizations of these objects, in the context and study of tamper resilient cryptography. These are seeded nonmalleable extractors, introduced by Dodis and Wichs; seedless nonmalleable extractors, introduced by Cheraghchi and Guruswami; and nonmalleable codes, introduced by Dziembowski, Pietrzak and Wichs. Besides being interesting on their own, they also have important applications in cryptography, e.g, privacy amplification with an active adversary, explicit nonmalleable codes etc, and often have unexpected connections to their nontampered analogues. However, the known constructions are far behind their nontampered counterparts. Indeed, the best known seeded nonmalleable extractor requires minentropy rate at least 0.49; while explicit constructions of nonmalleable twosource extractors were not known even if both sources have full minentropy, and was left as an open problem by Cheraghchi and Guruswami. In this paper we make progress towards solving the above problems and other related generalizations. Our contributions are as follows. (1) We construct an explicit seeded nonmalleable extractor for polylogarithmic minentropy. This dramatically improves all previous results and gives a simpler 2round privacy amplification protocol with optimal entropy loss, matching the best known result. In fact, we construct more general seeded nonmalleable extractors (that can handle multiple adversaries) which were used in the recent construction of explicit twosource extractors for polylogarithmic minentropy. (2) We construct the first explicit nonmalleable twosource extractor for almost full minentropy thus resolving the open question posed by Cheraghchi and Guruswami. (3) We motivate and initiate the study of two natural generalizations of seedless nonmalleable extractors and nonmalleable codes, where the sources or the codeword may be tampered many times. By using the connection found by Cheraghchi and Guruswami and providing efficient sampling algorithms, we obtain the first explicit nonmalleable codes with tampering degree t, with near optimal rate and error. We call these stronger notions onemany and manymanynonmalleable codes. This provides a stronger information theoretic analogue of a primitive known as continuous nonmalleable codes. Our basic technique used in all of our constructions can be seen as inspired, in part, by the techniques previously used to construct cryptographic nonmalleable commitments. 

Chechik, Shiri 
STOC'16: "Deterministic Decremental ..."
Deterministic Decremental Single Source Shortest Paths: Beyond the O(mn) Bound
Aaron Bernstein and Shiri Chechik (Columbia University, USA; Tel Aviv University, Israel) In this paper we consider the decremental singlesource shortest paths (SSSP) problem, where given a graph G and a source node s the goal is to maintain shortest paths between s and all other nodes in G under a sequence of online adversarial edge deletions. In their seminal work, Even and Shiloach [JACM 1981] presented an exact solution to the problem with only O(mn) total update time over all edge deletions. Their classic algorithm was the best known result for the decremental SSSP problem for three decades, even when approximate shortest paths are allowed. The first improvement over the EvenShiloach algorithm was given by Bernstein and Roditty [SODA 2011], who for the case of an unweighted and undirected graph presented an approximate (1+) algorithm with constant query time and a total update time of O(n^{2+O(1/√logn)}). This work triggered a series of new results, culminating in a recent breakthrough of Henzinger, Krinninger and Nanongkai [FOCS 14], who presented a approximate algorithm whose total update time is near linear O(m^{1+ O(1/√logn)}). In this paper they posed as a major open problem the question of derandomizing their result. In fact, all known improvements over the EvenShiloach algorithm are randomized. All these algorithms maintain some truncated shortest path trees from a small subset of nodes. While in the randomized setting it is possible to “hide” these nodes from the adversary, in the deterministic setting this is impossible: the adversary can delete all edges touching these nodes, thus forcing the algorithm to choose a new set of nodes and incur a new computation of shortest paths. In this paper we present the first deterministic decremental SSSP algorithm that breaks the EvenShiloach bound of O(mn) total update time, for unweighted and undirected graphs. Our algorithm is (1 + T) approximate and achieves a total update time of Õ(n^{2}). Our algorithm can also achieve the same bounds in the incremental setting. It is worth mentioning that for dense instances where 

Chen, Sitan 
STOC'16: "Basis Collapse for ..."
Basis Collapse for Holographic Algorithms over All Domain Sizes
Sitan Chen (Harvard University, USA) The theory of holographic algorithms introduced by Valiant represents a novel approach to achieving polynomialtime algorithms for seemingly intractable counting problems via a reduction to counting planar perfect matchings and a linear change of basis. Two fundamental parameters in holographic algorithms are the domain size and the basis size. Roughly, the domain size is the range of colors involved in the counting problem at hand (e.g. counting graph kcolorings is a problem over domain size k), while the basis size captures the dimensionality of the representation of those colors. A major open problem has been: for a given k, what is the smallest ℓ for which any holographic algorithm for a problem over domain size k "collapses to" (can be simulated by) a holographic algorithm with basis size ℓ? Cai and Lu showed in 2008 that over domain size 2, basis size 1 suffices, opening the door to an extensive line of work on the structural theory of holographic algorithms over the Boolean domain. Cai and Fu later showed for signatures of full rank that over domain sizes 3 and 4, basis sizes 1 and 2, respectively, suffice, and they conjectured that over domain size k there is a collapse to basis size ⌊log_{2} k⌋. In this work, we resolve this conjecture in the affirmative for signatures of full rank for all k. 

Chen, Xi 
STOC'16: "NearOptimal SmallDepth ..."
NearOptimal SmallDepth Lower Bounds for Small Distance Connectivity
Xi Chen, Igor C. Oliveira, Rocco A. Servedio, and LiYang Tan (Columbia University, USA; Charles University in Prague, Czechia; Toyota Technological Institute at Chicago, USA) We show that any depthd circuit for determining whether an nnode graph has an stot path of length at most k must have size n^{Ω(k1/d/d)} when k(n) ≤ n^{1/5}, and n^{Ω(k1/5d/d)} when k(n)≤ n. The previous best circuit size lower bounds were n^{kexp(−O(d))} (by Beame, Impagliazzo, and Pitassi (Computational Complexity 1998)) and n^{Ω((logk)/d)} (following from a recent formula size lower bound of Rossman (STOC 2014)). Our lower bound is quite close to optimal, as a simple construction gives depthd circuits of size n^{O(k2/d)} for this problem (and strengthening our bound even to n^{kΩ(1/d)} would require proving that undirected connectivity is not in NC^{1}). Our proof is by reduction to a new lower bound on the size of smalldepth circuits computing a skewed variant of the “Sipser functions” that have played an important role in classical circuit lower bounds. A key ingredient in our proof of the required lower bound for these Sipserlike functions is the use of random projections, an extension of random restrictions which were recently employed by Rossman, Servedio, and Tan (FOCS 2015). Random projections allow us to obtain sharper quantitative bounds while employing simpler arguments, both conceptually and technically, than in the previous works. 

Chestnut, Stephen R. 
STOC'16: "Beating CountSketch ..."
Beating CountSketch for Heavy Hitters in Insertion Streams
Vladimir Braverman, Stephen R. Chestnut, Nikita Ivkin, and David P. Woodruff (Johns Hopkins University, USA; ETH Zurich, Switzerland; IBM Research, USA) Given a stream p_{1}, …, p_{m} of items from a universe U, which, without loss of generality we identify with the set of integers {1, 2, …, n}, we consider the problem of returning all ℓ_{2}heavy hitters, i.e., those items j for which f_{j} ≥ T √F_{2}, where f_{j} is the number of occurrences of item j in the stream, and F_{2} = ∑_{i ∈ [n]} f_{i}^{2}. Such a guarantee is considerably stronger than the ℓ_{1}guarantee, which finds those j for which f_{j} ≥ T m. In 2002, Charikar, Chen, and FarachColton suggested the CountSketch data structure, which finds all such j using Θ(log^{2} n) bits of space (for constant T > 0). The only known lower bound is Ω(logn) bits of space, which comes from the need to specify the identities of the items found. In this paper we show one can achieve O(logn loglogn) bits of space for this problem. Our techniques, based on Gaussian processes, lead to a number of other new results for data streams, including: (1) The first algorithm for estimating F_{2} simultaneously at all points in a stream using only O(lognloglogn) bits of space, improving a natural union bound. (2) A way to estimate the ℓ_{∞} norm of a stream up to additive error T √F_{2} with O(lognloglogn) bits of space, resolving Open Question 3 from the IITK 2006 list for insertion only streams. 

Choudhary, Keerti 
STOC'16: "Fault Tolerant Subgraph ..."
Fault Tolerant Subgraph for Single Source Reachability: Generic and Optimal
Surender Baswana, Keerti Choudhary, and Liam Roditty (IIT Kanpur, India; BarIlan University, Israel) Let G=(V,E) be an nvertices medges directed graph. Let s∈ V be any designated source vertex. We address the problem of single source reachability (SSR) from s in presence of failures of vertices/edges. We show that for every k≥ 1, there is a subgraph H of G with at most 2^{k} n edges that preserves the reachability from s even after the failure of any k edges. Formally, given a set F of k edges, a vertex u∈ V is reachable from s in G∖ F if and only if u is reachable from s in H∖ F. We call H a kFault Tolerant Reachability Subgraph (kFTRS). We prove also a matching lower bound of Ω(2^{k}n) for such subgraphs. Our results extend to vertex failures without any extra overhead. The general construction of kFTRS is interesting from several different perspectives. From the Graph theory perspective it reveals a separation between SSR and single source shortest paths (SSSP) in directed graphs. More specifically, in the case of SSSP in weighted directed graphs, there is a lower bound of Ω(m) even for a single edge failure. In the case of unweighted graphs there is a lower bound of Ω(n^{3/2}) edges, again, even for a single edge failure. There is also a matching upper bound but nothing is known for two or more failures in the directed graphs. From the Algorithms perspective it implies fault tolerant solutions to other interesting problems, namely, (i) verifying if the strong connectivity of a graph is preserved after k edge or vertex failures, (ii) computing a dominator tree of a graph after kfailures. From the perspective of Techniques it makes an interesting usage of the concept of farthest mincut which was already introduced by Ford and Fulkerson in their pioneering work on flows and cuts. We show that there is a close relationship between the farthest mincut and the kFTRS. We believe that our new technique is of independent interest. 

Chuzhoy, Julia 
STOC'16: "Improved Approximation ..."
Improved Approximation for NodeDisjoint Paths in Planar Graphs
Julia Chuzhoy, David H. K. Kim, and Shi Li (Toyota Technological Institute at Chicago, USA; University of Chicago, USA; State University of New York at Buffalo, USA) We study the classical NodeDisjoint Paths (NDP) problem: given an nvertex graph G and a collection =(s_{1},t_{1}),…,(s_{k},t_{k}) of pairs of vertices of G called demand pairs, find a maximumcardinality set of nodedisjoint paths connecting the demand pairs. NDP is one of the most basic routing problems, that has been studied extensively. Despite this, there are still wide gaps in our understanding of its approximability: the best currently known upper bound of O(√n) on its approximation ratio is achieved via a simple greedy algorithm, while the best current negative result shows that the problem does not have a better than Ω(log^{1/2−δ}n)approximation for any constant δ, under standard complexity assumptions. Even for planar graphs no better approximation algorithms are known, and to the best of our knowledge, the best negative bound is APXhardness. Perhaps the biggest obstacle to obtaining better approximation algorithms for NDP is that most currently known approximation algorithms for this type of problems rely on the standard multicommodity flow relaxation, whose integrality gap is Ω(√n) for NDP, even in planar graphs. In this paper, we break the barrier of O(√n) on the approximability of NDP in planar graphs and obtain an Õ(n^{9/19})approximation. We introduce a new linear programming relaxation of the problem, and a number of new techniques, that we hope will be helpful in designing more powerful algorithms for this and related problems. 

Cohen, Aloni 
STOC'16: "Watermarking Cryptographic ..."
Watermarking Cryptographic Capabilities
Aloni Cohen, Justin Holmgren, Ryo Nishimaki, Vinod Vaikuntanathan, and Daniel Wichs (Massachusetts Institute of Technology, USA; NTT, Japan; Northeastern University, USA)
A watermarking scheme for programs embeds some information called a mark into a program while preserving its functionality. No adversary can remove the mark without damaging the functionality of the program. In this work, we study the problem of watermarking various cryptographic programs such as pseudorandom function (PRF) evaluation, decryption, and signing. For example, given a PRF key K, we create a marked program C that evaluates the PRF F(K,). An adversary that gets C cannot come up with any program C* in which the mark is removed but which still evaluates the PRF correctly on even a small fraction of the inputs. The work of Barak, Goldreich, Impagliazzo, Rudich, Sahai, Vadhan, and Yang (CRYPTO'01 and Journal of ACM 59(2)) shows that, assuming indistinguishability obfuscation (iO), such watermarking is impossible if the marked program C evaluates the original program with perfect correctness. In this work we show that, assuming iO, such watermarking is possible if the marked program C is allowed to err with even a negligible probability, which would be undetectable to the user. Our watermarking schemes are public key, namely we use a secret marking key to embed marks in programs, and a public detection key that allows anyone to detect marks in programs. Our schemes are secure against chosen program attacks, that is even if the adversary is given oracle access to the marking functionality. We emphasize that our security notion of watermark nonremovability considers arbitrary adversarial strategies to modify the marked program, in contrast to the prior works (Nishimaki, EUROCRYPT '13).
@InProceedings{STOC16p1115,
author = {Aloni Cohen and Justin Holmgren and Ryo Nishimaki and Vinod Vaikuntanathan and Daniel Wichs},
title = {Watermarking Cryptographic Capabilities},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11151127},
doi = {10.1145/2897518.2897651},
year = {2016},
}
Publisher's Version
Article Search


Cohen, Gil 
STOC'16: "TwoSource Dispersers ..."
TwoSource Dispersers for Polylogarithmic Entropy and Improved Ramsey Graphs
Gil Cohen (Weizmann Institute of Science, Israel) In his influential 1947 paper that inaugurated the probabilistic method, Erdős proved the existence of 2lognRamsey graphs on n vertices. Matching Erdős’ result with a constructive proof is considered a central problem in combinatorics, that has gained a significant attention in the literature. The state of the art result was obtained in the celebrated paper by Barak, Rao, Shaltiel, and Wigderson who constructed a 2^{2(loglogn)1−α}Ramsey graph, for some small universal constant α > 0. In this work, we significantly improve this result and construct 2^{(loglogn)c}Ramsey graphs, for some universal constant c. In the language of theoretical computer science, this resolves the problem of explicitly constructing dispersers for two nbit sources with entropy (n). In fact, our disperser is a zeroerror disperser that outputs a constant fraction of the entropy. Prior to this work, such dispersers could only support entropy Ω(n). 

Cohen, Michael B. 
STOC'16: "Geometric Median ..."
Geometric Median in Nearly Linear Time
Michael B. Cohen, Yin Tat Lee, Gary Miller, Jakub Pachocki, and Aaron Sidford (Massachusetts Institute of Technology, USA; Carnegie Mellon University, USA; Microsoft Research, USA) In this paper we provide faster algorithms for solving the geometric median problem: given n points in ^{d} compute a point that minimizes the sum of Euclidean distances to the points. This is one of the oldest nontrivial problems in computational geometry yet despite a long history of research the previous fastest running times for computing a (1+T)approximate geometric median were O(d· n^{4/3}T^{−8/3}) by Chin et. al, Õ(dexpT^{−4}logT^{−1}) by Badoiu et. al, O(nd+poly(d,T^{−1})) by Feldman and Langberg, and the polynomial running time of O((nd)^{O(1)}log1/T) by Parrilo and Sturmfels and Xue and Ye. In this paper we show how to compute such an approximate geometric median in time O(ndlog^{3}n/T) and O(dT^{−2}). While our O(dT^{−2}) is a fairly straightforward application of stochastic subgradient descent, our O(ndlog^{3}n/T) time algorithm is a novel long step interior point method. We start with a simple O((nd)^{O(1)}log1/T) time interior point method and show how to improve it, ultimately building an algorithm that is quite nonstandard from the perspective of interior point literature. Our result is one of few cases of outperforming standard interior point theory. Furthermore, it is the only case we know of where interior point methods yield a nearly linear time algorithm for a canonical optimization problem that traditionally requires superlinear time. 

CohenAddad, Vincent 
STOC'16: "Approximating Connectivity ..."
Approximating Connectivity Domination in Weighted BoundedGenus Graphs
Vincent CohenAddad, Éric Colin de Verdière, Philip N. Klein, Claire Mathieu, and David Meierfrankenfeld (ENS, France; CNRS, France; Brown University, USA)
We present a framework for addressing several problems on weighted planar graphs and graphs of bounded genus. With that framework, we derive polynomialtime approximation schemes for the following problems in planar graphs or graphs of bounded genus: edgeweighted tree cover and tour cover; vertexweighted connected dominating set, maxweightleaf spanning tree, and connected vertex cover. In addition, we obtain a polynomialtime approximation scheme for feedback vertex set in planar graphs. These are the first polynomialtime approximation schemes for all those problems in weighted embedded graphs. (For unweighted versions of some of these problems, polynomialtime approximation schemes were previously given using bidimensionality.)
@InProceedings{STOC16p584,
author = {Vincent CohenAddad and Éric Colin de Verdière and Philip N. Klein and Claire Mathieu and David Meierfrankenfeld},
title = {Approximating Connectivity Domination in Weighted BoundedGenus Graphs},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {584597},
doi = {10.1145/2897518.2897635},
year = {2016},
}
Publisher's Version
Article Search


Colin de Verdière, Éric 
STOC'16: "Approximating Connectivity ..."
Approximating Connectivity Domination in Weighted BoundedGenus Graphs
Vincent CohenAddad, Éric Colin de Verdière, Philip N. Klein, Claire Mathieu, and David Meierfrankenfeld (ENS, France; CNRS, France; Brown University, USA)
We present a framework for addressing several problems on weighted planar graphs and graphs of bounded genus. With that framework, we derive polynomialtime approximation schemes for the following problems in planar graphs or graphs of bounded genus: edgeweighted tree cover and tour cover; vertexweighted connected dominating set, maxweightleaf spanning tree, and connected vertex cover. In addition, we obtain a polynomialtime approximation scheme for feedback vertex set in planar graphs. These are the first polynomialtime approximation schemes for all those problems in weighted embedded graphs. (For unweighted versions of some of these problems, polynomialtime approximation schemes were previously given using bidimensionality.)
@InProceedings{STOC16p584,
author = {Vincent CohenAddad and Éric Colin de Verdière and Philip N. Klein and Claire Mathieu and David Meierfrankenfeld},
title = {Approximating Connectivity Domination in Weighted BoundedGenus Graphs},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {584597},
doi = {10.1145/2897518.2897635},
year = {2016},
}
Publisher's Version
Article Search


Czumaj, Artur 
STOC'16: "Relating Two Property ..."
Relating Two Property Testing Models for Bounded Degree Directed Graphs
Artur Czumaj, Pan Peng, and Christian Sohler (University of Warwick, UK; TU Dortmund, Germany; Institute of Software at Chinese Academy of Sciences, China) We study property testing algorithms in directed graphs (digraphs) with maximum indegree and maximum outdegree upper bounded by d. For directed graphs with bounded degree, there are two different models in property testing introduced by Bender and Ron (2002). In the bidirectional model, one can access both incoming and outgoing edges while in the unidirectional model one can only access outgoing edges. In our paper we provide a new relation between the two models: we prove that if a property can be tested with constant query complexity in the bidirectional model, then it can be tested with sublinear query complexity in the unidirectional model. A corollary of this result is that in the unidirectional model (the model allowing only queries to the outgoing neighbors), every property in hyperfinite digraphs is testable with sublinear query complexity. 

Daniely, Amit 
STOC'16: "Complexity Theoretic ..."
Complexity Theoretic Limitations on Learning Halfspaces
Amit Daniely (Google, USA) We study the problem of agnostically learning halfspaces which is defined by a fixed but unknown distribution D on Q^n X 1,1. We define Err_H(D) as the least error of a halfspace classifier for D. A learner who can access D has to return a hypothesis whose error is small compared to Err_H(D). Using the recently developed method of Daniely, Linial and ShalevShwartz we prove hardness of learning results assuming that random KXOR formulas are hard to (strongly) refute. We show that no efficient learning algorithm has nontrivial worstcase performance even under the guarantees that Err_H(D) <= eta for arbitrarily small constant eta>0, and that D is supported in the Boolean cube. Namely, even under these favorable conditions, and for every c>0, it is hard to return a hypothesis with error <= 1/2n^c. In particular, no efficient algorithm can achieve a constant approximation ratio. Under a stronger version of the assumption (where K can be polylogarithmic in n), we can take eta = 2^log^1nu(n) for arbitrarily small nu>0. These results substantially improve on previously known results, that only show hardness of exact learning. 

Dasgupta, Sanjoy 
STOC'16: "A Cost Function ..."
A Cost Function for SimilarityBased Hierarchical Clustering
Sanjoy Dasgupta (University of California at San Diego, USA)
The development of algorithms for hierarchical clustering has been hampered by a shortage of precise objective functions. To help address this situation, we introduce a simple cost function on hierarchies over a set of points, given pairwise similarities between those points. We show that this criterion behaves sensibly in canonical instances and that it admits a topdown construction procedure with a provably good approximation ratio.
@InProceedings{STOC16p118,
author = {Sanjoy Dasgupta},
title = {A Cost Function for SimilarityBased Hierarchical Clustering},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {118127},
doi = {10.1145/2897518.2897527},
year = {2016},
}
Publisher's Version
Article Search


Daskalakis, Constantinos 
STOC'16: "A SizeFree CLT ..."
A SizeFree CLT for Poisson Multinomials and Its Applications
Constantinos Daskalakis, Anindya De, Gautam Kamath, and Christos Tzamos (Massachusetts Institute of Technology, USA; Northwestern University, USA) An (n,k)Poisson Multinomial Distribution (PMD) is the distribution of the sum of n independent random vectors supported on the set B_{k}={e_{1},…,e_{k}} of standard basis vectors in ℝ^{k}. We show that any (n,k)PMD is poly(k/σ)close in total variation distance to the (appropriately discretized) multidimensional Gaussian with the same first two moments, removing the dependence on n from the Central Limit Theorem of Valiant and Valiant. Interestingly, our CLT is obtained by bootstrapping the ValiantValiant CLT itself through the structural characterization of PMDs shown in recent work by Daskalakis, Kamath and Tzamos. In turn, our stronger CLT can be leveraged to obtain an efficient PTAS for approximate Nash equilibria in anonymous games, significantly improving the state of the art, and matching qualitatively the running time dependence on n and 1/T of the best known algorithm for twostrategy anonymous games. Our new CLT also enables the construction of covers for the set of (n,k)PMDs, which are proper and whose size is shown to be essentially optimal. Our cover construction combines our CLT with the ShapleyFolkman theorem and recent sparsification results for Laplacian matrices by Batson, Spielman, and Srivastava. Our cover size lower bound is based on an algebraic geometric construction. Finally, leveraging the structural properties of the Fourier spectrum of PMDs we show that these distributions can be learned from O_{k}(1/T^{2}) samples in poly_{k}(1/T)time, removing the quasipolynomial dependence of the running time on 1/T from prior work. 

David, Roee 
STOC'16: "On the Effect of ..."
On the Effect of Randomness on Planted 3Coloring Models
Roee David and Uriel Feige (Weizmann Institute of Science, Israel)
We present the hosted coloring framework for studying al gorithmic and hardness results for the kcoloring problem. There is a class H of host graphs. One selects a graph H ∈ H and plants in it a balanced kcoloring (by partitioning the vertex set into k roughly equal parts, and removing all edges within each part). The resulting graph G is given as input to a polynomial time algorithm that needs to kcolor G (any legal kcoloring would do – the algorithm is not required to recover the planted kcoloring). Earlier planted models correspond to the case that H is the class of all nvertex dregular graphs, a member H ∈ H is chosen at random, and then a balanced kcoloring is planted at random. Blum and Spencer [1995] designed algorithms for this model when d = n δ (for 0 < δ ≤ 1), and Alon and Kahale [1997] managed to do so even when d is a sufficiently large constant. The new aspect in our framework is that it need not in volve randomness. In one model within the framework (with k = 3) H is a d regular spectral expander (meaning that ex cept for the largest eigenvalue of its adjacency matrix, every other eigenvalue has absolute value much smaller than d) chosen by an adversary, and the planted 3coloring is ran dom. We show that the 3coloring algorithm of Alon and Kahale [1997] can be modified to apply to this case. In an other model H is a random dregular graph but the planted balanced 3coloring is chosen by an adversary, after seeing H. We show that for a certain range of average degrees somewhat below √ n, finding a 3coloring is NPhard. To gether these results (and other results that we have) help clarify which aspects of randomness in the planted coloring model are the key to successful 3coloring algorithms.
@InProceedings{STOC16p77,
author = {Roee David and Uriel Feige},
title = {On the Effect of Randomness on Planted 3Coloring Models},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {7790},
doi = {10.1145/2897518.2897561},
year = {2016},
}
Publisher's Version
Article Search


De, Anindya 
STOC'16: "A SizeFree CLT ..."
A SizeFree CLT for Poisson Multinomials and Its Applications
Constantinos Daskalakis, Anindya De, Gautam Kamath, and Christos Tzamos (Massachusetts Institute of Technology, USA; Northwestern University, USA) An (n,k)Poisson Multinomial Distribution (PMD) is the distribution of the sum of n independent random vectors supported on the set B_{k}={e_{1},…,e_{k}} of standard basis vectors in ℝ^{k}. We show that any (n,k)PMD is poly(k/σ)close in total variation distance to the (appropriately discretized) multidimensional Gaussian with the same first two moments, removing the dependence on n from the Central Limit Theorem of Valiant and Valiant. Interestingly, our CLT is obtained by bootstrapping the ValiantValiant CLT itself through the structural characterization of PMDs shown in recent work by Daskalakis, Kamath and Tzamos. In turn, our stronger CLT can be leveraged to obtain an efficient PTAS for approximate Nash equilibria in anonymous games, significantly improving the state of the art, and matching qualitatively the running time dependence on n and 1/T of the best known algorithm for twostrategy anonymous games. Our new CLT also enables the construction of covers for the set of (n,k)PMDs, which are proper and whose size is shown to be essentially optimal. Our cover construction combines our CLT with the ShapleyFolkman theorem and recent sparsification results for Laplacian matrices by Batson, Spielman, and Srivastava. Our cover size lower bound is based on an algebraic geometric construction. Finally, leveraging the structural properties of the Fourier spectrum of PMDs we show that these distributions can be learned from O_{k}(1/T^{2}) samples in poly_{k}(1/T)time, removing the quasipolynomial dependence of the running time on 1/T from prior work. 

Demaine, Erik D. 
STOC'16: "A PTAS for Planar ..."
A PTAS for Planar Group Steiner Tree via Spanner Bootstrapping and Prize Collecting
MohammadHossein Bateni, Erik D. Demaine, MohammadTaghi Hajiaghayi, and Dániel Marx (Google, USA; Massachusetts Institute of Technology, USA; University of Maryland at College Park, USA; Hungarian Academy of Sciences, Hungary) We present the first polynomialtime approximation scheme (PTAS), i.e., (1+ε)approximation algorithm for any constant ε> 0, for the planar group Steiner tree problem (in which each group lies on a boundary of a face). This result improves on the best previous approximation factor of O(logn (loglogn)^{O(1)}). We achieve this result via a novel and powerful technique called spanner bootstrapping, which allows one to bootstrap from a superconstant approximation factor (even superpolynomial in the input size) all the way down to a PTAS. This is in contrast with the popular existing approach for planar PTASs of constructing lightweight spanners in one iteration, which notably requires a constantfactor approximate solution to start from. Spanner bootstrapping removes one of the main barriers for designing PTASs for problems which have no known constantfactor approximation (even on planar graphs), and thus can be used to obtain PTASs for several difficulttoapproximate problems. Our second major contribution required for the planar group Steiner tree PTAS is a spanner construction, which reduces the graph to have total weight within a factor of the optimal solution while approximately preserving the optimal solution. This is particularly challenging because group Steiner tree requires deciding which terminal in each group to connect by the tree, making it much harder than recent previous approaches to construct spanners for planar TSP by Klein [SIAM J. Computing 2008], subset TSP by Klein [STOC 2006], Steiner tree by Borradaile, Klein, and Mathieu [ACM Trans. Algorithms 2009], and Steiner forest by Bateni, Hajiaghayi, and Marx [J. ACM 2011] (and its improvement to an efficient PTAS by Eisenstat, Klein, and Mathieu [SODA 2012]. The main conceptual contribution here is realizing that selecting which terminals may be relevant is essentially a complicated prizecollecting process: we have to carefully weigh the cost and benefits of reaching or avoiding certain terminals in the spanner. Via a sequence of involved prizecollecting procedures, we can construct a spanner that reaches a set of terminals that is sufficient for an almostoptimal solution. Our PTAS for planar group Steiner tree implies the first PTAS for geometric Euclidean group Steiner tree with obstacles, as well as a (2+)approximation algorithm for group TSP with obstacles, improving over the best previous constantfactor approximation algorithms. By contrast, we show that planar group Steiner forest, a slight generalization of planar group Steiner tree, is APXhard on planar graphs of treewidth 3, even if the groups are pairwise disjoint and every group is a vertex or an edge. 

Devanur, Nikhil R. 
STOC'16: "The Sample Complexity ..."
The Sample Complexity of Auctions with Side Information
Nikhil R. Devanur, Zhiyi Huang, and ChristosAlexandros Psomas (Microsoft Research, USA; University of Hong Kong, China; University of California at Berkeley, USA)
Traditionally, the Bayesian optimal auction design problem has been considered either when the bidder values are i.i.d, or when each bidder is individually identifiable via her value distribution. The latter is a reasonable approach when the bidders can be classified into a few categories, but there are many instances where the classification of bidders is a continuum. For example, the classification of the bidders may be based on their annual income, their propensity to buy an item based on past behavior, or in the case of ad auctions, the click through rate of their ads. We introduce an alternate model that captures this aspect, where bidders are a priori identical, but can be distinguished based (only) on some side information the auctioneer obtains at the time of the auction. We extend the sample complexity approach of Dhangwatnotai et al. and Cole and Roughgarden to this model and obtain almost matching upper and lower bounds. As an aside, we obtain a revenue monotonicity lemma which may be of independent interest. We also show how to use Empirical Risk Minimization techniques to improve the sample complexity bound of Cole and Roughgarden for the nonidentical but independent value distribution case.
@InProceedings{STOC16p426,
author = {Nikhil R. Devanur and Zhiyi Huang and ChristosAlexandros Psomas},
title = {The Sample Complexity of Auctions with Side Information},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {426439},
doi = {10.1145/2897518.2897553},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "A Duality Based ..."
A Duality Based Unified Approach to Bayesian Mechanism Design
Yang Cai, Nikhil R. Devanur, and S. Matthew Weinberg (McGill University, Canada; Microsoft Research, USA; Princeton University, USA)
We provide a unified view of many recent developments in Bayesian mechanism design, including the blackbox reductions of Cai et. al., simple auctions for additive buyers, and postedprice mechanisms for unitdemand buyers. Additionally, we show that viewing these three previously disjoint lines of work through the same lens leads to new developments as well. First, we provide a duality framework for Bayesian mechanism design, which naturally accommodates multiple agents and arbitrary objectives/feasibility constraints. Using this, we prove that either a postedprice mechanism or the VCG auction with perbidder entry fees achieves a constantfactor of the optimal Bayesian IC revenue whenever buyers are unitdemand or additive, unifying previous breakthroughs of Chawla et. al. and Yao, and improving both approximation ratios (from 33.75 to 24 and 69 to 8). Finally, we show that this view also leads to improved structural characterizations in the Cai et. al. framework.
@InProceedings{STOC16p926,
author = {Yang Cai and Nikhil R. Devanur and S. Matthew Weinberg},
title = {A Duality Based Unified Approach to Bayesian Mechanism Design},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {926939},
doi = {10.1145/2897518.2897645},
year = {2016},
}
Publisher's Version
Article Search


Diakonikolas, Ilias 
STOC'16: "The Fourier Transform ..."
The Fourier Transform of Poisson Multinomial Distributions and Its Algorithmic Applications
Ilias Diakonikolas, Daniel M. Kane, and Alistair Stewart (University of Southern California, USA; University of California at San Diego, USA) An (n, k)Poisson Multinomial Distribution (PMD) is a random variable of the form X = ∑_{i=1}^{n} X_{i}, where the X_{i}’s are independent random vectors supported on the set of standard basis vectors in ^{k}. In this paper, we obtain a refined structural understanding of PMDs by analyzing their Fourier transform. As our core structural result, we prove that the Fourier transform of PMDs is approximately sparse, i.e., its L_{1}norm is small outside a small set. By building on this result, we obtain the following applications: Learning Theory. We give the first computationally efficient learning algorithm for PMDs under the total variation distance. Our algorithm learns an (n, k)PMD within variation distance ε using a nearoptimal sample size of O_{k}(1/ε^{2}), and runs in time O_{k}(1/ε^{2}) · logn. Previously, no algorithm with a (1/ε) runtime was known, even for k=3. Game Theory. We give the first efficient polynomialtime approximation scheme (EPTAS) for computing Nash equilibria in anonymous games. For normalized anonymous games with n players and k strategies, our algorithm computes a wellsupported εNash equilibrium in time n^{O(k3)} · (k/ε)^{O(k3log(k/ε)/loglog(k/ε))k−1}. The best previous algorithm for this problem [, ] had running time n^{(f(k)/ε)k}, where f(k) = Ω(k^{k2}), for any k>2. Statistics. We prove a multivariate central limit theorem (CLT) that relates an arbitrary PMD to a discretized multivariate Gaussian with the same mean and covariance, in total variation distance. Our new CLT strengthens the CLT of Valiant and Valiant [, ] by removing the dependence on n in the error bound. Along the way we prove several new structural results of independent interest about PMDs. These include: (i) a robust momentmatching lemma, roughly stating that two PMDs that approximately agree on their lowdegree parameter moments are close in variation distance; (ii) nearoptimal size proper εcovers for PMDs in total variation distance (constructive upper bound and nearlymatching lower bound). In addition to Fourier analysis, we employ a number of analytic tools, including the saddlepoint method from complex analysis, that may find other applications. 

Dobzinski, Shahar 
STOC'16: "Breaking the Logarithmic ..."
Breaking the Logarithmic Barrier for Truthful Combinatorial Auctions with Submodular Bidders
Shahar Dobzinski (Weizmann Institute of Science, Israel)
We study a central problem in Algorithmic Mechanism Design: constructing truthful mechanisms for welfare maximization in combinatorial auctions with submodular bidders. Dobzinski, Nisan, and Schapira provided the first mechanism that guarantees a nontrivial approximation ratio of O(log^2 m) [STOC'06], where m is the number of items. This was subsequently improved to O( log m log log m) [Dobzinski, APPROX'07] and then to O(m) [Krysta and Vocking, ICALP'12]. In this paper we develop the first mechanism that breaks the logarithmic barrier. Specifically, the mechanism provides an approximation ratio of O( m). Similarly to previous constructions, our mechanism uses polynomially many value and demand queries, and in fact provides the same approximation ratio for the larger class of XOS (a.k.a. fractionally subadditive) valuations. We also develop a computationally efficient implementation of the mechanism for combinatorial auctions with budget additive bidders. Although in general computing a demand query is NPhard for budget additive valuations, we observe that the specific form of demand queries that our mechanism uses can be efficiently computed when bidders are budget additive.
@InProceedings{STOC16p940,
author = {Shahar Dobzinski},
title = {Breaking the Logarithmic Barrier for Truthful Combinatorial Auctions with Submodular Bidders},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {940948},
doi = {10.1145/2897518.2897569},
year = {2016},
}
Publisher's Version
Article Search


Dughmi, Shaddin 
STOC'16: "Algorithmic Bayesian ..."
Algorithmic Bayesian Persuasion
Shaddin Dughmi and Haifeng Xu (University of Southern California, USA)
Persuasion, defined as the act of exploiting an informational advantage in order to effect the decisions of others, is ubiquitous. Indeed, persuasive communication has been estimated to account for almost a third of all economic activity in the US. This paper examines persuasion through a computational lens, focusing on what is perhaps the most basic and fundamental model in this space: the celebrated Bayesian persuasion model of Kamenica and Gentzkow. Here there are two players, a sender and a receiver. The receiver must take one of a number of actions with apriori unknown payoff, and the sender has access to additional information regarding the payoffs of the various actions for both players. The sender can commit to revealing a noisy signal regarding the realization of the payoffs of various actions, and would like to do so as to maximize her own payoff in expectation assuming that the receiver rationally acts to maximize his own payoff. When the payoffs of various actions follow a joint distribution (the common prior), the sender's problem is nontrivial, and its computational complexity depends on the representation of this prior. We examine the sender's optimization task in three of the most natural input models for this problem, and essentially pin down its computational complexity in each. When the payoff distributions of the different actions are i.i.d. and given explicitly, we exhibit a polynomialtime (exact) algorithmic solution, and a ``simple'' (11/e)approximation algorithm. Our optimal scheme for the i.i.d. setting involves an analogy to auction theory, and makes use of Border's characterization of the space of reducedforms for singleitem auctions. When action payoffs are independent but nonidentical with marginal distributions given explicitly, we show that it is #Phard to compute the optimal expected sender utility. In doing so, we rule out a generalized Border's theorem, as defined by Gopalan et al, for this setting. Finally, we consider a general (possibly correlated) joint distribution of action payoffs presented by a black box sampling oracle, and exhibit a fully polynomialtime approximation scheme (FPTAS) with a bicriteria guarantee. Our FPTAS is based on MonteCarlo sampling, and its analysis relies on the principle of deferred decisions. Moreover, we show that this result is the best possible in the blackbox model for informationtheoretic reasons.
@InProceedings{STOC16p412,
author = {Shaddin Dughmi and Haifeng Xu},
title = {Algorithmic Bayesian Persuasion},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {412425},
doi = {10.1145/2897518.2897583},
year = {2016},
}
Publisher's Version
Article Search


Efremenko, Klim 
STOC'16: "ConstantRate Coding ..."
ConstantRate Coding for Multiparty Interactive Communication Is Impossible
Mark Braverman, Klim Efremenko, Ran Gelles, and Bernhard Haeupler (Princeton University, USA; Tel Aviv University, Israel; Carnegie Mellon University, USA)
We study coding schemes for multiparty interactive communication over synchronous networks that suffer from stochastic noise, where each bit is independently flipped with probability ε. We analyze the minimal overhead that must be added by the coding scheme in order to succeed in performing the computation despite the noise. Our main result is a lower bound on the communication of any noiseresilient protocol over a synchronous star network with nparties (where all parties communicate in every round). Specifically, we show a task that can be solved by communicating T bits over the noisefree network, but for which any protocol with success probability of 1o(1) must communicate at least Ω(T log n / log log n) bits when the channels are noisy. By a 1994 result of Rajagopalan and Schulman, the slowdown we prove is the highest one can obtain on any topology, up to a log log n factor. We complete our lower bound with a matching coding scheme that achieves the same overhead; thus, the capacity of (synchronous) star networks is Θ(log log n / log n). Our bounds prove that, despite several previous coding schemes with rate Ω(1) for certain topologies, no coding scheme with constant rate Ω(1) exists for arbitrary nparty noisy networks.
@InProceedings{STOC16p999,
author = {Mark Braverman and Klim Efremenko and Ran Gelles and Bernhard Haeupler},
title = {ConstantRate Coding for Multiparty Interactive Communication Is Impossible},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {9991010},
doi = {10.1145/2897518.2897563},
year = {2016},
}
Publisher's Version
Article Search


EmamjomehZadeh, Ehsan 
STOC'16: "Deterministic and ..."
Deterministic and Probabilistic Binary Search in Graphs
Ehsan EmamjomehZadeh, David Kempe, and Vikrant Singhal (University of Southern California, USA) We consider the following natural generalization of Binary Search: in a given undirected, positively weighted graph, one vertex is a target. The algorithm’s task is to identify the target by adaptively querying vertices. In response to querying a node q, the algorithm learns either that q is the target, or is given an edge out of q that lies on a shortest path from q to the target. We study this problem in a general noisy model in which each query independently receives a correct answer with probability p > 1/2 (a known constant), and an (adversarial) incorrect one with probability 1 − p. Our main positive result is that when p = 1 (i.e., all answers are correct), log_{2} n queries are always sufficient. For general p, we give an (almost informationtheoretically optimal) algorithm that uses, in expectation, no more than (1 − δ) logn/1 − H(p) + o(logn) + O(log^{2} (1/δ)) queries, and identifies the target correctly with probability at leas 1 − δ. Here, H(p) = −(p logp + (1 − p) log(1 − p)) denotes the entropy. The first bound is achieved by the algorithm that iteratively queries a 1median of the nodes not ruled out yet; the second bound by careful repeated invocations of a multiplicative weights algorithm. Even for p = 1, we show several hardness results for the problem of determining whether a target can be found using K queries. Our upper bound of log_{2} n implies a quasipolynomialtime algorithm for undirected connected graphs; we show that this is bestpossible under the Strong Exponential Time Hypothesis (SETH). Furthermore, for directed graphs, or for undirected graphs with nonuniform node querying costs, the problem is PSPACEcomplete. For a semiadaptive version, in which one may query r nodes each in k rounds, we show membership in Σ_{2k−1} in the polynomial hierarchy, and hardness for Σ_{2k−5}. 

Emek, Yuval 
STOC'16: "Online Matching: ..."
Online Matching: Haste Makes Waste!
Yuval Emek, Shay Kutten, and Roger Wattenhofer (Technion, Israel; ETH Zurich, Switzerland) This paper studies a new online problem, referred to as mincost perfect matching with delays (MPMD), defined over a finite metric space (i.e., a complete graph with positive edge weights obeying the triangle inequality) M that is known to the algorithm in advance. Requests arrive in a continuous time online fashion at the points of M and should be served by matching them to each other. The algorithm is allowed to delay its request matching commitments, but this does not come for free: the total cost of the algorithm is the sum of metric distances between matched requests plus the sum of times each request waited since it arrived until it was matched. A randomized online MPMD algorithm is presented whose competitive ratio is O (log^{2} n + logΔ), where n is the number of points in M and Δ is its aspect ratio. The analysis is based on a machinery developed in the context of a new stochastic process that can be viewed as two interleaved Poisson processes; surprisingly, this new process captures precisely the behavior of our algorithm. A related problem in which the algorithm is allowed to clear any unmatched request at a fixed penalty is also addressed. It is suggested that the MPMD problem is merely the tip of the iceberg for a general framework of online problems with delayed service that captures many more natural problems. 

Ene, Alina 
STOC'16: "Routing under Balance ..."
Routing under Balance
Alina Ene, Gary Miller, Jakub Pachocki, and Aaron Sidford (University of Warwick, UK; Carnegie Mellon University, USA; Microsoft Research, USA) We introduce the notion of balance for directed graphs: a weighted directed graph is αbalanced if for every cut S ⊆ V, the total weight of edges going from S to V∖ S is within factor α of the total weight of edges going from V∖ S to S. Several important families of graphs are nearly balanced, in particular, Eulerian graphs (with α = 1) and residual graphs of (1+T)approximate undirected maximum flows (with α=O(1/T)). We use the notion of balance to give a more finegrained understanding of several wellstudied routing questions that are considerably harder in directed graphs. We first revisit oblivious routings in directed graphs. Our main algorithmic result is an oblivious routing scheme for singlesource instances that achieve an O(α · log^{3} n / loglogn) competitive ratio. In the process, we make several technical contributions which may be of independent interest. In particular, we give an efficient algorithm for computing lowradius decompositions of directed graphs parameterized by balance. We also define and construct lowstretch arborescences, a generalization of lowstretch spanning trees to directed graphs. On the negative side, we present new lower bounds for oblivious routing problems on directed graphs. We show that the competitive ratio of oblivious routing algorithms for directed graphs is Ω(n) in general; this result improves upon the longstanding best known lower bound of Ω(√n) by Hajiaghayi et al. We also show that our restriction to singlesource instances is necessary by showing an Ω(√n) lower bound for multiplesource oblivious routing in Eulerian graphs. We also study the maximum flow problem in balanced directed graphs with arbitrary capacities. We develop an efficient algorithm that finds an (1+T)approximate maximum flows in αbalanced graphs in time O(m α^{2} / T^{2}). We show that, using our approximate maximum flow algorithm, we can efficiently determine whether a given directed graph is αbalanced. Additionally, we give an application to the directed sparsest cut problem. 

Evra, Shai 
STOC'16: "Bounded Degree Cosystolic ..."
Bounded Degree Cosystolic Expanders of Every Dimension
Shai Evra and Tali Kaufman (Hebrew University of Jerusalem, Israel; BarIlan University, Israel)
In recent years a high dimensional theory of expanders has emerged. The notion of combinatorial expansion of graphs (i.e. the Cheeger constant of a graph) has seen two generalizations to high dimensional simplicial complexes. One generalization, known as coboundary expansion, is due to Linial and Meshulem; the other, which we term here cosystolic expansion, is due to Gromov, who showed that cosystolic expanders have the topological overlapping property. No construction (either random or explicit) of bounded degree combinational expanders (according to either definition) were known until a recent work of Kaufman, Kazhdan and Lubotzky, which provided the first bounded degree cosystolic expanders of dimension two. No bounded degree combinatorial expanders are known in higher dimensions. In this work we present explicit bounded degree cosystolic expanders of every dimension. This solves affirmatively an open question raised by Gromov, who asked whether there exist bounded degree complexes with the topological overlapping property in every dimension. Moreover, we provide a local to global criterion on a complex that implies cosystolic expansion: Namely, for a ddimensional complex, X, if its underlying graph is a good expander, and all its links are both coboundary expanders and good expander graphs, then the (d1)dimensional skeleton of the complex is a cosystolic expander.
@InProceedings{STOC16p36,
author = {Shai Evra and Tali Kaufman},
title = {Bounded Degree Cosystolic Expanders of Every Dimension},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {3648},
doi = {10.1145/2897518.2897543},
year = {2016},
}
Publisher's Version
Article Search


Feige, Uriel 
STOC'16: "On the Effect of ..."
On the Effect of Randomness on Planted 3Coloring Models
Roee David and Uriel Feige (Weizmann Institute of Science, Israel)
We present the hosted coloring framework for studying al gorithmic and hardness results for the kcoloring problem. There is a class H of host graphs. One selects a graph H ∈ H and plants in it a balanced kcoloring (by partitioning the vertex set into k roughly equal parts, and removing all edges within each part). The resulting graph G is given as input to a polynomial time algorithm that needs to kcolor G (any legal kcoloring would do – the algorithm is not required to recover the planted kcoloring). Earlier planted models correspond to the case that H is the class of all nvertex dregular graphs, a member H ∈ H is chosen at random, and then a balanced kcoloring is planted at random. Blum and Spencer [1995] designed algorithms for this model when d = n δ (for 0 < δ ≤ 1), and Alon and Kahale [1997] managed to do so even when d is a sufficiently large constant. The new aspect in our framework is that it need not in volve randomness. In one model within the framework (with k = 3) H is a d regular spectral expander (meaning that ex cept for the largest eigenvalue of its adjacency matrix, every other eigenvalue has absolute value much smaller than d) chosen by an adversary, and the planted 3coloring is ran dom. We show that the 3coloring algorithm of Alon and Kahale [1997] can be modified to apply to this case. In an other model H is a random dregular graph but the planted balanced 3coloring is chosen by an adversary, after seeing H. We show that for a certain range of average degrees somewhat below √ n, finding a 3coloring is NPhard. To gether these results (and other results that we have) help clarify which aspects of randomness in the planted coloring model are the key to successful 3coloring algorithms.
@InProceedings{STOC16p77,
author = {Roee David and Uriel Feige},
title = {On the Effect of Randomness on Planted 3Coloring Models},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {7790},
doi = {10.1145/2897518.2897561},
year = {2016},
}
Publisher's Version
Article Search


Feldman, Michal 
STOC'16: "The Price of Anarchy ..."
The Price of Anarchy in Large Games
Michal Feldman, Nicole Immorlica, Brendan Lucier, Tim Roughgarden, and Vasilis Syrgkanis (Tel Aviv University, Israel; Microsoft Research, USA; Stanford University, USA)
We present an analysis framework for bounding the price of anarchy (POA) in games that have many players, as in many of the games most pertinent to computer science applications. We use this framework to demonstrate that, in many of the models in which the POA has been studied, the POA in large games is much smaller than the worstcase bound. Our framework also differentiates between mechanisms with similar worstcase performance, such as simultaneous uniformprice auctions and greedy combinatorial auctions, thereby providing new insights about which mechanisms are likely to perform well in realistic settings.
@InProceedings{STOC16p963,
author = {Michal Feldman and Nicole Immorlica and Brendan Lucier and Tim Roughgarden and Vasilis Syrgkanis},
title = {The Price of Anarchy in Large Games},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {963976},
doi = {10.1145/2897518.2897580},
year = {2016},
}
Publisher's Version
Article Search


Fenner, Stephen 
STOC'16: "Bipartite Perfect ..."
Bipartite Perfect Matching Is in QuasiNC
Stephen Fenner, Rohit Gurjar, and Thomas Thierauf (University of South Carolina, USA; Aalen University, Germany) We show that the bipartite perfect matching problem is in quasi NC^{2}. That is, it has uniform circuits of quasipolynomial size n^{O(logn)}, and O(log^{2} n) depth. Previously, only an exponential upper bound was known on the size of such circuits with polylogarithmic depth. We obtain our result by an almost complete derandomization of the famous Isolation Lemma when applied to yield an efficient randomized parallel algorithm for the bipartite perfect matching problem. 

Fischer, Orr 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Fomin, Fedor V. 
STOC'16: "Exact Algorithms ..."
Exact Algorithms via Monotone Local Search
Fedor V. Fomin, Serge Gaspers, Daniel Lokshtanov, and Saket Saurabh (University of Bergen, Norway; UNSW, Australia; Data61, Australia; Institute of Mathematical Sciences at Chennai, India) We give a new general approach for designing exact exponentialtime algorithms for subset problems. In a subset problem the input implicitly describes a family of sets over a universe of size n and the task is to determine whether the family contains at least one set. A typical example of a subset problem is Weighted dSAT. Here, the input is a CNFformula with clauses of size at most d, and an integer W. The universe is the set of variables and the variables have integer weights. The family contains all the subsets S of variables such that the total weight of the variables in S does not exceed W, and setting the variables in S to 1 and the remaining variables to 0 satisfies the formula. Our approach is based on “monotone local search”, where the goal is to extend a partial solution to a solution by adding as few elements as possible. More formally, in the extension problem we are also given as input a subset X of the universe and an integer k. The task is to determine whether one can add at most k elements to X to obtain a set in the (implicitly defined) family. Our main result is that a c^{k}n^{O(1)} time algorithm for the extension problem immediately yields a randomized algorithm for finding a solution of any size with running time O((2−1/c)^{n}). In many cases, the extension problem can be reduced to simply finding a solution of size at most k. Furthermore, efficient algorithms for finding small solutions have been extensively studied in the field of parameterized algorithms. Directly applying these algorithms, our theorem yields in one stroke significant improvements over the best known exponentialtime algorithms for several wellstudied problems, including dHitting Set, Feedback Vertex Set, Node Unique Label Cover, and Weighted dSAT. Our results demonstrate an interesting and very concrete connection between parameterized algorithms and exact exponentialtime algorithms. We also show how to derandomize our algorithms at the cost of a subexponential multiplicative factor in the running time. Our derandomization is based on an efficient construction of a new pseudorandom object that might be of independent interest. Finally, we extend our methods to establish new combinatorial upper bounds and develop enumeration algorithms. 

Fraigniaud, Pierre 
STOC'16: "Parallel Exhaustive ..."
Parallel Exhaustive Search without Coordination
Pierre Fraigniaud, Amos Korman, and Yoav Rodeh (CNRS, France; University of Paris Diderot, France; Weizmann Institute of Science, Israel) We analyse parallel algorithms in the context of exhaustive search over totally ordered sets. Imagine an infinite list of “boxes”, with a “treasure” hidden in one of them, where the boxes’ order reflects the importance of finding the treasure in a given box. At each time step, a search protocol executed by a searcher has the ability to peek into one box, and see whether the treasure is present or not. Clearly, the best strategy of a single searcher would be to open the boxes one by one, in increasing order. Moreover, by equally dividing the workload between them, k searchers can trivially find the treasure k times faster than one searcher. However, this straightforward strategy is very sensitive to failures (e.g., crashes of processors), and overcoming this issue seems to require a large amount of communication. We therefore address the question of designing parallel search algorithms maximizing their speedup and maintaining high levels of robustness, while minimizing the amount of resources for coordination. Based on the observation that algorithms that avoid communication are inherently robust, we focus our attention on identifying the best running time performance of noncoordinating algorithms. Specifically, we devise noncoordinating algorithms that achieve a speedup of 9/8 for two searchers, a speedup of 4/3 for three searchers, and in general, a speedup of k/4(1+1/k)^{2} for any k≥ 1 searchers. Thus, asymptotically, the speedup is only four times worse compared to the case of full coordination. Moreover, these bounds are tight in a strong sense as no noncoordinating search algorithm can achieve better speedups. Our algorithms are surprisingly simple and hence applicable. However they are memory intensive and so we suggest a practical, memory efficient version, with a speedup of (k^{2} − 1)/4k. That is, it is only a factor of (k+1)/(k−1) slower than the optimal algorithm. Overall, we highlight that, in faulty contexts in which coordination between the searchers is technically difficult to implement, intrusive with respect to privacy, and/or costly in term of resources, it might well be worth giving up on coordination, and simply run our noncoordinating exhaustive search algorithms. 

Frieze, Alan 
STOC'16: "Separating Subadditive ..."
Separating Subadditive Euclidean Functionals
Alan Frieze and Wesley Pegden (Carnegie Mellon University, USA)
The classical BeardwoodHaltonHammersly theorem (1959) asserts the existence of an asymptotic formula of the form constant times square root n for the minimum length of a Traveling Salesperson Tour through n random points in the unit square, and in the decades since it was proved, the existence of such formulas has been shown for other such Euclidean functionals on random points in the unit square as well. Despite more than 50 years of attention, however, it remained unknown whether the minimum length TSP through n random points in the unit square was asymptotically distinct from its natural lower bounds, such as the minimum length spanning tree, the minimum length 2factor, or, as raised by Goemans and Bertsimas, from its linear programming relaxation. We prove that the TSP on random points in Euclidean space is indeed asymptotically distinct from these and other natural lower bounds, and show that this separation implies that branchandbound algorithms based on these natural lower bounds must take nearly exponential time to solve the TSP to optimality, even in average case. This is the first averagecase superpolynomial lower bound for these branchandbound algorithms.
@InProceedings{STOC16p22,
author = {Alan Frieze and Wesley Pegden},
title = {Separating Subadditive Euclidean Functionals},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {2235},
doi = {10.1145/2897518.2897571},
year = {2016},
}
Publisher's Version
Article Search


Ganor, Anat 
STOC'16: "Exponential Separation ..."
Exponential Separation of Communication and External Information
Anat Ganor, Gillat Kol, and Ran Raz (Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) We show an exponential gap between communication complexity and external information complexity, by analyzing a communication task suggested as a candidate by Braverman. Previously, only a separation of communication complexity and internal information complexity was known. More precisely, we obtain an explicit example of a search problem with external information complexity ≤ O(k), with respect to any input distribution, and distributional communication complexity ≥ 2^{k}, with respect to some input distribution. In particular, this shows that a communication protocol cannot always be compressed to its external information. By a result of Braverman, our gap is the largest possible. Moreover, since the upper bound of O(k) on the external information complexity of the problem is obtained with respect to any input distribution, our result implies an exponential gap between communication complexity and information complexity (both internal and external) in the nondistributional setting of Braverman. In this setting, no gap was previously known, even for internal information complexity. 

Garg, Ankit 
STOC'16: "Communication Lower ..."
Communication Lower Bounds for Statistical Estimation Problems via a Distributed Data Processing Inequality
Mark Braverman, Ankit Garg, Tengyu Ma, Huy L. Nguyen, and David P. Woodruff (Princeton University, USA; Toyota Technological Institute at Chicago, USA; IBM Research, USA) We study the tradeoff between the statistical error and communication cost of distributed statistical estimation problems in high dimensions. In the distributed sparse Gaussian mean estimation problem, each of the m machines receives n data points from a ddimensional Gaussian distribution with unknown mean θ which is promised to be ksparse. The machines communicate by message passing and aim to estimate the mean θ. We provide a tight (up to logarithmic factors) tradeoff between the estimation error and the number of bits communicated between the machines. This directly leads to a lower bound for the distributed sparse linear regression problem: to achieve the statistical minimax error, the total communication is at least Ω(minn,dm), where n is the number of observations that each machine receives and d is the ambient dimension. These lower results improve upon Shamir (NIPS’14) and SteinhardtDuchi (COLT’15) by allowing multiround iterative communication model. We also give the first optimal simultaneous protocol in the dense case for mean estimation. As our main technique, we prove a distributed data processing inequality, as a generalization of usual data processing inequalities, which might be of independent interest and useful for other problems. 

Gaspers, Serge 
STOC'16: "Exact Algorithms ..."
Exact Algorithms via Monotone Local Search
Fedor V. Fomin, Serge Gaspers, Daniel Lokshtanov, and Saket Saurabh (University of Bergen, Norway; UNSW, Australia; Data61, Australia; Institute of Mathematical Sciences at Chennai, India) We give a new general approach for designing exact exponentialtime algorithms for subset problems. In a subset problem the input implicitly describes a family of sets over a universe of size n and the task is to determine whether the family contains at least one set. A typical example of a subset problem is Weighted dSAT. Here, the input is a CNFformula with clauses of size at most d, and an integer W. The universe is the set of variables and the variables have integer weights. The family contains all the subsets S of variables such that the total weight of the variables in S does not exceed W, and setting the variables in S to 1 and the remaining variables to 0 satisfies the formula. Our approach is based on “monotone local search”, where the goal is to extend a partial solution to a solution by adding as few elements as possible. More formally, in the extension problem we are also given as input a subset X of the universe and an integer k. The task is to determine whether one can add at most k elements to X to obtain a set in the (implicitly defined) family. Our main result is that a c^{k}n^{O(1)} time algorithm for the extension problem immediately yields a randomized algorithm for finding a solution of any size with running time O((2−1/c)^{n}). In many cases, the extension problem can be reduced to simply finding a solution of size at most k. Furthermore, efficient algorithms for finding small solutions have been extensively studied in the field of parameterized algorithms. Directly applying these algorithms, our theorem yields in one stroke significant improvements over the best known exponentialtime algorithms for several wellstudied problems, including dHitting Set, Feedback Vertex Set, Node Unique Label Cover, and Weighted dSAT. Our results demonstrate an interesting and very concrete connection between parameterized algorithms and exact exponentialtime algorithms. We also show how to derandomize our algorithms at the cost of a subexponential multiplicative factor in the running time. Our derandomization is based on an efficient construction of a new pseudorandom object that might be of independent interest. Finally, we extend our methods to establish new combinatorial upper bounds and develop enumeration algorithms. 

Gavinsky, Dmitry 
STOC'16: "Entangled Simultaneity ..."
Entangled Simultaneity versus Classical Interactivity in Communication Complexity
Dmitry Gavinsky (Czech Academy of Sciences, Czechia)
In 1999 Raz demonstrated a partial function that had an efficient quantum twoway communication protocol but no efficient classical twoway protocol and asked, whether there existed a function with an efficient quantum oneway protocol, but still no efficient classical twoway protocol. In 2010 Klartag and Regev demonstrated such a function and asked, whether there existed a function with an efficient quantum simultaneousmessages protocol, but still no efficient classical twoway protocol. In this work we answer the latter question affirmatively and present a partial function Shape, which can be computed by a protocol sending entangled simultaneous messages of polylogarithmic size, and whose classical twoway complexity is lower bounded by a polynomial.
@InProceedings{STOC16p877,
author = {Dmitry Gavinsky},
title = {Entangled Simultaneity versus Classical Interactivity in Communication Complexity},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {877884},
doi = {10.1145/2897518.2897545},
year = {2016},
}
Publisher's Version
Article Search


Gelles, Ran 
STOC'16: "ConstantRate Coding ..."
ConstantRate Coding for Multiparty Interactive Communication Is Impossible
Mark Braverman, Klim Efremenko, Ran Gelles, and Bernhard Haeupler (Princeton University, USA; Tel Aviv University, Israel; Carnegie Mellon University, USA)
We study coding schemes for multiparty interactive communication over synchronous networks that suffer from stochastic noise, where each bit is independently flipped with probability ε. We analyze the minimal overhead that must be added by the coding scheme in order to succeed in performing the computation despite the noise. Our main result is a lower bound on the communication of any noiseresilient protocol over a synchronous star network with nparties (where all parties communicate in every round). Specifically, we show a task that can be solved by communicating T bits over the noisefree network, but for which any protocol with success probability of 1o(1) must communicate at least Ω(T log n / log log n) bits when the channels are noisy. By a 1994 result of Rajagopalan and Schulman, the slowdown we prove is the highest one can obtain on any topology, up to a log log n factor. We complete our lower bound with a matching coding scheme that achieves the same overhead; thus, the capacity of (synchronous) star networks is Θ(log log n / log n). Our bounds prove that, despite several previous coding schemes with rate Ω(1) for certain topologies, no coding scheme with constant rate Ω(1) exists for arbitrary nparty noisy networks.
@InProceedings{STOC16p999,
author = {Mark Braverman and Klim Efremenko and Ran Gelles and Bernhard Haeupler},
title = {ConstantRate Coding for Multiparty Interactive Communication Is Impossible},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {9991010},
doi = {10.1145/2897518.2897563},
year = {2016},
}
Publisher's Version
Article Search


Goldenberg, Elazar 
STOC'16: "Streaming Algorithms ..."
Streaming Algorithms for Embedding and Computing Edit Distance in the Low Distance Regime
Diptarka Chakraborty, Elazar Goldenberg, and Michal Koucký (IIT Kanpur, India; Charles University in Prague, Czechia) The Hamming and the edit metrics are two common notions of measuring distances between pairs of strings x,y lying in the Boolean hypercube. The edit distance between x and y is defined as the minimum number of character insertion, deletion, and bit flips needed for converting x into y. Whereas, the Hamming distance between x and y is the number of bit flips needed for converting x to y. In this paper we study a randomized injective embedding of the edit distance into the Hamming distance with a small distortion. We show a randomized embedding with quadratic distortion. Namely, for any x,y satisfying that their edit distance equals k, the Hamming distance between the embedding of x and y is O(k^{2}) with high probability. This improves over the distortion ratio of O( n ^{*} n) obtained by Jowhari (2012) for small values of k. Moreover, the embedding output size is linear in the input size and the embedding can be computed using a single pass over the input. We provide several applications for this embedding. Among our results we provide a onepass (streaming) algorithm for edit distance running in space O(s) and computing edit distance exactly upto distance s^{1}/6. This algorithm is based on kernelization for edit distance that is of independent interest. 

Goldreich, Oded 
STOC'16: "Matrix Rigidity ..."
Matrix Rigidity of Random Toeplitz Matrices
Oded Goldreich and Avishay Tal (Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) We prove that random nbyn Toeplitz (alternatively Hankel) matrices over F_{2} have rigidity Ω(n^{3}/r^{2}logn) for rank r ≥ √n, with high probability. For r = o(n/logn · loglogn), this improves over the Ω(n^{2}/r · log(n/r)) bound that is known for many explicit matrices. Our result implies that the explicit trilinear [n]× [n] × [2n] function defined by F(x,y,z) = ∑_{i,j}x_{i} y_{j} z_{i+j} has complexity Ω(n^{3/5}) in the multilinear circuit model suggested by Goldreich and Wigderson (ECCC, 2013), which yields an exp(n^{3/5}) lower bound on the size of the socalled canonical depththree circuits for F. We also prove that F has complexity Ω(n^{2/3}) if the multilinear circuits are further restricted to be of depth 2. In addition, we show that a matrix whose entries are sampled from a 2^{−n}biased distribution has complexity Ω(n^{2/3}), regardless of depth restrictions, almost matching the known O(n^{2/3}) upper bound for any matrix. We turn this randomized construction into an explicit 4linear construction with similar lower bounds, using the quadratic smallbiased construction of Mossel et al. (RS&A, 2006). 

Goyal, Vipul 
STOC'16: "Nonmalleable Extractors ..."
Nonmalleable Extractors and Codes, with Their Many Tampered Extensions
Eshan Chattopadhyay, Vipul Goyal, and Xin Li (University of Texas at Austin, USA; Microsoft Research, India; Johns Hopkins University, USA) Randomness extractors and error correcting codes are fundamental objects in computer science. Recently, there have been several natural generalizations of these objects, in the context and study of tamper resilient cryptography. These are seeded nonmalleable extractors, introduced by Dodis and Wichs; seedless nonmalleable extractors, introduced by Cheraghchi and Guruswami; and nonmalleable codes, introduced by Dziembowski, Pietrzak and Wichs. Besides being interesting on their own, they also have important applications in cryptography, e.g, privacy amplification with an active adversary, explicit nonmalleable codes etc, and often have unexpected connections to their nontampered analogues. However, the known constructions are far behind their nontampered counterparts. Indeed, the best known seeded nonmalleable extractor requires minentropy rate at least 0.49; while explicit constructions of nonmalleable twosource extractors were not known even if both sources have full minentropy, and was left as an open problem by Cheraghchi and Guruswami. In this paper we make progress towards solving the above problems and other related generalizations. Our contributions are as follows. (1) We construct an explicit seeded nonmalleable extractor for polylogarithmic minentropy. This dramatically improves all previous results and gives a simpler 2round privacy amplification protocol with optimal entropy loss, matching the best known result. In fact, we construct more general seeded nonmalleable extractors (that can handle multiple adversaries) which were used in the recent construction of explicit twosource extractors for polylogarithmic minentropy. (2) We construct the first explicit nonmalleable twosource extractor for almost full minentropy thus resolving the open question posed by Cheraghchi and Guruswami. (3) We motivate and initiate the study of two natural generalizations of seedless nonmalleable extractors and nonmalleable codes, where the sources or the codeword may be tampered many times. By using the connection found by Cheraghchi and Guruswami and providing efficient sampling algorithms, we obtain the first explicit nonmalleable codes with tampering degree t, with near optimal rate and error. We call these stronger notions onemany and manymanynonmalleable codes. This provides a stronger information theoretic analogue of a primitive known as continuous nonmalleable codes. Our basic technique used in all of our constructions can be seen as inspired, in part, by the techniques previously used to construct cryptographic nonmalleable commitments. Vipul Goyal, Omkant Pandey, and Silas Richelson (Microsoft Research, India; Drexel University, USA; Massachusetts Institute of Technology, USA)
We present a new nonmalleable commitment protocol. Our protocol has the following features: itemize The protocol has only three rounds of interaction. Pass (TCC 2013) showed an impossibility result for a tworound nonmalleable commitment scheme w.r.t. a blackbox reduction to any ``standard" intractability reduction. Thus, this resolves the round complexity of nonmalleable commitment at least w.r.t. blackbox security reductions. Our construction is secure as per the standard notion of nonmalleability w.r.t. commitment. Our protocol is truly efficient. In our basic protocol, the entire computation of the committer is dominated by just three invocations of a noninteractive statically binding commitment scheme, while, the receiver computation (in the commitment stage) is limited to just sampling a random string. Unlike many previous works, we directly construct a protocol for large tags and hence avoid any nonmalleability amplification steps. Our protocol is based on a blackbox use of any noninteractive statistically binding commitment scheme. Such schemes, in turn, can be based on any onetoone oneway function (or any oneway function at the cost of an extra initialization round). Previously, the best known blackbox construction of nonmalleable commitments required a larger (constant) number of rounds. Our construction is publiccoin and makes use of only blackbox simulation. Prior to our work, no publiccoin constant round nonmalleable commitment schemes were known based on blackbox simulation. itemize Our techniques depart significantly from the techniques used previously to construct nonmalleable commitment schemes. As a main technical tool, we rely on nonmalleable codes in the split state model. Our proofs of security are purely combinatorial in nature. In addition, we also present a simple construction of constant round nonmalleable commitments from any oneway function. While this result is not new, the main feature is its simplicity compared to any previous construction of nonmalleable commitments (in any number of rounds). We believe the construction is simple enough to be covered in a graduate level course on cryptography. The construction uses nonmalleable codes in the split state model in a blackbox way.
@InProceedings{STOC16p1128,
author = {Vipul Goyal and Omkant Pandey and Silas Richelson},
title = {Textbook Nonmalleable Commitments},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11281141},
doi = {10.1145/2897518.2897657},
year = {2016},
}
Publisher's Version
Article Search


Gurjar, Rohit 
STOC'16: "Bipartite Perfect ..."
Bipartite Perfect Matching Is in QuasiNC
Stephen Fenner, Rohit Gurjar, and Thomas Thierauf (University of South Carolina, USA; Aalen University, Germany) We show that the bipartite perfect matching problem is in quasi NC^{2}. That is, it has uniform circuits of quasipolynomial size n^{O(logn)}, and O(log^{2} n) depth. Previously, only an exponential upper bound was known on the size of such circuits with polylogarithmic depth. We obtain our result by an almost complete derandomization of the famous Isolation Lemma when applied to yield an efficient randomized parallel algorithm for the bipartite perfect matching problem. 

Guruswami, Venkatesan 
STOC'16: "Repairing ReedSolomon ..."
Repairing ReedSolomon Codes
Venkatesan Guruswami and Mary Wootters (Carnegie Mellon University, USA)
A fundamental fact about polynomial interpolation is that k evaluations of a degree(k1) polynomial f are sufficient to determine f. This is also necessary in a strong sense: given k1 evaluations, we learn nothing about the value of f on any k'th point. In this paper, we study a variant of the polynomial interpolation problem. Instead of querying entire evaluations of f (which are elements of a large field F), we are allowed to query partial evaluations; that is, each evaluation delivers a few elements from a small subfield of F, rather than a single element from F. We show that in this model, one can do significantly better than in the traditional setting, in terms of the amount of information required to determine the missing evaluation. More precisely, we show that only O(k) bits are necessary to recover a missing evaluation. In contrast, the traditional method of looking at k evaluations requires Omega(k log(k)) bits. We also show that our result is optimal for linear methods, even up to the leading constants. Our motivation comes from the use of ReedSolomon (RS) codes for distributed storage systems, in particular for the exact repair problem. The traditional use of RS codes in this setting is analogous to the traditional interpolation problem. Each node in a system stores an evaluation of f, and if one node fails we can recover it by reading k other nodes. However, each node is free to send less information, leading to the modified problem above. The quicklydeveloping field of regenerating codes has yielded several codes which take advantage of this freedom. However, these codes are not RS codes, and RS codes are still often used in practice; in 2011, Dimakis et al. asked how well RS codes could perform in this setting. Our results imply that RS codes can also take advantage of this freedom to download partial symbols. In some parameter regimesthose with small levels of subpacketizationour scheme for RS codes outperforms all known regenerating codes. Even with a high degree of subpacketization, our methods give nontrivial schemes, and we give an improved repair scheme for a specific (14,10)RS code used in the Facebook Hadoop Analytics cluster.
@InProceedings{STOC16p216,
author = {Venkatesan Guruswami and Mary Wootters},
title = {Repairing ReedSolomon Codes},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {216226},
doi = {10.1145/2897518.2897525},
year = {2016},
}
Publisher's Version
Article Search


Haah, Jeongwan 
STOC'16: "SampleOptimal Tomography ..."
SampleOptimal Tomography of Quantum States
Jeongwan Haah, Aram W. Harrow, Zhengfeng Ji, Xiaodi Wu, and Nengkun Yu (Massachusetts Institute of Technology, USA; University of Technology Sydney, Australia; University of Waterloo, Canada; Institute of Software at Chinese Academy of Sciences, China; University of Oregon, USA; University of Guelph, Canada) It is a fundamental problem to decide how many copies of an unknown mixed quantum state are necessary and sufficient to determine the state. This is the quantum analogue of the problem of estimating a probability distribution given some number of samples. Previously, it was known only that estimating states to error T in trace distance required O(dr^{2}/T^{2}) copies for a ddimensional density matrix of rank r. Here, we give a measurement scheme (POVM) that uses O( (dr/ δ ) ln(d/δ) ) copies to estimate ρ to error δ in infidelity. This implies O( (dr / T^{2})· ln(d/T) ) copies suffice to achieve error T in trace distance. For fixed d, our measurement can be implemented on a quantum computer in time polynomial in n. We also use the Holevo bound from quantum information theory to prove a lower bound of Ω(dr/T^{2})/ log(d/rT) copies needed to achieve error T in trace distance. This implies a lower bound Ω(dr/δ)/log(d/rδ) for the estimation error δ in infidelity. These match our upper bounds up to log factors. Our techniques can also show an Ω(r^{2}d/δ) lower bound for measurement strategies in which each copy is measured individually and then the outcomes are classically postprocessed to produce an estimate. This matches the known achievability results and proves for the first time that such “product” measurements have asymptotically suboptimal scaling with d and r. 

Haeupler, Bernhard 
STOC'16: "ConstantRate Coding ..."
ConstantRate Coding for Multiparty Interactive Communication Is Impossible
Mark Braverman, Klim Efremenko, Ran Gelles, and Bernhard Haeupler (Princeton University, USA; Tel Aviv University, Israel; Carnegie Mellon University, USA)
We study coding schemes for multiparty interactive communication over synchronous networks that suffer from stochastic noise, where each bit is independently flipped with probability ε. We analyze the minimal overhead that must be added by the coding scheme in order to succeed in performing the computation despite the noise. Our main result is a lower bound on the communication of any noiseresilient protocol over a synchronous star network with nparties (where all parties communicate in every round). Specifically, we show a task that can be solved by communicating T bits over the noisefree network, but for which any protocol with success probability of 1o(1) must communicate at least Ω(T log n / log log n) bits when the channels are noisy. By a 1994 result of Rajagopalan and Schulman, the slowdown we prove is the highest one can obtain on any topology, up to a log log n factor. We complete our lower bound with a matching coding scheme that achieves the same overhead; thus, the capacity of (synchronous) star networks is Θ(log log n / log n). Our bounds prove that, despite several previous coding schemes with rate Ω(1) for certain topologies, no coding scheme with constant rate Ω(1) exists for arbitrary nparty noisy networks.
@InProceedings{STOC16p999,
author = {Mark Braverman and Klim Efremenko and Ran Gelles and Bernhard Haeupler},
title = {ConstantRate Coding for Multiparty Interactive Communication Is Impossible},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {9991010},
doi = {10.1145/2897518.2897563},
year = {2016},
}
Publisher's Version
Article Search


Hajiaghayi, MohammadTaghi 
STOC'16: "A PTAS for Planar ..."
A PTAS for Planar Group Steiner Tree via Spanner Bootstrapping and Prize Collecting
MohammadHossein Bateni, Erik D. Demaine, MohammadTaghi Hajiaghayi, and Dániel Marx (Google, USA; Massachusetts Institute of Technology, USA; University of Maryland at College Park, USA; Hungarian Academy of Sciences, Hungary) We present the first polynomialtime approximation scheme (PTAS), i.e., (1+ε)approximation algorithm for any constant ε> 0, for the planar group Steiner tree problem (in which each group lies on a boundary of a face). This result improves on the best previous approximation factor of O(logn (loglogn)^{O(1)}). We achieve this result via a novel and powerful technique called spanner bootstrapping, which allows one to bootstrap from a superconstant approximation factor (even superpolynomial in the input size) all the way down to a PTAS. This is in contrast with the popular existing approach for planar PTASs of constructing lightweight spanners in one iteration, which notably requires a constantfactor approximate solution to start from. Spanner bootstrapping removes one of the main barriers for designing PTASs for problems which have no known constantfactor approximation (even on planar graphs), and thus can be used to obtain PTASs for several difficulttoapproximate problems. Our second major contribution required for the planar group Steiner tree PTAS is a spanner construction, which reduces the graph to have total weight within a factor of the optimal solution while approximately preserving the optimal solution. This is particularly challenging because group Steiner tree requires deciding which terminal in each group to connect by the tree, making it much harder than recent previous approaches to construct spanners for planar TSP by Klein [SIAM J. Computing 2008], subset TSP by Klein [STOC 2006], Steiner tree by Borradaile, Klein, and Mathieu [ACM Trans. Algorithms 2009], and Steiner forest by Bateni, Hajiaghayi, and Marx [J. ACM 2011] (and its improvement to an efficient PTAS by Eisenstat, Klein, and Mathieu [SODA 2012]. The main conceptual contribution here is realizing that selecting which terminals may be relevant is essentially a complicated prizecollecting process: we have to carefully weigh the cost and benefits of reaching or avoiding certain terminals in the spanner. Via a sequence of involved prizecollecting procedures, we can construct a spanner that reaches a set of terminals that is sufficient for an almostoptimal solution. Our PTAS for planar group Steiner tree implies the first PTAS for geometric Euclidean group Steiner tree with obstacles, as well as a (2+)approximation algorithm for group TSP with obstacles, improving over the best previous constantfactor approximation algorithms. By contrast, we show that planar group Steiner forest, a slight generalization of planar group Steiner tree, is APXhard on planar graphs of treewidth 3, even if the groups are pairwise disjoint and every group is a vertex or an edge. 

Hall, Chris 
STOC'16: "Ramanujan Coverings ..."
Ramanujan Coverings of Graphs
Chris Hall, Doron Puder, and William F. Sawin (University of Wyoming, USA; Institute for Advanced Study at Princeton, USA; Princeton University, USA) Let G be a finite connected graph, and let ρ be the spectral radius of its universal cover. For example, if G is kregular then ρ=2√k−1. We show that for every r, there is an rcovering (a.k.a. an rlift) of G where all the new eigenvalues are bounded from above by ρ. It follows that a bipartite Ramanujan graph has a Ramanujan rcovering for every r. This generalizes the r=2 case due to Marcus, Spielman and Srivastava (2013). Every rcovering of G corresponds to a labeling of the edges of G by elements of the symmetric group S_{r}. We generalize this notion to labeling the edges by elements of various groups and present a broader scenario where Ramanujan coverings are guaranteed to exist. In particular, this shows the existence of richer families of bipartite Ramanujan graphs than was known before. Inspired by MarcusSpielmanSrivastava, a crucial component of our proof is the existence of interlacing families of polynomials for complex reflection groups. The core argument of this component is taken from MarcusSpielmanSrivastava (2015). Another important ingredient of our proof is a new generalization of the matching polynomial of a graph. We define the rth matching polynomial of G to be the average matching polynomial of all rcoverings of G. We show this polynomial shares many properties with the original matching polynomial. For example, it is real rooted with all its roots inside [−ρ,ρ]. 

Hansen, Thomas Dueholm 
STOC'16: "Simulating Branching ..."
Simulating Branching Programs with Edit Distance and Friends: Or: A Polylog Shaved Is a Lower Bound Made
Amir Abboud, Thomas Dueholm Hansen, Virginia Vassilevska Williams, and Ryan Williams (Stanford University, USA; Aarhus University, Denmark) A recent, active line of work achieves tight lower bounds for fundamental problems under the Strong Exponential Time Hypothesis (SETH). A celebrated result of Backurs and Indyk (STOC’15) proves that computing the Edit Distance of two sequences of length n in truly subquadratic O(n^{2−ε}) time, for some ε>0, is impossible under SETH. The result was extended by followup works to simpler looking problems like finding the Longest Common Subsequence (LCS). SETH is a very strong assumption, asserting that even linear size CNF formulas cannot be analyzed for satisfiability with an exponential speedup over exhaustive search. We consider much safer assumptions, e.g. that such a speedup is impossible for SAT on more expressive representations, like subexponentialsize NC circuits. Intuitively, this assumption is much more plausible: NC circuits can implement linear algebra and complex cryptographic primitives, while CNFs cannot even approximately compute an XOR of bits. Our main result is a surprising reduction from SAT on Branching Programs to fundamental problems in P like Edit Distance, LCS, and many others. Truly subquadratic algorithms for these problems therefore have far more remarkable consequences than merely faster CNFSAT algorithms. For example, SAT on arbitrary o(n)depth bounded fanin circuits (and therefore also NCCircuitSAT) can be solved in (2−ε)^{n} time. An interesting feature of our work is that we get major consequences even from mildly subquadratic algorithms for Edit Distance or LCS. For example, we show that if an arbitrarily large polylog factor is shaved from n^{2} for Edit Distance then NEXP does not have nonuniform NC^{1} circuits. 

Harris, David G. 
STOC'16: "Distributed (Δ+1)Coloring ..."
Distributed (Δ+1)Coloring in Sublogarithmic Rounds
David G. Harris, Johannes Schneider, and HsinHao Su (University of Maryland, USA; ABB Corporate Research, USA; Massachusetts Institute of Technology, USA)
The (∆+1)coloring problem is a fundamental symmetry breaking problem in distributed computing. We give a new randomized coloring algorithm for (∆+1)coloring running in O(√log ∆)+ 2^O(√log log n) rounds with probability 11/n^Ω(1) in a graph with n nodes and maximum degree ∆. This implies that the (∆+1)coloring problem is easier than the maximal independent set problem and the maximal matching problem, due to their lower bounds by Kuhn, Moscibroda, and Wattenhofer [PODC'04]. Our algorithm also extends to the listcoloring problem where the palette of each node contains ∆+1 colors.
@InProceedings{STOC16p465,
author = {David G. Harris and Johannes Schneider and HsinHao Su},
title = {Distributed (Δ+1)Coloring in Sublogarithmic Rounds},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {465478},
doi = {10.1145/2897518.2897533},
year = {2016},
}
Publisher's Version
Article Search


Harrow, Aram W. 
STOC'16: "SampleOptimal Tomography ..."
SampleOptimal Tomography of Quantum States
Jeongwan Haah, Aram W. Harrow, Zhengfeng Ji, Xiaodi Wu, and Nengkun Yu (Massachusetts Institute of Technology, USA; University of Technology Sydney, Australia; University of Waterloo, Canada; Institute of Software at Chinese Academy of Sciences, China; University of Oregon, USA; University of Guelph, Canada) It is a fundamental problem to decide how many copies of an unknown mixed quantum state are necessary and sufficient to determine the state. This is the quantum analogue of the problem of estimating a probability distribution given some number of samples. Previously, it was known only that estimating states to error T in trace distance required O(dr^{2}/T^{2}) copies for a ddimensional density matrix of rank r. Here, we give a measurement scheme (POVM) that uses O( (dr/ δ ) ln(d/δ) ) copies to estimate ρ to error δ in infidelity. This implies O( (dr / T^{2})· ln(d/T) ) copies suffice to achieve error T in trace distance. For fixed d, our measurement can be implemented on a quantum computer in time polynomial in n. We also use the Holevo bound from quantum information theory to prove a lower bound of Ω(dr/T^{2})/ log(d/rT) copies needed to achieve error T in trace distance. This implies a lower bound Ω(dr/δ)/log(d/rδ) for the estimation error δ in infidelity. These match our upper bounds up to log factors. Our techniques can also show an Ω(r^{2}d/δ) lower bound for measurement strategies in which each copy is measured individually and then the outcomes are classically postprocessed to produce an estimate. This matches the known achievability results and proves for the first time that such “product” measurements have asymptotically suboptimal scaling with d and r. 

Hazan, Elad 
STOC'16: "The Computational ..."
The Computational Power of Optimization in Online Learning
Elad Hazan and Tomer Koren (Princeton University, USA; Technion, Israel) We consider the fundamental problem of prediction with expert advice where the experts are “optimizable”: there is a blackbox optimization oracle that can be used to compute, in constant time, the leading expert in retrospect at any point in time. In this setting, we give a novel online algorithm that attains vanishing regret with respect to N experts in total O(√N) computation time. We also give a lower bound showing that this running time cannot be improved (up to log factors) in the oracle model, thereby exhibiting a quadratic speedup as compared to the standard, oraclefree setting where the required time for vanishing regret is Θ(N). These results demonstrate an exponential gap between the power of optimization in online learning and its power in statistical learning: in the latter, an optimization oracle—i.e., an efficient empirical risk minimizer—allows to learn a finite hypothesis class of size N in time O(logN). We also study the implications of our results to learning in repeated zerosum games, in a setting where the players have access to oracles that compute, in constant time, their bestresponse to any mixed strategy of their opponent. We show that the runtime required for approximating the minimax value of the game in this setting is Θ(√N), yielding again a quadratic improvement upon the oraclefree setting, where Θ(N) is known to be tight. 

Henzinger, Monika 
STOC'16: "A Deterministic ..."
A Deterministic AlmostTight Distributed Algorithm for Approximating SingleSource Shortest Paths
Monika Henzinger, Sebastian Krinninger, and Danupon Nanongkai (University of Vienna, Austria; Max Planck Institute for Informatics, Germany; KTH, Sweden) We present a deterministic (1+o(1))approximation O(n^{1/2+o(1)}+D^{1+o(1)})time algorithm for solving the singlesource shortest paths problem on distributed weighted networks (the CONGEST model); here n is the number of nodes in the network and D is its (hop) diameter. This is the first nontrivial deterministic algorithm for this problem. It also improves (i) the running time of the randomized (1+o(1))approximation Õ(n^{1/2}D^{1/4}+D)time algorithm of Nanongkai [STOC 2014] by a factor of as large as n^{1/8}, and (ii) the O(T^{−1}logT^{−1})approximation factor of Lenzen and PattShamir’s Õ(n^{1/2+T}+D)time algorithm [STOC 2013] within the same running time. Our running time matches the known time lower bound of Ω(n^{1/2}/logn + D) [Das Sarma et al. STOC 2011] modulo some lowerorder terms, thus essentially settling the status of this problem which was raised at least a decade ago [Elkin SIGACT News 2004]. It also implies a (2+o(1))approximation O(n^{1/2+o(1)}+D^{1+o(1)})time algorithm for approximating a network’s weighted diameter which almost matches the lower bound by Holzer et al. [PODC 2012]. In achieving this result, we develop two techniques which might be of independent interest and useful in other settings: (i) a deterministic process that replaces the “hitting set argument” commonly used for shortest paths computation in various settings, and (ii) a simple, deterministic, construction of an (n^{o(1)}, o(1))hop set of size O(n^{1+o(1)}). We combine these techniques with many distributed algorithmic techniques, some of which from problems that are not directly related to shortest paths, e.g. ruling sets [Goldberg et al. STOC 1987], source detection [Lenzen, Peleg PODC 2013], and partial distance estimation [Lenzen, PattShamir PODC 2015]. Our hop set construction also leads to singlesource shortest paths algorithms in two other settings: (i) a (1+o(1))approximation O(n^{o(1)})time algorithm on congested cliques, and (ii) a (1+o(1))approximation O(n^{o(1)}logW)pass O(n^{1+o(1)}logW)space streaming algorithm, when edge weights are in {1, 2, …, W}. The first result answers an open problem in [Nanongkai, STOC 2014]. The second result partially answers an open problem raised by McGregor in 2006 [sublinear.info, Problem 14]. Sayan Bhattacharya, Monika Henzinger, and Danupon Nanongkai (Institute of Mathematical Sciences at Chennai, India; University of Vienna, Austria; KTH, Sweden) We present two deterministic dynamic algorithms for the maximum matching problem. (1) An algorithm that maintains a (2+T)approximate maximum matching in general graphs with O(poly(logn, 1/T)) update time. (2) An algorithm that maintains an α_{K} approximation of the value of the maximum matching with O(n^{2/K}) update time in bipartite graphs, for every sufficiently large constant positive integer K. Here, 1≤ α_{K} < 2 is a constant determined by the value of K. Result (1) is the first deterministic algorithm that can maintain an o(logn)approximate maximum matching with polylogarithmic update time, improving the seminal result of Onak et al. [STOC 2010]. Its approximation guarantee almost matches the guarantee of the best randomized polylogarithmic update time algorithm [Baswana et al. FOCS 2011]. Result (2) achieves a betterthantwo approximation with arbitrarily small polynomial update time on bipartite graphs. Previously the best update time for this problem was O(m^{1/4}) [Bernstein et al. ICALP 2015], where m is the current number of edges in the graph. 

Hirvonen, Juho 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Holmgren, Justin 
STOC'16: "Watermarking Cryptographic ..."
Watermarking Cryptographic Capabilities
Aloni Cohen, Justin Holmgren, Ryo Nishimaki, Vinod Vaikuntanathan, and Daniel Wichs (Massachusetts Institute of Technology, USA; NTT, Japan; Northeastern University, USA)
A watermarking scheme for programs embeds some information called a mark into a program while preserving its functionality. No adversary can remove the mark without damaging the functionality of the program. In this work, we study the problem of watermarking various cryptographic programs such as pseudorandom function (PRF) evaluation, decryption, and signing. For example, given a PRF key K, we create a marked program C that evaluates the PRF F(K,). An adversary that gets C cannot come up with any program C* in which the mark is removed but which still evaluates the PRF correctly on even a small fraction of the inputs. The work of Barak, Goldreich, Impagliazzo, Rudich, Sahai, Vadhan, and Yang (CRYPTO'01 and Journal of ACM 59(2)) shows that, assuming indistinguishability obfuscation (iO), such watermarking is impossible if the marked program C evaluates the original program with perfect correctness. In this work we show that, assuming iO, such watermarking is possible if the marked program C is allowed to err with even a negligible probability, which would be undetectable to the user. Our watermarking schemes are public key, namely we use a secret marking key to embed marks in programs, and a public detection key that allows anyone to detect marks in programs. Our schemes are secure against chosen program attacks, that is even if the adversary is given oracle access to the marking functionality. We emphasize that our security notion of watermark nonremovability considers arbitrary adversarial strategies to modify the marked program, in contrast to the prior works (Nishimaki, EUROCRYPT '13).
@InProceedings{STOC16p1115,
author = {Aloni Cohen and Justin Holmgren and Ryo Nishimaki and Vinod Vaikuntanathan and Daniel Wichs},
title = {Watermarking Cryptographic Capabilities},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11151127},
doi = {10.1145/2897518.2897651},
year = {2016},
}
Publisher's Version
Article Search


Hopkins, Samuel B. 
STOC'16: "Fast Spectral Algorithms ..."
Fast Spectral Algorithms from SumofSquares Proofs: Tensor Decomposition and Planted Sparse Vectors
Samuel B. Hopkins, Tselil Schramm, Jonathan Shi, and David Steurer (Cornell University, USA; University of California at Berkeley, USA) We consider two problems that arise in machine learning applications: the problem of recovering a planted sparse vector in a random linear subspace and the problem of decomposing a random lowrank overcomplete 3tensor. For both problems, the best known guarantees are based on the sumofsquares method. We develop new algorithms inspired by analyses of the sumofsquares method. Our algorithms achieve the same or similar guarantees as sumofsquares for these problems but the running time is significantly faster. For the planted sparse vector problem, we give an algorithm with running time nearly linear in the input size that approximately recovers a planted sparse vector with up to constant relative sparsity in a random subspace of ℝ^{n} of dimension up to Ω(√n). These recovery guarantees match the best known ones of Barak, Kelner, and Steurer (STOC 2014) up to logarithmic factors. For tensor decomposition, we give an algorithm with running time close to linear in the input size (with exponent ≈ 1.125) that approximately recovers a component of a random 3tensor over ℝ^{n} of rank up to Ω(n^{4/3}). The best previous algorithm for this problem due to Ge and Ma (RANDOM 2015) works up to rank Ω(n^{3/2}) but requires quasipolynomial time. 

Hsu, Justin 
STOC'16: "Do Prices Coordinate ..."
Do Prices Coordinate Markets?
Justin Hsu, Jamie Morgenstern, Ryan Rogers, Aaron Roth, and Rakesh Vohra (University of Pennsylvania, USA)
Walrasian equilibrium prices have a remarkable property: they allow each buyer to purchase a bundle of goods that she finds the most desirable, while guaranteeing that the induced allocation over all buyers will globally maximize social welfare. However, this clean story has two caveats. * First, the prices may induce indifferences. In fact, the minimal equilibrium prices necessarily induce indifferences. Accordingly, buyers may need to coordinate with one another to arrive at a socially optimal outcomethe prices alone are not sufficient to coordinate the market. * Second, although natural procedures converge to Walrasian equilibrium prices on a fixed population, in practice buyers typically observe prices without participating in a price computation process. These prices cannot be perfect Walrasian equilibrium prices, but instead somehow reflect distributional information about the market. To better understand the performance of Walrasian prices when facing these two problems, we give two results. First, we propose a mild genericity condition on valuations under which the minimal Walrasian equilibrium prices induce allocations which result in low overdemand, no matter how the buyers break ties. In fact, under genericity the overdemand of any good can be bounded by 1, which is the best possible at the minimal prices. We demonstrate our results for unit demand valuations and give an extension to matroid based valuations (MBV), conjectured to be equivalent to gross substitute valuations (GS). Second, we use techniques from learning theory to argue that the overdemand and welfare induced by a price vector converge to their expectations uniformly over the class of all price vectors, with respective sample complexity linear and quadratic in the number of goods in the market. These results make no assumption on the form of the valuation functions. These two results imply that under a mild genericity condition, the exact Walrasian equilibrium prices computed in a market are guaranteed to induce both low overdemand and high welfare when used in a new market where agents are sampled independently from the same distribution, whenever the number of agents is larger than the number of commodities in the market.
@InProceedings{STOC16p440,
author = {Justin Hsu and Jamie Morgenstern and Ryan Rogers and Aaron Roth and Rakesh Vohra},
title = {Do Prices Coordinate Markets?},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {440453},
doi = {10.1145/2897518.2897559},
year = {2016},
}
Publisher's Version
Article Search
Video


Huang, Zhiyi 
STOC'16: "The Sample Complexity ..."
The Sample Complexity of Auctions with Side Information
Nikhil R. Devanur, Zhiyi Huang, and ChristosAlexandros Psomas (Microsoft Research, USA; University of Hong Kong, China; University of California at Berkeley, USA)
Traditionally, the Bayesian optimal auction design problem has been considered either when the bidder values are i.i.d, or when each bidder is individually identifiable via her value distribution. The latter is a reasonable approach when the bidders can be classified into a few categories, but there are many instances where the classification of bidders is a continuum. For example, the classification of the bidders may be based on their annual income, their propensity to buy an item based on past behavior, or in the case of ad auctions, the click through rate of their ads. We introduce an alternate model that captures this aspect, where bidders are a priori identical, but can be distinguished based (only) on some side information the auctioneer obtains at the time of the auction. We extend the sample complexity approach of Dhangwatnotai et al. and Cole and Roughgarden to this model and obtain almost matching upper and lower bounds. As an aside, we obtain a revenue monotonicity lemma which may be of independent interest. We also show how to use Empirical Risk Minimization techniques to improve the sample complexity bound of Cole and Roughgarden for the nonidentical but independent value distribution case.
@InProceedings{STOC16p426,
author = {Nikhil R. Devanur and Zhiyi Huang and ChristosAlexandros Psomas},
title = {The Sample Complexity of Auctions with Side Information},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {426439},
doi = {10.1145/2897518.2897553},
year = {2016},
}
Publisher's Version
Article Search


Immorlica, Nicole 
STOC'16: "The Price of Anarchy ..."
The Price of Anarchy in Large Games
Michal Feldman, Nicole Immorlica, Brendan Lucier, Tim Roughgarden, and Vasilis Syrgkanis (Tel Aviv University, Israel; Microsoft Research, USA; Stanford University, USA)
We present an analysis framework for bounding the price of anarchy (POA) in games that have many players, as in many of the games most pertinent to computer science applications. We use this framework to demonstrate that, in many of the models in which the POA has been studied, the POA in large games is much smaller than the worstcase bound. Our framework also differentiates between mechanisms with similar worstcase performance, such as simultaneous uniformprice auctions and greedy combinatorial auctions, thereby providing new insights about which mechanisms are likely to perform well in realistic settings.
@InProceedings{STOC16p963,
author = {Michal Feldman and Nicole Immorlica and Brendan Lucier and Tim Roughgarden and Vasilis Syrgkanis},
title = {The Price of Anarchy in Large Games},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {963976},
doi = {10.1145/2897518.2897580},
year = {2016},
}
Publisher's Version
Article Search


Ivkin, Nikita 
STOC'16: "Beating CountSketch ..."
Beating CountSketch for Heavy Hitters in Insertion Streams
Vladimir Braverman, Stephen R. Chestnut, Nikita Ivkin, and David P. Woodruff (Johns Hopkins University, USA; ETH Zurich, Switzerland; IBM Research, USA) Given a stream p_{1}, …, p_{m} of items from a universe U, which, without loss of generality we identify with the set of integers {1, 2, …, n}, we consider the problem of returning all ℓ_{2}heavy hitters, i.e., those items j for which f_{j} ≥ T √F_{2}, where f_{j} is the number of occurrences of item j in the stream, and F_{2} = ∑_{i ∈ [n]} f_{i}^{2}. Such a guarantee is considerably stronger than the ℓ_{1}guarantee, which finds those j for which f_{j} ≥ T m. In 2002, Charikar, Chen, and FarachColton suggested the CountSketch data structure, which finds all such j using Θ(log^{2} n) bits of space (for constant T > 0). The only known lower bound is Ω(logn) bits of space, which comes from the need to specify the identities of the items found. In this paper we show one can achieve O(logn loglogn) bits of space for this problem. Our techniques, based on Gaussian processes, lead to a number of other new results for data streams, including: (1) The first algorithm for estimating F_{2} simultaneously at all points in a stream using only O(lognloglogn) bits of space, improving a natural union bound. (2) A way to estimate the ℓ_{∞} norm of a stream up to additive error T √F_{2} with O(lognloglogn) bits of space, resolving Open Question 3 from the IITK 2006 list for insertion only streams. 

Ji, Zhengfeng 
STOC'16: "Classical Verification ..."
Classical Verification of Quantum Proofs
Zhengfeng Ji (University of Technology Sydney, Australia; University of Waterloo, Canada; Institute of Software at Chinese Academy of Sciences, China)
We present a classical interactive protocol that verifies the validity of a quantum witness state for the local Hamiltonian problem. It follows from this protocol that approximating the nonlocal value of a multiplayer oneround game to inverse polynomial precision is QMAhard. Our work makes an interesting connection between the theory of QMAcompleteness and Hamiltonian complexity on one hand and the study of nonlocal games and Bell inequalities on the other.
@InProceedings{STOC16p885,
author = {Zhengfeng Ji},
title = {Classical Verification of Quantum Proofs},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {885898},
doi = {10.1145/2897518.2897634},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "SampleOptimal Tomography ..."
SampleOptimal Tomography of Quantum States
Jeongwan Haah, Aram W. Harrow, Zhengfeng Ji, Xiaodi Wu, and Nengkun Yu (Massachusetts Institute of Technology, USA; University of Technology Sydney, Australia; University of Waterloo, Canada; Institute of Software at Chinese Academy of Sciences, China; University of Oregon, USA; University of Guelph, Canada) It is a fundamental problem to decide how many copies of an unknown mixed quantum state are necessary and sufficient to determine the state. This is the quantum analogue of the problem of estimating a probability distribution given some number of samples. Previously, it was known only that estimating states to error T in trace distance required O(dr^{2}/T^{2}) copies for a ddimensional density matrix of rank r. Here, we give a measurement scheme (POVM) that uses O( (dr/ δ ) ln(d/δ) ) copies to estimate ρ to error δ in infidelity. This implies O( (dr / T^{2})· ln(d/T) ) copies suffice to achieve error T in trace distance. For fixed d, our measurement can be implemented on a quantum computer in time polynomial in n. We also use the Holevo bound from quantum information theory to prove a lower bound of Ω(dr/T^{2})/ log(d/rT) copies needed to achieve error T in trace distance. This implies a lower bound Ω(dr/δ)/log(d/rδ) for the estimation error δ in infidelity. These match our upper bounds up to log factors. Our techniques can also show an Ω(r^{2}d/δ) lower bound for measurement strategies in which each copy is measured individually and then the outcomes are classically postprocessed to produce an estimate. This matches the known achievability results and proves for the first time that such “product” measurements have asymptotically suboptimal scaling with d and r. 

Kamath, Gautam 
STOC'16: "A SizeFree CLT ..."
A SizeFree CLT for Poisson Multinomials and Its Applications
Constantinos Daskalakis, Anindya De, Gautam Kamath, and Christos Tzamos (Massachusetts Institute of Technology, USA; Northwestern University, USA) An (n,k)Poisson Multinomial Distribution (PMD) is the distribution of the sum of n independent random vectors supported on the set B_{k}={e_{1},…,e_{k}} of standard basis vectors in ℝ^{k}. We show that any (n,k)PMD is poly(k/σ)close in total variation distance to the (appropriately discretized) multidimensional Gaussian with the same first two moments, removing the dependence on n from the Central Limit Theorem of Valiant and Valiant. Interestingly, our CLT is obtained by bootstrapping the ValiantValiant CLT itself through the structural characterization of PMDs shown in recent work by Daskalakis, Kamath and Tzamos. In turn, our stronger CLT can be leveraged to obtain an efficient PTAS for approximate Nash equilibria in anonymous games, significantly improving the state of the art, and matching qualitatively the running time dependence on n and 1/T of the best known algorithm for twostrategy anonymous games. Our new CLT also enables the construction of covers for the set of (n,k)PMDs, which are proper and whose size is shown to be essentially optimal. Our cover construction combines our CLT with the ShapleyFolkman theorem and recent sparsification results for Laplacian matrices by Batson, Spielman, and Srivastava. Our cover size lower bound is based on an algebraic geometric construction. Finally, leveraging the structural properties of the Fourier spectrum of PMDs we show that these distributions can be learned from O_{k}(1/T^{2}) samples in poly_{k}(1/T)time, removing the quasipolynomial dependence of the running time on 1/T from prior work. 

Kane, Daniel M. 
STOC'16: "SuperLinear Gate ..."
SuperLinear Gate and SuperQuadratic Wire Lower Bounds for DepthTwo and DepthThree Threshold Circuits
Daniel M. Kane and Ryan Williams (University of California at San Diego, USA; Stanford University, USA) In order to formally understand the power of neural computing, we first need to crack the frontier of threshold circuits with two and three layers, a regime that has been surprisingly intractable to analyze. We prove the first superlinear gate lower bounds and the first superquadratic wire lower bounds for depthtwo linear threshold circuits with arbitrary weights, and depththree majority circuits computing an explicit function. (1) We prove that for all ε ≪ √log(n)/n, the lineartime computable Andreev’s function cannot be computed on a (1/2+ε)fraction of nbit inputs by depthtwo circuits of o(ε^{3} n^{3/2}/log^{3} n) gates, nor can it be computed with o(ε^{3} n^{5/2}/log^{7/2} n) wires. This establishes an averagecase “size hierarchy” for threshold circuits, as Andreev’s function is computable by uniform depthtwo circuits of o(n^{3}) linear threshold gates, and by uniform depththree circuits of O(n) majority gates. (2) We present a new function in P based on smallbiased sets, which we prove cannot be computed by a majority vote of depthtwo threshold circuits of o(n^{3/2}/log^{3} n) gates, nor with o(n^{5/2}/log^{7/2}n) wires. (3) We give tight averagecase (gate and wire) complexity results for computing PARITY with depthtwo threshold circuits; the answer turns out to be the same as for depthtwo majority circuits. The key is a new method for analyzing random restrictions to linear threshold functions. Our main analytical tool is the LittlewoodOfford Lemma from additive combinatorics. Ilias Diakonikolas, Daniel M. Kane, and Alistair Stewart (University of Southern California, USA; University of California at San Diego, USA) An (n, k)Poisson Multinomial Distribution (PMD) is a random variable of the form X = ∑_{i=1}^{n} X_{i}, where the X_{i}’s are independent random vectors supported on the set of standard basis vectors in ^{k}. In this paper, we obtain a refined structural understanding of PMDs by analyzing their Fourier transform. As our core structural result, we prove that the Fourier transform of PMDs is approximately sparse, i.e., its L_{1}norm is small outside a small set. By building on this result, we obtain the following applications: Learning Theory. We give the first computationally efficient learning algorithm for PMDs under the total variation distance. Our algorithm learns an (n, k)PMD within variation distance ε using a nearoptimal sample size of O_{k}(1/ε^{2}), and runs in time O_{k}(1/ε^{2}) · logn. Previously, no algorithm with a (1/ε) runtime was known, even for k=3. Game Theory. We give the first efficient polynomialtime approximation scheme (EPTAS) for computing Nash equilibria in anonymous games. For normalized anonymous games with n players and k strategies, our algorithm computes a wellsupported εNash equilibrium in time n^{O(k3)} · (k/ε)^{O(k3log(k/ε)/loglog(k/ε))k−1}. The best previous algorithm for this problem [, ] had running time n^{(f(k)/ε)k}, where f(k) = Ω(k^{k2}), for any k>2. Statistics. We prove a multivariate central limit theorem (CLT) that relates an arbitrary PMD to a discretized multivariate Gaussian with the same mean and covariance, in total variation distance. Our new CLT strengthens the CLT of Valiant and Valiant [, ] by removing the dependence on n in the error bound. Along the way we prove several new structural results of independent interest about PMDs. These include: (i) a robust momentmatching lemma, roughly stating that two PMDs that approximately agree on their lowdegree parameter moments are close in variation distance; (ii) nearoptimal size proper εcovers for PMDs in total variation distance (constructive upper bound and nearlymatching lower bound). In addition to Fourier analysis, we employ a number of analytic tools, including the saddlepoint method from complex analysis, that may find other applications. 

Kapralov, Michael 
STOC'16: "Sparse Fourier Transform ..."
Sparse Fourier Transform in Any Constant Dimension with NearlyOptimal Sample Complexity in Sublinear Time
Michael Kapralov (EPFL, Switzerland) We consider the problem of computing a ksparse approximation to the Fourier transform of a length N signal. Our main result is a randomized algorithm for computing such an approximation (i.e. achieving ℓ_{2}/ℓ_{2} sparse recovery guarantees using Fourier measurements) using O_{d}(klogNloglogN) samples of the signal in time domain and O_{d}(klog^{d+3} N) runtime, where d≥ 1 is the dimensionality of the Fourier transform. The sample complexity matches the Ω(klog(N/k)) lower bound for nonadaptive algorithms due to [DIPW] for any k≤ N^{1−δ} for a constant δ>0 up to an O(loglogN) factor. Prior to our work a result with comparable sample complexity klogN log^{O(1)}logN and sublinear runtime was known for the Fourier transform on the line [IKP], but for any dimension d≥ 2 previously known techniques either suffered from a (logN) factor loss in sample complexity or required Ω(N) runtime. 

Karger, David R. 
STOC'16: "Enumerating Parametric ..."
Enumerating Parametric Global Minimum Cuts by Random Interleaving
David R. Karger (Massachusetts Institute of Technology, USA)
Recently, Aissi et al. gave new counting and algorithmic bounds for parametric minimum cuts in a graph, where each edge cost is a linear combination of multiple cost criteria and different cuts become minimum as the coefficients of the linear combination are varied. In this article, we derive better bounds using a mathematically simpler argument. We provide faster algorithms for enumerating these cuts. We give a lower bound showing our upper bounds have roughly the right degree. Our results also immediately generalize to parametric versions of other problems solved by the Contraction Algorithm, including approximate mincuts, multiway cuts, and a matroid optimization problem. We also give a first generalization to nonlinear parametric minimum cuts.
@InProceedings{STOC16p542,
author = {David R. Karger},
title = {Enumerating Parametric Global Minimum Cuts by Random Interleaving},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {542555},
doi = {10.1145/2897518.2897578},
year = {2016},
}
Publisher's Version
Article Search


Kaufman, Tali 
STOC'16: "Bounded Degree Cosystolic ..."
Bounded Degree Cosystolic Expanders of Every Dimension
Shai Evra and Tali Kaufman (Hebrew University of Jerusalem, Israel; BarIlan University, Israel)
In recent years a high dimensional theory of expanders has emerged. The notion of combinatorial expansion of graphs (i.e. the Cheeger constant of a graph) has seen two generalizations to high dimensional simplicial complexes. One generalization, known as coboundary expansion, is due to Linial and Meshulem; the other, which we term here cosystolic expansion, is due to Gromov, who showed that cosystolic expanders have the topological overlapping property. No construction (either random or explicit) of bounded degree combinational expanders (according to either definition) were known until a recent work of Kaufman, Kazhdan and Lubotzky, which provided the first bounded degree cosystolic expanders of dimension two. No bounded degree combinatorial expanders are known in higher dimensions. In this work we present explicit bounded degree cosystolic expanders of every dimension. This solves affirmatively an open question raised by Gromov, who asked whether there exist bounded degree complexes with the topological overlapping property in every dimension. Moreover, we provide a local to global criterion on a complex that implies cosystolic expansion: Namely, for a ddimensional complex, X, if its underlying graph is a good expander, and all its links are both coboundary expanders and good expander graphs, then the (d1)dimensional skeleton of the complex is a cosystolic expander.
@InProceedings{STOC16p36,
author = {Shai Evra and Tali Kaufman},
title = {Bounded Degree Cosystolic Expanders of Every Dimension},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {3648},
doi = {10.1145/2897518.2897543},
year = {2016},
}
Publisher's Version
Article Search


Kayal, Neeraj 
STOC'16: "On the Size of Homogeneous ..."
On the Size of Homogeneous and of Depth Four Formulas with Low Individual Degree
Neeraj Kayal, Chandan Saha, and Sébastien Tavenas (Microsoft Research, India; Indian Institute of Science, India)
Let r be an integer. Let us call a polynomial f as a multiric polynomial if the degree of f with respect to any variable is at most r (this generalizes the notion of multilinear polynomials). We investigate arithmetic circuits in which the output is syntactically forced to be a multiric polynomial and refer to these as multiric circuits.
Specifically, first define the formal degree of a node a with respect to a variable x inductively as follows. For a leaf it is 1 if a is labelled with x and zero otherwise; for an internal node labelled with * (respectively +) it is the sum of (respectively the maximum of) the formal degrees of the children with respect to x. We call an arithmetic circuit as a multiric circuit if the formal degree of the output node with respect to any variable is at most r. We prove lower bounds for various subclasses of multiric circuits.
@InProceedings{STOC16p626,
author = {Neeraj Kayal and Chandan Saha and Sébastien Tavenas},
title = {On the Size of Homogeneous and of Depth Four Formulas with Low Individual Degree},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {626632},
doi = {10.1145/2897518.2897550},
year = {2016},
}
Publisher's Version
Article Search


Keller, Barbara 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Kempe, David 
STOC'16: "Deterministic and ..."
Deterministic and Probabilistic Binary Search in Graphs
Ehsan EmamjomehZadeh, David Kempe, and Vikrant Singhal (University of Southern California, USA) We consider the following natural generalization of Binary Search: in a given undirected, positively weighted graph, one vertex is a target. The algorithm’s task is to identify the target by adaptively querying vertices. In response to querying a node q, the algorithm learns either that q is the target, or is given an edge out of q that lies on a shortest path from q to the target. We study this problem in a general noisy model in which each query independently receives a correct answer with probability p > 1/2 (a known constant), and an (adversarial) incorrect one with probability 1 − p. Our main positive result is that when p = 1 (i.e., all answers are correct), log_{2} n queries are always sufficient. For general p, we give an (almost informationtheoretically optimal) algorithm that uses, in expectation, no more than (1 − δ) logn/1 − H(p) + o(logn) + O(log^{2} (1/δ)) queries, and identifies the target correctly with probability at leas 1 − δ. Here, H(p) = −(p logp + (1 − p) log(1 − p)) denotes the entropy. The first bound is achieved by the algorithm that iteratively queries a 1median of the nodes not ruled out yet; the second bound by careful repeated invocations of a multiplicative weights algorithm. Even for p = 1, we show several hardness results for the problem of determining whether a target can be found using K queries. Our upper bound of log_{2} n implies a quasipolynomialtime algorithm for undirected connected graphs; we show that this is bestpossible under the Strong Exponential Time Hypothesis (SETH). Furthermore, for directed graphs, or for undirected graphs with nonuniform node querying costs, the problem is PSPACEcomplete. For a semiadaptive version, in which one may query r nodes each in k rounds, we show membership in Σ_{2k−1} in the polynomial hierarchy, and hardness for Σ_{2k−5}. 

Khanna, Sanjeev 
STOC'16: "Tight Bounds for ..."
Tight Bounds for SinglePass Streaming Complexity of the Set Cover Problem
Sepehr Assadi, Sanjeev Khanna, and Yang Li (University of Pennsylvania, USA) We resolve the space complexity of singlepass streaming algorithms for approximating the classic set cover problem. For finding an αapproximate set cover (for α= o(√n)) via a singlepass streaming algorithm, we show that Θ(mn/α) space is both sufficient and necessary (up to an O(logn) factor); here m denotes number of the sets and n denotes size of the universe. This provides a strong negative answer to the open question posed by Indyk (2015) regarding the possibility of having a singlepass algorithm with a small approximation factor that uses sublinear space. We further study the problem of estimating the size of a minimum set cover (as opposed to finding the actual sets), and establish that an additional factor of α saving in the space is achievable in this case and that this is the best possible. In other words, we show that Θ(mn/α^{2}) space is both sufficient and necessary (up to logarithmic factors) for estimating the size of a minimum set cover to within a factor of α. Our algorithm in fact works for the more general problem of estimating the optimal value of a covering integer program. On the other hand, our lower bound holds even for set cover instances where the sets are presented in a random order. 

Khot, Subhash 
STOC'16: "Candidate Hard Unique ..."
Candidate Hard Unique Game
Subhash Khot and Dana Moshkovitz (New York University, USA; Massachusetts Institute of Technology, USA)
We propose a candidate reduction for ruling out polynomialtime algorithms for unique games, either under plausible complexity assumptions, or unconditionally for Lasserre semidefinite programs with a constant number of rounds. We analyze the completeness and Lasserre solution of our construction, and provide a soundness analysis in a certain setting of interest. Addressing general settings is tightly connected to a question on Gaussian isoperimetry. Our construction is based on our previous work on the complexity of approximately solving a system of linear equations over reals, which we suggested as an avenue towards a (positive) resolution of the Unique Games Conjecture. The construction employs a new encoding scheme that we call the real code. The real code has two useful properties: like the long code, it has a unique local test, and like the Hadamard code, it has the socalled subcode covering property.
@InProceedings{STOC16p63,
author = {Subhash Khot and Dana Moshkovitz},
title = {Candidate Hard Unique Game},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {6376},
doi = {10.1145/2897518.2897531},
year = {2016},
}
Publisher's Version
Article Search


Kim, David H. K. 
STOC'16: "Improved Approximation ..."
Improved Approximation for NodeDisjoint Paths in Planar Graphs
Julia Chuzhoy, David H. K. Kim, and Shi Li (Toyota Technological Institute at Chicago, USA; University of Chicago, USA; State University of New York at Buffalo, USA) We study the classical NodeDisjoint Paths (NDP) problem: given an nvertex graph G and a collection =(s_{1},t_{1}),…,(s_{k},t_{k}) of pairs of vertices of G called demand pairs, find a maximumcardinality set of nodedisjoint paths connecting the demand pairs. NDP is one of the most basic routing problems, that has been studied extensively. Despite this, there are still wide gaps in our understanding of its approximability: the best currently known upper bound of O(√n) on its approximation ratio is achieved via a simple greedy algorithm, while the best current negative result shows that the problem does not have a better than Ω(log^{1/2−δ}n)approximation for any constant δ, under standard complexity assumptions. Even for planar graphs no better approximation algorithms are known, and to the best of our knowledge, the best negative bound is APXhardness. Perhaps the biggest obstacle to obtaining better approximation algorithms for NDP is that most currently known approximation algorithms for this type of problems rely on the standard multicommodity flow relaxation, whose integrality gap is Ω(√n) for NDP, even in planar graphs. In this paper, we break the barrier of O(√n) on the approximability of NDP in planar graphs and obtain an Õ(n^{9/19})approximation. We introduce a new linear programming relaxation of the problem, and a number of new techniques, that we hope will be helpful in designing more powerful algorithms for this and related problems. 

Klein, Philip N. 
STOC'16: "Approximating Connectivity ..."
Approximating Connectivity Domination in Weighted BoundedGenus Graphs
Vincent CohenAddad, Éric Colin de Verdière, Philip N. Klein, Claire Mathieu, and David Meierfrankenfeld (ENS, France; CNRS, France; Brown University, USA)
We present a framework for addressing several problems on weighted planar graphs and graphs of bounded genus. With that framework, we derive polynomialtime approximation schemes for the following problems in planar graphs or graphs of bounded genus: edgeweighted tree cover and tour cover; vertexweighted connected dominating set, maxweightleaf spanning tree, and connected vertex cover. In addition, we obtain a polynomialtime approximation scheme for feedback vertex set in planar graphs. These are the first polynomialtime approximation schemes for all those problems in weighted embedded graphs. (For unweighted versions of some of these problems, polynomialtime approximation schemes were previously given using bidimensionality.)
@InProceedings{STOC16p584,
author = {Vincent CohenAddad and Éric Colin de Verdière and Philip N. Klein and Claire Mathieu and David Meierfrankenfeld},
title = {Approximating Connectivity Domination in Weighted BoundedGenus Graphs},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {584597},
doi = {10.1145/2897518.2897635},
year = {2016},
}
Publisher's Version
Article Search


Kol, Gillat 
STOC'16: "Exponential Separation ..."
Exponential Separation of Communication and External Information
Anat Ganor, Gillat Kol, and Ran Raz (Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) We show an exponential gap between communication complexity and external information complexity, by analyzing a communication task suggested as a candidate by Braverman. Previously, only a separation of communication complexity and internal information complexity was known. More precisely, we obtain an explicit example of a search problem with external information complexity ≤ O(k), with respect to any input distribution, and distributional communication complexity ≥ 2^{k}, with respect to some input distribution. In particular, this shows that a communication protocol cannot always be compressed to its external information. By a result of Braverman, our gap is the largest possible. Moreover, since the upper bound of O(k) on the external information complexity of the problem is obtained with respect to any input distribution, our result implies an exponential gap between communication complexity and information complexity (both internal and external) in the nondistributional setting of Braverman. In this setting, no gap was previously known, even for internal information complexity. Gillat Kol (Institute for Advanced Study at Princeton, USA)
We study the interactive compression problem: Given a twoparty communication protocol with small information cost, can it be compressed so that the total number of bits communicated is also small? We consider the case where the parties have inputs that are independent of each other, and give a simulation protocol that communicates I^2 * polylog(I) bits, where I is the information cost of the original protocol. Our protocol is the first simulation protocol whose communication complexity is bounded by a polynomial in the information cost of the original protocol.
@InProceedings{STOC16p987,
author = {Gillat Kol},
title = {Interactive Compression for Product Distributions},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {987998},
doi = {10.1145/2897518.2897537},
year = {2016},
}
Publisher's Version
Article Search


Kopelowitz, Tsvi 
STOC'16: "Contention Resolution ..."
Contention Resolution with LogLogstar Channel Accesses
Michael A. Bender, Tsvi Kopelowitz, Seth Pettie, and Maxwell Young (Stony Brook University, USA; University of Michigan, USA; Mississippi State University, USA)
For decades, randomized exponential backoff has provided a critical
algorithmic building block in situations where multiple devices seek
access to a shared resource. Surprisingly, despite this history,
the performance of standard backoff is poor under worstcase
scheduling of demands on the resource: (i) subconstant throughput
can occur under plausible scenarios, and (ii) each of N devices
requires Omega(log N) access attempts before obtaining the
resource.
In this paper, we address these shortcomings by offering a new
backoff protocol for a shared communications channel that guarantees
expected constant throughput with only O(log(log* N)) access
attempts in expectation. Central to this result are new algorithms
for approximate counting and leader election with the
same performance guarantees.
@InProceedings{STOC16p499,
author = {Michael A. Bender and Tsvi Kopelowitz and Seth Pettie and Maxwell Young},
title = {Contention Resolution with LogLogstar Channel Accesses},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {499508},
doi = {10.1145/2897518.2897655},
year = {2016},
}
Publisher's Version
Article Search


Kopparty, Swastik 
STOC'16: "HighRate LocallyCorrectable ..."
HighRate LocallyCorrectable and LocallyTestable Codes with Subpolynomial Query Complexity
Swastik Kopparty, Or Meir, Noga RonZewi, and Shubhangi Saraf (Rutgers University, USA; Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) In this work, we construct the first locallycorrectable codes (LCCs), and locallytestable codes (LTCs) with constant rate, constant relative distance, and subpolynomial query complexity. Specifically, we show that there exist LCCs and LTCs with block length n, constant rate (which can even be taken arbitrarily close to 1) and constant relative distance, whose query complexity is exp(Õ(√logn)) (for LCCs) and (logn)^{O(loglogn)} (for LTCs). Previously such codes were known to exist only with Ω(n^{β}) query complexity (for constant β>0). In addition to having small query complexity, our codes also achieve better tradeoffs between the rate and the relative distance than were previously known to be achievable by LCCs or LTCs. Specifically, over large (but constant size) alphabet, our codes approach the Singleton bound, that is, they have almost the bestpossible relationship between their rate and distance. This has the surprising consequence that asking for a largealphabet errorcorrecting code to further be an LCC or LTC with subpolynomial query complexity does not require any sacrifice in terms of rate and distance! Over the binary alphabet, our codes meet the Zyablov bound. Such tradeoffs between the rate and the relative distance were previously not known for any o(n) query complexity. Our results on LCCs also immediately give locallydecodable codes (LDCs) with the same parameters. Our codes are based on a technique of Alon, Edmonds and Luby. We observe that this technique can be used as a general distanceamplification method, and show that it interacts well with local correctors and testers. We obtain our main results by applying this method to suitably constructed LCCs and LTCs in the nonstandard regime of subconstant relative distance. 

Koren, Tomer 
STOC'16: "The Computational ..."
The Computational Power of Optimization in Online Learning
Elad Hazan and Tomer Koren (Princeton University, USA; Technion, Israel) We consider the fundamental problem of prediction with expert advice where the experts are “optimizable”: there is a blackbox optimization oracle that can be used to compute, in constant time, the leading expert in retrospect at any point in time. In this setting, we give a novel online algorithm that attains vanishing regret with respect to N experts in total O(√N) computation time. We also give a lower bound showing that this running time cannot be improved (up to log factors) in the oracle model, thereby exhibiting a quadratic speedup as compared to the standard, oraclefree setting where the required time for vanishing regret is Θ(N). These results demonstrate an exponential gap between the power of optimization in online learning and its power in statistical learning: in the latter, an optimization oracle—i.e., an efficient empirical risk minimizer—allows to learn a finite hypothesis class of size N in time O(logN). We also study the implications of our results to learning in repeated zerosum games, in a setting where the players have access to oracles that compute, in constant time, their bestresponse to any mixed strategy of their opponent. We show that the runtime required for approximating the minimax value of the game in this setting is Θ(√N), yielding again a quadratic improvement upon the oraclefree setting, where Θ(N) is known to be tight. 

Korman, Amos 
STOC'16: "Parallel Exhaustive ..."
Parallel Exhaustive Search without Coordination
Pierre Fraigniaud, Amos Korman, and Yoav Rodeh (CNRS, France; University of Paris Diderot, France; Weizmann Institute of Science, Israel) We analyse parallel algorithms in the context of exhaustive search over totally ordered sets. Imagine an infinite list of “boxes”, with a “treasure” hidden in one of them, where the boxes’ order reflects the importance of finding the treasure in a given box. At each time step, a search protocol executed by a searcher has the ability to peek into one box, and see whether the treasure is present or not. Clearly, the best strategy of a single searcher would be to open the boxes one by one, in increasing order. Moreover, by equally dividing the workload between them, k searchers can trivially find the treasure k times faster than one searcher. However, this straightforward strategy is very sensitive to failures (e.g., crashes of processors), and overcoming this issue seems to require a large amount of communication. We therefore address the question of designing parallel search algorithms maximizing their speedup and maintaining high levels of robustness, while minimizing the amount of resources for coordination. Based on the observation that algorithms that avoid communication are inherently robust, we focus our attention on identifying the best running time performance of noncoordinating algorithms. Specifically, we devise noncoordinating algorithms that achieve a speedup of 9/8 for two searchers, a speedup of 4/3 for three searchers, and in general, a speedup of k/4(1+1/k)^{2} for any k≥ 1 searchers. Thus, asymptotically, the speedup is only four times worse compared to the case of full coordination. Moreover, these bounds are tight in a strong sense as no noncoordinating search algorithm can achieve better speedups. Our algorithms are surprisingly simple and hence applicable. However they are memory intensive and so we suggest a practical, memory efficient version, with a speedup of (k^{2} − 1)/4k. That is, it is only a factor of (k+1)/(k−1) slower than the optimal algorithm. Overall, we highlight that, in faulty contexts in which coordination between the searchers is technically difficult to implement, intrusive with respect to privacy, and/or costly in term of resources, it might well be worth giving up on coordination, and simply run our noncoordinating exhaustive search algorithms. 

Kothari, Robin 
STOC'16: "Separations in Query ..."
Separations in Query Complexity using Cheat Sheets
Scott Aaronson, Shalev BenDavid, and Robin Kothari (Massachusetts Institute of Technology, USA)
We show a power 2.5 separation between boundederror randomized and quantum query complexity for a total Boolean function, refuting the widely believed conjecture that the best such separation could only be quadratic (from Grover's algorithm). We also present a total function with a power 4 separation between quantum query complexity and approximate polynomial degree, showing severe limitations on the power of the polynomial method. Finally, we exhibit a total function with a quadratic gap between quantum query complexity and certificate complexity, which is optimal (up to log factors). These separations are shown using a new, general technique that we call the cheat sheet technique, which builds upon the techniques of Ambainis et al. [STOC 2016]. The technique is based on a generic transformation that converts any (possibly partial) function into a new total function with desirable properties for showing separations. The framework also allows many known separations, including some recent breakthrough results of Ambainis et al. [STOC 2016], to be shown in a unified manner.
@InProceedings{STOC16p863,
author = {Scott Aaronson and Shalev BenDavid and Robin Kothari},
title = {Separations in Query Complexity using Cheat Sheets},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {863876},
doi = {10.1145/2897518.2897644},
year = {2016},
}
Publisher's Version
Article Search


Koucký, Michal 
STOC'16: "Streaming Algorithms ..."
Streaming Algorithms for Embedding and Computing Edit Distance in the Low Distance Regime
Diptarka Chakraborty, Elazar Goldenberg, and Michal Koucký (IIT Kanpur, India; Charles University in Prague, Czechia) The Hamming and the edit metrics are two common notions of measuring distances between pairs of strings x,y lying in the Boolean hypercube. The edit distance between x and y is defined as the minimum number of character insertion, deletion, and bit flips needed for converting x into y. Whereas, the Hamming distance between x and y is the number of bit flips needed for converting x to y. In this paper we study a randomized injective embedding of the edit distance into the Hamming distance with a small distortion. We show a randomized embedding with quadratic distortion. Namely, for any x,y satisfying that their edit distance equals k, the Hamming distance between the embedding of x and y is O(k^{2}) with high probability. This improves over the distortion ratio of O( n ^{*} n) obtained by Jowhari (2012) for small values of k. Moreover, the embedding output size is linear in the input size and the embedding can be computed using a single pass over the input. We provide several applications for this embedding. Among our results we provide a onepass (streaming) algorithm for edit distance running in space O(s) and computing edit distance exactly upto distance s^{1}/6. This algorithm is based on kernelization for edit distance that is of independent interest. 

Krinninger, Sebastian 
STOC'16: "A Deterministic ..."
A Deterministic AlmostTight Distributed Algorithm for Approximating SingleSource Shortest Paths
Monika Henzinger, Sebastian Krinninger, and Danupon Nanongkai (University of Vienna, Austria; Max Planck Institute for Informatics, Germany; KTH, Sweden) We present a deterministic (1+o(1))approximation O(n^{1/2+o(1)}+D^{1+o(1)})time algorithm for solving the singlesource shortest paths problem on distributed weighted networks (the CONGEST model); here n is the number of nodes in the network and D is its (hop) diameter. This is the first nontrivial deterministic algorithm for this problem. It also improves (i) the running time of the randomized (1+o(1))approximation Õ(n^{1/2}D^{1/4}+D)time algorithm of Nanongkai [STOC 2014] by a factor of as large as n^{1/8}, and (ii) the O(T^{−1}logT^{−1})approximation factor of Lenzen and PattShamir’s Õ(n^{1/2+T}+D)time algorithm [STOC 2013] within the same running time. Our running time matches the known time lower bound of Ω(n^{1/2}/logn + D) [Das Sarma et al. STOC 2011] modulo some lowerorder terms, thus essentially settling the status of this problem which was raised at least a decade ago [Elkin SIGACT News 2004]. It also implies a (2+o(1))approximation O(n^{1/2+o(1)}+D^{1+o(1)})time algorithm for approximating a network’s weighted diameter which almost matches the lower bound by Holzer et al. [PODC 2012]. In achieving this result, we develop two techniques which might be of independent interest and useful in other settings: (i) a deterministic process that replaces the “hitting set argument” commonly used for shortest paths computation in various settings, and (ii) a simple, deterministic, construction of an (n^{o(1)}, o(1))hop set of size O(n^{1+o(1)}). We combine these techniques with many distributed algorithmic techniques, some of which from problems that are not directly related to shortest paths, e.g. ruling sets [Goldberg et al. STOC 1987], source detection [Lenzen, Peleg PODC 2013], and partial distance estimation [Lenzen, PattShamir PODC 2015]. Our hop set construction also leads to singlesource shortest paths algorithms in two other settings: (i) a (1+o(1))approximation O(n^{o(1)})time algorithm on congested cliques, and (ii) a (1+o(1))approximation O(n^{o(1)}logW)pass O(n^{1+o(1)}logW)space streaming algorithm, when edge weights are in {1, 2, …, W}. The first result answers an open problem in [Nanongkai, STOC 2014]. The second result partially answers an open problem raised by McGregor in 2006 [sublinear.info, Problem 14]. 

Kudekar, Shrinivas 
STOC'16: "ReedMuller Codes ..."
ReedMuller Codes Achieve Capacity on Erasure Channels
Shrinivas Kudekar, Santhosh Kumar, Marco Mondelli, Henry D. Pfister, Eren Şaşoğlu, and Rüdiger Urbanke (Qualcomm Research, USA; Texas A&M University, USA; EPFL, Switzerland; Duke University, USA; Intel, USA)
We introduce a new approach to proving that a sequence of deterministic linear codes achieves capacity on an erasure channel under maximum a posteriori decoding. Rather than relying on the precise structure of the codes, our method exploits code symmetry. In particular, the technique applies to any sequence of linear codes where the block lengths are strictly increasing, the code rates converge, and the permutation group of each code is doubly transitive. In a nutshell, we show that symmetry alone implies nearoptimal performance.
An important consequence of this result is that a sequence of ReedMuller codes with increasing block length and converging rate achieves capacity. This possibility has been suggested previously in the literature, but it has only been proven for cases where the limiting code rate is 0 or 1. Moreover, these results extend naturally to affineinvariant codes and, thus, to all extended primitive narrowsense BCH codes. This is used to resolve, in the affirmative, the existence question for capacityachieving sequences of binary cyclic codes. The primary tools used in the proofs are the sharp threshold property for symmetric monotone boolean functions and the area theorem for extrinsic information transfer (EXIT) functions.
@InProceedings{STOC16p658,
author = {Shrinivas Kudekar and Santhosh Kumar and Marco Mondelli and Henry D. Pfister and Eren Şaşoğlu and Rüdiger Urbanke},
title = {ReedMuller Codes Achieve Capacity on Erasure Channels},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {658669},
doi = {10.1145/2897518.2897584},
year = {2016},
}
Publisher's Version
Article Search


Kumar, Santhosh 
STOC'16: "ReedMuller Codes ..."
ReedMuller Codes Achieve Capacity on Erasure Channels
Shrinivas Kudekar, Santhosh Kumar, Marco Mondelli, Henry D. Pfister, Eren Şaşoğlu, and Rüdiger Urbanke (Qualcomm Research, USA; Texas A&M University, USA; EPFL, Switzerland; Duke University, USA; Intel, USA)
We introduce a new approach to proving that a sequence of deterministic linear codes achieves capacity on an erasure channel under maximum a posteriori decoding. Rather than relying on the precise structure of the codes, our method exploits code symmetry. In particular, the technique applies to any sequence of linear codes where the block lengths are strictly increasing, the code rates converge, and the permutation group of each code is doubly transitive. In a nutshell, we show that symmetry alone implies nearoptimal performance.
An important consequence of this result is that a sequence of ReedMuller codes with increasing block length and converging rate achieves capacity. This possibility has been suggested previously in the literature, but it has only been proven for cases where the limiting code rate is 0 or 1. Moreover, these results extend naturally to affineinvariant codes and, thus, to all extended primitive narrowsense BCH codes. This is used to resolve, in the affirmative, the existence question for capacityachieving sequences of binary cyclic codes. The primary tools used in the proofs are the sharp threshold property for symmetric monotone boolean functions and the area theorem for extrinsic information transfer (EXIT) functions.
@InProceedings{STOC16p658,
author = {Shrinivas Kudekar and Santhosh Kumar and Marco Mondelli and Henry D. Pfister and Eren Şaşoğlu and Rüdiger Urbanke},
title = {ReedMuller Codes Achieve Capacity on Erasure Channels},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {658669},
doi = {10.1145/2897518.2897584},
year = {2016},
}
Publisher's Version
Article Search


Kutten, Shay 
STOC'16: "Online Matching: ..."
Online Matching: Haste Makes Waste!
Yuval Emek, Shay Kutten, and Roger Wattenhofer (Technion, Israel; ETH Zurich, Switzerland) This paper studies a new online problem, referred to as mincost perfect matching with delays (MPMD), defined over a finite metric space (i.e., a complete graph with positive edge weights obeying the triangle inequality) M that is known to the algorithm in advance. Requests arrive in a continuous time online fashion at the points of M and should be served by matching them to each other. The algorithm is allowed to delay its request matching commitments, but this does not come for free: the total cost of the algorithm is the sum of metric distances between matched requests plus the sum of times each request waited since it arrived until it was matched. A randomized online MPMD algorithm is presented whose competitive ratio is O (log^{2} n + logΔ), where n is the number of points in M and Δ is its aspect ratio. The analysis is based on a machinery developed in the context of a new stochastic process that can be viewed as two interleaved Poisson processes; surprisingly, this new process captures precisely the behavior of our algorithm. A related problem in which the algorithm is allowed to clear any unmatched request at a fixed penalty is also addressed. It is suggested that the MPMD problem is merely the tip of the iceberg for a general framework of online problems with delayed service that captures many more natural problems. 

Kyng, Rasmus 
STOC'16: "Sparsified Cholesky ..."
Sparsified Cholesky and Multigrid Solvers for Connection Laplacians
Rasmus Kyng, Yin Tat Lee, Richard Peng, Sushant Sachdeva, and Daniel A. Spielman (Yale University, USA; Massachusetts Institute of Technology, USA; Georgia Tech, USA)
We introduce the sparsified Cholesky and sparsified multigrid algorithms for solving systems of linear equations. These algorithms accelerate Gaussian elimination by sparsifying the nonzero matrix entries created by the elimination process. We use these new algorithms to derive the first nearly linear time algorithms for solving systems of equations in connection Laplaciansa generalization of Laplacian matrices that arise in many problems in image and signal processing. We also prove that every connection Laplacian has a linear sized approximate inverse. This is an LU factorization with a linear number of nonzero entries that is a strong approximation of the original matrix. Using such a factorization one can solve systems of equations in a connection Laplacian in linear time. Such a factorization was unknown even for ordinary graph Laplacians.
@InProceedings{STOC16p842,
author = {Rasmus Kyng and Yin Tat Lee and Richard Peng and Sushant Sachdeva and Daniel A. Spielman},
title = {Sparsified Cholesky and Multigrid Solvers for Connection Laplacians},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {842850},
doi = {10.1145/2897518.2897640},
year = {2016},
}
Publisher's Version
Article Search


Lee, Troy 
STOC'16: "Separations in Query ..."
Separations in Query Complexity Based on Pointer Functions
Andris Ambainis, Kaspars Balodis, Aleksandrs Belovs, Troy Lee, Miklos Santha, and Juris Smotrovs (University of Latvia, Latvia; CWI, Netherlands; Nanyang Technological University, Singapore; National University of Singapore, Singapore; CNRS, France) In 1986, Saks and Wigderson conjectured that the largest separation between deterministic and zeroerror randomized query complexity for a total boolean function is given by the function f on n=2^{k} bits defined by a complete binary tree of NAND gates of depth k, which achieves R_{0}(f) = O(D(f)^{0.7537…}). We show this is false by giving an example of a total boolean function f on n bits whose deterministic query complexity is Ω(n/log(n)) while its zeroerror randomized query complexity is Õ(√n). We further show that the quantum query complexity of the same function is Õ(n^{1/4}), giving the first example of a total function with a superquadratic gap between its quantum and deterministic query complexities. We also construct a total boolean function g on n variables that has zeroerror randomized query complexity Ω(n/log(n)) and boundederror randomized query complexity R(g) = Õ(√n). This is the first superlinear separation between these two complexity measures. The exact quantum query complexity of the same function is Q_{E}(g) = Õ(√n). These functions show that the relations D(f) = O(R_{1}(f)^{2}) and R_{0}(f) = Õ(R(f)^{2}) are optimal, up to polylogarithmic factors. Further variations of these functions give additional separations between other query complexity measures: a cubic separation between Q and R_{0}, a 3/2power separation between Q_{E} and R, and a 4th power separation between approximate degree and boundederror randomized query complexity. All of these examples are variants of a function recently introduced by Goos, Pitassi, and Watson which they used to separate the unambiguous 1certificate complexity from deterministic query complexity and to resolve the famous Clique versus Independent Set problem in communication complexity. 

Lee, Yin Tat 
STOC'16: "Geometric Median ..."
Geometric Median in Nearly Linear Time
Michael B. Cohen, Yin Tat Lee, Gary Miller, Jakub Pachocki, and Aaron Sidford (Massachusetts Institute of Technology, USA; Carnegie Mellon University, USA; Microsoft Research, USA) In this paper we provide faster algorithms for solving the geometric median problem: given n points in ^{d} compute a point that minimizes the sum of Euclidean distances to the points. This is one of the oldest nontrivial problems in computational geometry yet despite a long history of research the previous fastest running times for computing a (1+T)approximate geometric median were O(d· n^{4/3}T^{−8/3}) by Chin et. al, Õ(dexpT^{−4}logT^{−1}) by Badoiu et. al, O(nd+poly(d,T^{−1})) by Feldman and Langberg, and the polynomial running time of O((nd)^{O(1)}log1/T) by Parrilo and Sturmfels and Xue and Ye. In this paper we show how to compute such an approximate geometric median in time O(ndlog^{3}n/T) and O(dT^{−2}). While our O(dT^{−2}) is a fairly straightforward application of stochastic subgradient descent, our O(ndlog^{3}n/T) time algorithm is a novel long step interior point method. We start with a simple O((nd)^{O(1)}log1/T) time interior point method and show how to improve it, ultimately building an algorithm that is quite nonstandard from the perspective of interior point literature. Our result is one of few cases of outperforming standard interior point theory. Furthermore, it is the only case we know of where interior point methods yield a nearly linear time algorithm for a canonical optimization problem that traditionally requires superlinear time. Rasmus Kyng, Yin Tat Lee, Richard Peng, Sushant Sachdeva, and Daniel A. Spielman (Yale University, USA; Massachusetts Institute of Technology, USA; Georgia Tech, USA)
We introduce the sparsified Cholesky and sparsified multigrid algorithms for solving systems of linear equations. These algorithms accelerate Gaussian elimination by sparsifying the nonzero matrix entries created by the elimination process. We use these new algorithms to derive the first nearly linear time algorithms for solving systems of equations in connection Laplaciansa generalization of Laplacian matrices that arise in many problems in image and signal processing. We also prove that every connection Laplacian has a linear sized approximate inverse. This is an LU factorization with a linear number of nonzero entries that is a strong approximation of the original matrix. Using such a factorization one can solve systems of equations in a connection Laplacian in linear time. Such a factorization was unknown even for ordinary graph Laplacians.
@InProceedings{STOC16p842,
author = {Rasmus Kyng and Yin Tat Lee and Richard Peng and Sushant Sachdeva and Daniel A. Spielman},
title = {Sparsified Cholesky and Multigrid Solvers for Connection Laplacians},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {842850},
doi = {10.1145/2897518.2897640},
year = {2016},
}
Publisher's Version
Article Search


Lempiäinen, Tuomo 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Levey, Elaine 
STOC'16: "A (1+epsilon)Approximation ..."
A (1+epsilon)Approximation for Makespan Scheduling with Precedence Constraints using LP Hierarchies
Elaine Levey and Thomas Rothvoss (University of Washington, USA) In a classical problem in scheduling, one has n unit size jobs with a precedence order and the goal is to find a schedule of those jobs on m identical machines as to minimize the makespan. It is one of the remaining four open problems from the book of Garey & Johnson whether or not this problem is NPhard for m=3. We prove that for any fixed ε and m, an LPhierarchy lift of the timeindex LP with a slightly super polylogarithmic number of r = (log(n))^{Θ(loglogn)} rounds provides a (1 + ε)approximation. For example SheraliAdams suffices as hierarchy. This implies an algorithm that yields a (1+ε)approximation in time n^{O(r)}. The previously best approximation algorithms guarantee a 2 − 7/3m+1approximation in polynomial time for m ≥ 4 and 4/3 for m=3. Our algorithm is based on a recursive scheduling approach where in each step we reduce the correlation in form of long chains. Our method adds to the rather short list of examples where hierarchies are actually useful to obtain better approximation algorithms. 

Li, Shi 
STOC'16: "Improved Approximation ..."
Improved Approximation for NodeDisjoint Paths in Planar Graphs
Julia Chuzhoy, David H. K. Kim, and Shi Li (Toyota Technological Institute at Chicago, USA; University of Chicago, USA; State University of New York at Buffalo, USA) We study the classical NodeDisjoint Paths (NDP) problem: given an nvertex graph G and a collection =(s_{1},t_{1}),…,(s_{k},t_{k}) of pairs of vertices of G called demand pairs, find a maximumcardinality set of nodedisjoint paths connecting the demand pairs. NDP is one of the most basic routing problems, that has been studied extensively. Despite this, there are still wide gaps in our understanding of its approximability: the best currently known upper bound of O(√n) on its approximation ratio is achieved via a simple greedy algorithm, while the best current negative result shows that the problem does not have a better than Ω(log^{1/2−δ}n)approximation for any constant δ, under standard complexity assumptions. Even for planar graphs no better approximation algorithms are known, and to the best of our knowledge, the best negative bound is APXhardness. Perhaps the biggest obstacle to obtaining better approximation algorithms for NDP is that most currently known approximation algorithms for this type of problems rely on the standard multicommodity flow relaxation, whose integrality gap is Ω(√n) for NDP, even in planar graphs. In this paper, we break the barrier of O(√n) on the approximability of NDP in planar graphs and obtain an Õ(n^{9/19})approximation. We introduce a new linear programming relaxation of the problem, and a number of new techniques, that we hope will be helpful in designing more powerful algorithms for this and related problems. 

Li, Xin 
STOC'16: "Extractors for Sumset ..."
Extractors for Sumset Sources
Eshan Chattopadhyay and Xin Li (University of Texas at Austin, USA; Johns Hopkins University, USA)
We propose a new model of weak random sources which we call sumset sources. A sumset source X is the sum of C independent sources, with each source on n bits source having minentropy k. We show that extractors for this class of sources can be used to give extractors for most classes of weak sources that have been studied previously, including independent sources, affine sources (which generalizes oblivious bitfixing sources), small space sources, total entropy independent sources, and interleaved sources. This provides a unified approach for randomness extraction.
A known extractor for this class of sources, prior to our work, is the Paley graph function introduced by Chor and Goldreich, which works for the sum of 2 independent sources, where one has minentropy at least 0.51n and the other has logarithmic minentropy. To the best of our knowledge, the only other known construction is from the work of Kamp, Rao, Vadhan and Zuckerman, which can extract from the sum of exponentially many independent sources.
Our main result is an explicit extractor for the sum of C independent sources for some large enough constant C, where each source has polylogarithmic minentropy. We then use this extractor to obtain improved extractors for other well studied classes of sources including smallspace sources, affine sources and interleaved sources.
@InProceedings{STOC16p299,
author = {Eshan Chattopadhyay and Xin Li},
title = {Extractors for Sumset Sources},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {299311},
doi = {10.1145/2897518.2897643},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "Nonmalleable Extractors ..."
Nonmalleable Extractors and Codes, with Their Many Tampered Extensions
Eshan Chattopadhyay, Vipul Goyal, and Xin Li (University of Texas at Austin, USA; Microsoft Research, India; Johns Hopkins University, USA) Randomness extractors and error correcting codes are fundamental objects in computer science. Recently, there have been several natural generalizations of these objects, in the context and study of tamper resilient cryptography. These are seeded nonmalleable extractors, introduced by Dodis and Wichs; seedless nonmalleable extractors, introduced by Cheraghchi and Guruswami; and nonmalleable codes, introduced by Dziembowski, Pietrzak and Wichs. Besides being interesting on their own, they also have important applications in cryptography, e.g, privacy amplification with an active adversary, explicit nonmalleable codes etc, and often have unexpected connections to their nontampered analogues. However, the known constructions are far behind their nontampered counterparts. Indeed, the best known seeded nonmalleable extractor requires minentropy rate at least 0.49; while explicit constructions of nonmalleable twosource extractors were not known even if both sources have full minentropy, and was left as an open problem by Cheraghchi and Guruswami. In this paper we make progress towards solving the above problems and other related generalizations. Our contributions are as follows. (1) We construct an explicit seeded nonmalleable extractor for polylogarithmic minentropy. This dramatically improves all previous results and gives a simpler 2round privacy amplification protocol with optimal entropy loss, matching the best known result. In fact, we construct more general seeded nonmalleable extractors (that can handle multiple adversaries) which were used in the recent construction of explicit twosource extractors for polylogarithmic minentropy. (2) We construct the first explicit nonmalleable twosource extractor for almost full minentropy thus resolving the open question posed by Cheraghchi and Guruswami. (3) We motivate and initiate the study of two natural generalizations of seedless nonmalleable extractors and nonmalleable codes, where the sources or the codeword may be tampered many times. By using the connection found by Cheraghchi and Guruswami and providing efficient sampling algorithms, we obtain the first explicit nonmalleable codes with tampering degree t, with near optimal rate and error. We call these stronger notions onemany and manymanynonmalleable codes. This provides a stronger information theoretic analogue of a primitive known as continuous nonmalleable codes. Our basic technique used in all of our constructions can be seen as inspired, in part, by the techniques previously used to construct cryptographic nonmalleable commitments. 

Li, Yang 
STOC'16: "Tight Bounds for ..."
Tight Bounds for SinglePass Streaming Complexity of the Set Cover Problem
Sepehr Assadi, Sanjeev Khanna, and Yang Li (University of Pennsylvania, USA) We resolve the space complexity of singlepass streaming algorithms for approximating the classic set cover problem. For finding an αapproximate set cover (for α= o(√n)) via a singlepass streaming algorithm, we show that Θ(mn/α) space is both sufficient and necessary (up to an O(logn) factor); here m denotes number of the sets and n denotes size of the universe. This provides a strong negative answer to the open question posed by Indyk (2015) regarding the possibility of having a singlepass algorithm with a small approximation factor that uses sublinear space. We further study the problem of estimating the size of a minimum set cover (as opposed to finding the actual sets), and establish that an additional factor of α saving in the space is achievable in this case and that this is the best possible. In other words, we show that Θ(mn/α^{2}) space is both sufficient and necessary (up to logarithmic factors) for estimating the size of a minimum set cover to within a factor of α. Our algorithm in fact works for the more general problem of estimating the optimal value of a covering integer program. On the other hand, our lower bound holds even for set cover instances where the sets are presented in a random order. 

Li, Yi 
STOC'16: "On Approximating ..."
On Approximating Functions of the Singular Values in a Stream
Yi Li and David P. Woodruff (Facebook, USA; IBM Research, USA) For any real number p > 0, we nearly completely characterize the space complexity of estimating A_{p}^{p} = ∑_{i=1}^{n} σ_{i}^{p} for n × n matrices A in which each row and each column has O(1) nonzero entries and whose entries are presented one at a time in a data stream model. Here the σ_{i} are the singular values of A, and when p ≥ 1, A_{p}^{p} is the pth power of the Schatten pnorm. We show that when p is not an even integer, to obtain a (1+T)approximation to A_{p}^{p} with constant probability, any 1pass algorithm requires n^{1−g(T)} bits of space, where g(T) → 0 as T → 0 and T > 0 is a constant independent of n. However, when p is an even integer, we give an upper bound of n^{1−2/p} (T^{−1}logn) bits of space, which holds even in the turnstile data stream model. The latter is optimal up to (T^{−1} logn) factors. Our results considerably strengthen lower bounds in previous work for arbitrary (not necessarily sparse) matrices A: the previous best lower bound was Ω(logn) for p∈ (0,1), Ω(n^{1/p−1/2}/logn) for p∈ [1,2) and Ω(n^{1−2/p}) for p∈ (2,∞). We note for p ∈ (2, ∞), while our lower bound for even integers is the same, for other p in this range our lower bound is n^{1−g(T)}, which is considerably stronger than the previous n^{1−2/p} for small enough constant T > 0. We obtain similar nearlinear lower bounds for KyFan norms, eigenvalue shrinkers, and Mestimators, many of which could have been solvable in logarithmic space prior to our work. 

Lokshtanov, Daniel 
STOC'16: "Exact Algorithms ..."
Exact Algorithms via Monotone Local Search
Fedor V. Fomin, Serge Gaspers, Daniel Lokshtanov, and Saket Saurabh (University of Bergen, Norway; UNSW, Australia; Data61, Australia; Institute of Mathematical Sciences at Chennai, India) We give a new general approach for designing exact exponentialtime algorithms for subset problems. In a subset problem the input implicitly describes a family of sets over a universe of size n and the task is to determine whether the family contains at least one set. A typical example of a subset problem is Weighted dSAT. Here, the input is a CNFformula with clauses of size at most d, and an integer W. The universe is the set of variables and the variables have integer weights. The family contains all the subsets S of variables such that the total weight of the variables in S does not exceed W, and setting the variables in S to 1 and the remaining variables to 0 satisfies the formula. Our approach is based on “monotone local search”, where the goal is to extend a partial solution to a solution by adding as few elements as possible. More formally, in the extension problem we are also given as input a subset X of the universe and an integer k. The task is to determine whether one can add at most k elements to X to obtain a set in the (implicitly defined) family. Our main result is that a c^{k}n^{O(1)} time algorithm for the extension problem immediately yields a randomized algorithm for finding a solution of any size with running time O((2−1/c)^{n}). In many cases, the extension problem can be reduced to simply finding a solution of size at most k. Furthermore, efficient algorithms for finding small solutions have been extensively studied in the field of parameterized algorithms. Directly applying these algorithms, our theorem yields in one stroke significant improvements over the best known exponentialtime algorithms for several wellstudied problems, including dHitting Set, Feedback Vertex Set, Node Unique Label Cover, and Weighted dSAT. Our results demonstrate an interesting and very concrete connection between parameterized algorithms and exact exponentialtime algorithms. We also show how to derandomize our algorithms at the cost of a subexponential multiplicative factor in the running time. Our derandomization is based on an efficient construction of a new pseudorandom object that might be of independent interest. Finally, we extend our methods to establish new combinatorial upper bounds and develop enumeration algorithms. 

Lovett, Shachar 
STOC'16: "Algebraic Attacks ..."
Algebraic Attacks against Random Local Functions and Their Countermeasures
Benny Applebaum and Shachar Lovett (Tel Aviv University, Israel; University of California at San Diego, USA) Suppose that you have n truly random bits x=(x_{1},…,x_{n}) and you wish to use them to generate m≫ n pseudorandom bits y=(y_{1},…, y_{m}) using a local mapping, i.e., each y_{i} should depend on at most d=O(1) bits of x. In the polynomial regime of m=n^{s}, s>1, the only known solution, originates from (Goldreich, ECCC 2000), is based on Random Local Functions: Compute y_{i} by applying some fixed (public) dary predicate P to a random (public) tuple of distinct inputs (x_{i1},…,x_{id}). Our goal in this paper is to understand, for any value of s, how the pseudorandomness of the resulting sequence depends on the choice of the underlying predicate. We derive the following results: (1) We show that pseudorandomness against F_{2}linear adversaries (i.e., the distribution y has lowbias) is achieved if the predicate is (a) k=Ω(s)resilience, i.e., uncorrelated with any ksubset of its inputs, and (b) has algebraic degree of Ω(s) even after fixing Ω(s) of its inputs. We also show that these requirements are necessary, and so they form a tight characterization (up to constants) of security against linear attacks. Our positive result shows that a dlocal lowbias generator can have output length of n^{Ω(d)}, answering an open question of Mossel, Shpilka and Trevisan (FOCS, 2003). Our negative result shows that a candidate for pseudorandom generator proposed by the first author (computational complexity, 2015) and by O’Donnell and Witmer (CCC 2014) is insecure. We use similar techniques to refute a conjecture of Feldman, Perkins and Vempala (STOC 2015) regarding the hardness of planted constraint satisfaction problems. (2) Motivated by the cryptanalysis literature, we consider security against algebraic attacks. We provide the first theoretical treatment of such attacks by formalizing a general notion of algebraic inversion and distinguishing attacks based on the Polynomial Calculus proof system. We show that algebraic attacks succeed if and only if there exist a degree e=O(s) nonzero polynomial Q whose roots cover the roots of P or cover the roots of P’s complement. As a corollary, we obtain the first example of a predicate P for which the generated sequence y passes all linear tests but fails to pass some polynomialtime computable test, answering an open question posed by the first author (Question 4.9, computational complexity 2015). 

Lucier, Brendan 
STOC'16: "The Price of Anarchy ..."
The Price of Anarchy in Large Games
Michal Feldman, Nicole Immorlica, Brendan Lucier, Tim Roughgarden, and Vasilis Syrgkanis (Tel Aviv University, Israel; Microsoft Research, USA; Stanford University, USA)
We present an analysis framework for bounding the price of anarchy (POA) in games that have many players, as in many of the games most pertinent to computer science applications. We use this framework to demonstrate that, in many of the models in which the POA has been studied, the POA in large games is much smaller than the worstcase bound. Our framework also differentiates between mechanisms with similar worstcase performance, such as simultaneous uniformprice auctions and greedy combinatorial auctions, thereby providing new insights about which mechanisms are likely to perform well in realistic settings.
@InProceedings{STOC16p963,
author = {Michal Feldman and Nicole Immorlica and Brendan Lucier and Tim Roughgarden and Vasilis Syrgkanis},
title = {The Price of Anarchy in Large Games},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {963976},
doi = {10.1145/2897518.2897580},
year = {2016},
}
Publisher's Version
Article Search


Ma, Tengyu 
STOC'16: "Communication Lower ..."
Communication Lower Bounds for Statistical Estimation Problems via a Distributed Data Processing Inequality
Mark Braverman, Ankit Garg, Tengyu Ma, Huy L. Nguyen, and David P. Woodruff (Princeton University, USA; Toyota Technological Institute at Chicago, USA; IBM Research, USA) We study the tradeoff between the statistical error and communication cost of distributed statistical estimation problems in high dimensions. In the distributed sparse Gaussian mean estimation problem, each of the m machines receives n data points from a ddimensional Gaussian distribution with unknown mean θ which is promised to be ksparse. The machines communicate by message passing and aim to estimate the mean θ. We provide a tight (up to logarithmic factors) tradeoff between the estimation error and the number of bits communicated between the machines. This directly leads to a lower bound for the distributed sparse linear regression problem: to achieve the statistical minimax error, the total communication is at least Ω(minn,dm), where n is the number of observations that each machine receives and d is the ambient dimension. These lower results improve upon Shamir (NIPS’14) and SteinhardtDuchi (COLT’15) by allowing multiround iterative communication model. We also give the first optimal simultaneous protocol in the dense case for mean estimation. As our main technique, we prove a distributed data processing inequality, as a generalization of usual data processing inequalities, which might be of independent interest and useful for other problems. 

Mackenzie, Simon 
STOC'16: "A Discrete and Bounded ..."
A Discrete and Bounded EnvyFree Cake Cutting Protocol for Four Agents
Haris Aziz and Simon Mackenzie (Data61, Australia; UNSW, Australia)
We consider the wellstudied cake cutting problem in which the goal is to identify an envyfree allocation based on a minimal number of queries from the agents. The problem has attracted considerable attention within various branches of computer science, mathematics, and economics. Although, the elegant SelfridgeConway envyfree protocol for three agents has been known since 1960, it has been a major open problem to obtain a bounded envyfree protocol for more than three agents. The problem has been termed the central open problem in cake cutting. We solve this problem by proposing a discrete and bounded envyfree protocol for four agents.
@InProceedings{STOC16p454,
author = {Haris Aziz and Simon Mackenzie},
title = {A Discrete and Bounded EnvyFree Cake Cutting Protocol for Four Agents},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {454464},
doi = {10.1145/2897518.2897522},
year = {2016},
}
Publisher's Version
Article Search


Mao, Jieming 
STOC'16: "Parallel Algorithms ..."
Parallel Algorithms for Select and Partition with Noisy Comparisons
Mark Braverman, Jieming Mao, and S. Matthew Weinberg (Princeton University, USA)
We consider the problem of finding the kth highest element in a totally ordered set of n elements (Select), and partitioning a totally ordered set into the top k and bottom n − k elements (Partition) using pairwise comparisons. Motivated by settings like peer grading or crowdsourcing, where multiple rounds of interaction are costly and queried comparisons may be inconsistent with the ground truth, we evaluate algorithms based both on their total runtime and the number of interactive rounds in three comparison models: noiseless (where the comparisons are correct), erasure (where comparisons are erased with probability 1 − γ), and noisy (where comparisons are correct with probability 1/2 + γ/2 and incorrect otherwise). We provide numerous matching upper and lower bounds in all three models. Even our results in the noiseless model, which is quite wellstudied in the TCS literature on parallel algorithms, are novel.
@InProceedings{STOC16p851,
author = {Mark Braverman and Jieming Mao and S. Matthew Weinberg},
title = {Parallel Algorithms for Select and Partition with Noisy Comparisons},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {851862},
doi = {10.1145/2897518.2897642},
year = {2016},
}
Publisher's Version
Article Search


Marx, Dániel 
STOC'16: "A PTAS for Planar ..."
A PTAS for Planar Group Steiner Tree via Spanner Bootstrapping and Prize Collecting
MohammadHossein Bateni, Erik D. Demaine, MohammadTaghi Hajiaghayi, and Dániel Marx (Google, USA; Massachusetts Institute of Technology, USA; University of Maryland at College Park, USA; Hungarian Academy of Sciences, Hungary) We present the first polynomialtime approximation scheme (PTAS), i.e., (1+ε)approximation algorithm for any constant ε> 0, for the planar group Steiner tree problem (in which each group lies on a boundary of a face). This result improves on the best previous approximation factor of O(logn (loglogn)^{O(1)}). We achieve this result via a novel and powerful technique called spanner bootstrapping, which allows one to bootstrap from a superconstant approximation factor (even superpolynomial in the input size) all the way down to a PTAS. This is in contrast with the popular existing approach for planar PTASs of constructing lightweight spanners in one iteration, which notably requires a constantfactor approximate solution to start from. Spanner bootstrapping removes one of the main barriers for designing PTASs for problems which have no known constantfactor approximation (even on planar graphs), and thus can be used to obtain PTASs for several difficulttoapproximate problems. Our second major contribution required for the planar group Steiner tree PTAS is a spanner construction, which reduces the graph to have total weight within a factor of the optimal solution while approximately preserving the optimal solution. This is particularly challenging because group Steiner tree requires deciding which terminal in each group to connect by the tree, making it much harder than recent previous approaches to construct spanners for planar TSP by Klein [SIAM J. Computing 2008], subset TSP by Klein [STOC 2006], Steiner tree by Borradaile, Klein, and Mathieu [ACM Trans. Algorithms 2009], and Steiner forest by Bateni, Hajiaghayi, and Marx [J. ACM 2011] (and its improvement to an efficient PTAS by Eisenstat, Klein, and Mathieu [SODA 2012]. The main conceptual contribution here is realizing that selecting which terminals may be relevant is essentially a complicated prizecollecting process: we have to carefully weigh the cost and benefits of reaching or avoiding certain terminals in the spanner. Via a sequence of involved prizecollecting procedures, we can construct a spanner that reaches a set of terminals that is sufficient for an almostoptimal solution. Our PTAS for planar group Steiner tree implies the first PTAS for geometric Euclidean group Steiner tree with obstacles, as well as a (2+)approximation algorithm for group TSP with obstacles, improving over the best previous constantfactor approximation algorithms. By contrast, we show that planar group Steiner forest, a slight generalization of planar group Steiner tree, is APXhard on planar graphs of treewidth 3, even if the groups are pairwise disjoint and every group is a vertex or an edge. 

Mathieu, Claire 
STOC'16: "Approximating Connectivity ..."
Approximating Connectivity Domination in Weighted BoundedGenus Graphs
Vincent CohenAddad, Éric Colin de Verdière, Philip N. Klein, Claire Mathieu, and David Meierfrankenfeld (ENS, France; CNRS, France; Brown University, USA)
We present a framework for addressing several problems on weighted planar graphs and graphs of bounded genus. With that framework, we derive polynomialtime approximation schemes for the following problems in planar graphs or graphs of bounded genus: edgeweighted tree cover and tour cover; vertexweighted connected dominating set, maxweightleaf spanning tree, and connected vertex cover. In addition, we obtain a polynomialtime approximation scheme for feedback vertex set in planar graphs. These are the first polynomialtime approximation schemes for all those problems in weighted embedded graphs. (For unweighted versions of some of these problems, polynomialtime approximation schemes were previously given using bidimensionality.)
@InProceedings{STOC16p584,
author = {Vincent CohenAddad and Éric Colin de Verdière and Philip N. Klein and Claire Mathieu and David Meierfrankenfeld},
title = {Approximating Connectivity Domination in Weighted BoundedGenus Graphs},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {584597},
doi = {10.1145/2897518.2897635},
year = {2016},
}
Publisher's Version
Article Search


Meierfrankenfeld, David 
STOC'16: "Approximating Connectivity ..."
Approximating Connectivity Domination in Weighted BoundedGenus Graphs
Vincent CohenAddad, Éric Colin de Verdière, Philip N. Klein, Claire Mathieu, and David Meierfrankenfeld (ENS, France; CNRS, France; Brown University, USA)
We present a framework for addressing several problems on weighted planar graphs and graphs of bounded genus. With that framework, we derive polynomialtime approximation schemes for the following problems in planar graphs or graphs of bounded genus: edgeweighted tree cover and tour cover; vertexweighted connected dominating set, maxweightleaf spanning tree, and connected vertex cover. In addition, we obtain a polynomialtime approximation scheme for feedback vertex set in planar graphs. These are the first polynomialtime approximation schemes for all those problems in weighted embedded graphs. (For unweighted versions of some of these problems, polynomialtime approximation schemes were previously given using bidimensionality.)
@InProceedings{STOC16p584,
author = {Vincent CohenAddad and Éric Colin de Verdière and Philip N. Klein and Claire Mathieu and David Meierfrankenfeld},
title = {Approximating Connectivity Domination in Weighted BoundedGenus Graphs},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {584597},
doi = {10.1145/2897518.2897635},
year = {2016},
}
Publisher's Version
Article Search


Meir, Or 
STOC'16: "HighRate LocallyCorrectable ..."
HighRate LocallyCorrectable and LocallyTestable Codes with Subpolynomial Query Complexity
Swastik Kopparty, Or Meir, Noga RonZewi, and Shubhangi Saraf (Rutgers University, USA; Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) In this work, we construct the first locallycorrectable codes (LCCs), and locallytestable codes (LTCs) with constant rate, constant relative distance, and subpolynomial query complexity. Specifically, we show that there exist LCCs and LTCs with block length n, constant rate (which can even be taken arbitrarily close to 1) and constant relative distance, whose query complexity is exp(Õ(√logn)) (for LCCs) and (logn)^{O(loglogn)} (for LTCs). Previously such codes were known to exist only with Ω(n^{β}) query complexity (for constant β>0). In addition to having small query complexity, our codes also achieve better tradeoffs between the rate and the relative distance than were previously known to be achievable by LCCs or LTCs. Specifically, over large (but constant size) alphabet, our codes approach the Singleton bound, that is, they have almost the bestpossible relationship between their rate and distance. This has the surprising consequence that asking for a largealphabet errorcorrecting code to further be an LCC or LTC with subpolynomial query complexity does not require any sacrifice in terms of rate and distance! Over the binary alphabet, our codes meet the Zyablov bound. Such tradeoffs between the rate and the relative distance were previously not known for any o(n) query complexity. Our results on LCCs also immediately give locallydecodable codes (LDCs) with the same parameters. Our codes are based on a technique of Alon, Edmonds and Luby. We observe that this technique can be used as a general distanceamplification method, and show that it interacts well with local correctors and testers. We obtain our main results by applying this method to suitably constructed LCCs and LTCs in the nonstandard regime of subconstant relative distance. 

Miller, Gary 
STOC'16: "Geometric Median ..."
Geometric Median in Nearly Linear Time
Michael B. Cohen, Yin Tat Lee, Gary Miller, Jakub Pachocki, and Aaron Sidford (Massachusetts Institute of Technology, USA; Carnegie Mellon University, USA; Microsoft Research, USA) In this paper we provide faster algorithms for solving the geometric median problem: given n points in ^{d} compute a point that minimizes the sum of Euclidean distances to the points. This is one of the oldest nontrivial problems in computational geometry yet despite a long history of research the previous fastest running times for computing a (1+T)approximate geometric median were O(d· n^{4/3}T^{−8/3}) by Chin et. al, Õ(dexpT^{−4}logT^{−1}) by Badoiu et. al, O(nd+poly(d,T^{−1})) by Feldman and Langberg, and the polynomial running time of O((nd)^{O(1)}log1/T) by Parrilo and Sturmfels and Xue and Ye. In this paper we show how to compute such an approximate geometric median in time O(ndlog^{3}n/T) and O(dT^{−2}). While our O(dT^{−2}) is a fairly straightforward application of stochastic subgradient descent, our O(ndlog^{3}n/T) time algorithm is a novel long step interior point method. We start with a simple O((nd)^{O(1)}log1/T) time interior point method and show how to improve it, ultimately building an algorithm that is quite nonstandard from the perspective of interior point literature. Our result is one of few cases of outperforming standard interior point theory. Furthermore, it is the only case we know of where interior point methods yield a nearly linear time algorithm for a canonical optimization problem that traditionally requires superlinear time. Alina Ene, Gary Miller, Jakub Pachocki, and Aaron Sidford (University of Warwick, UK; Carnegie Mellon University, USA; Microsoft Research, USA) We introduce the notion of balance for directed graphs: a weighted directed graph is αbalanced if for every cut S ⊆ V, the total weight of edges going from S to V∖ S is within factor α of the total weight of edges going from V∖ S to S. Several important families of graphs are nearly balanced, in particular, Eulerian graphs (with α = 1) and residual graphs of (1+T)approximate undirected maximum flows (with α=O(1/T)). We use the notion of balance to give a more finegrained understanding of several wellstudied routing questions that are considerably harder in directed graphs. We first revisit oblivious routings in directed graphs. Our main algorithmic result is an oblivious routing scheme for singlesource instances that achieve an O(α · log^{3} n / loglogn) competitive ratio. In the process, we make several technical contributions which may be of independent interest. In particular, we give an efficient algorithm for computing lowradius decompositions of directed graphs parameterized by balance. We also define and construct lowstretch arborescences, a generalization of lowstretch spanning trees to directed graphs. On the negative side, we present new lower bounds for oblivious routing problems on directed graphs. We show that the competitive ratio of oblivious routing algorithms for directed graphs is Ω(n) in general; this result improves upon the longstanding best known lower bound of Ω(√n) by Hajiaghayi et al. We also show that our restriction to singlesource instances is necessary by showing an Ω(√n) lower bound for multiplesource oblivious routing in Eulerian graphs. We also study the maximum flow problem in balanced directed graphs with arbitrary capacities. We develop an efficient algorithm that finds an (1+T)approximate maximum flows in αbalanced graphs in time O(m α^{2} / T^{2}). We show that, using our approximate maximum flow algorithm, we can efficiently determine whether a given directed graph is αbalanced. Additionally, we give an application to the directed sparsest cut problem. 

Moitra, Ankur 
STOC'16: "How Robust Are Reconstruction ..."
How Robust Are Reconstruction Thresholds for Community Detection?
Ankur Moitra, William Perry, and Alexander S. Wein (Massachusetts Institute of Technology, USA)
The stochastic block model is one of the oldest and most ubiquitous models for studying clustering and community detection. In an exciting sequence of developments, motivated by deep but nonrigorous ideas from statistical physics, Decelle et al. conjectured a sharp threshold for when community detection is possible in the sparse regime. Mossel, Neeman and Sly and Massoulie proved the conjecture and gave matching algorithms and lower bounds.
Here we revisit the stochastic block model from the perspective of semirandom models where we allow an adversary to make `helpful' changes that strengthen ties within each community and break ties between them. We show a surprising result that these `helpful' changes can shift the informationtheoretic threshold, making the community detection problem strictly harder. We complement this by showing that an algorithm based on semidefinite programming (which was known to get close to the threshold) continues to work in the semirandom model (even for partial recovery). This suggests that algorithms based on semidefinite programming are robust in ways that any algorithm meeting the informationtheoretic threshold cannot be.
These results point to an interesting new direction: Can we find robust, semirandom analogues to some of the classical, averagecase thresholds in statistics? We also explore this question in the broadcast tree model, and we show that the viewpoint of semirandom models can help explain why some algorithms are preferred to others in practice, in spite of the gaps in their statistical performance on random models.
@InProceedings{STOC16p828,
author = {Ankur Moitra and William Perry and Alexander S. Wein},
title = {How Robust Are Reconstruction Thresholds for Community Detection?},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {828841},
doi = {10.1145/2897518.2897573},
year = {2016},
}
Publisher's Version
Article Search


Mondelli, Marco 
STOC'16: "ReedMuller Codes ..."
ReedMuller Codes Achieve Capacity on Erasure Channels
Shrinivas Kudekar, Santhosh Kumar, Marco Mondelli, Henry D. Pfister, Eren Şaşoğlu, and Rüdiger Urbanke (Qualcomm Research, USA; Texas A&M University, USA; EPFL, Switzerland; Duke University, USA; Intel, USA)
We introduce a new approach to proving that a sequence of deterministic linear codes achieves capacity on an erasure channel under maximum a posteriori decoding. Rather than relying on the precise structure of the codes, our method exploits code symmetry. In particular, the technique applies to any sequence of linear codes where the block lengths are strictly increasing, the code rates converge, and the permutation group of each code is doubly transitive. In a nutshell, we show that symmetry alone implies nearoptimal performance.
An important consequence of this result is that a sequence of ReedMuller codes with increasing block length and converging rate achieves capacity. This possibility has been suggested previously in the literature, but it has only been proven for cases where the limiting code rate is 0 or 1. Moreover, these results extend naturally to affineinvariant codes and, thus, to all extended primitive narrowsense BCH codes. This is used to resolve, in the affirmative, the existence question for capacityachieving sequences of binary cyclic codes. The primary tools used in the proofs are the sharp threshold property for symmetric monotone boolean functions and the area theorem for extrinsic information transfer (EXIT) functions.
@InProceedings{STOC16p658,
author = {Shrinivas Kudekar and Santhosh Kumar and Marco Mondelli and Henry D. Pfister and Eren Şaşoğlu and Rüdiger Urbanke},
title = {ReedMuller Codes Achieve Capacity on Erasure Channels},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {658669},
doi = {10.1145/2897518.2897584},
year = {2016},
}
Publisher's Version
Article Search


Montanari, Andrea 
STOC'16: "Semidefinite Programs ..."
Semidefinite Programs on Sparse Random Graphs and Their Application to Community Detection
Andrea Montanari and Subhabrata Sen (Stanford University, USA)
Denote by A the adjacency matrix of an ErdosRenyi graph with bounded average degree. We consider the problem of maximizing <AEA,X> over the set of positive semidefinite matrices X with diagonal entries X_ii=1. We prove that for large (bounded) average degree d, the value of this semidefinite program (SDP) is with high probability 2n*sqrt(d) + n, o(sqrt(d))+o(n). For a random regular graph of degree d, we prove that the SDP value is 2n*sqrt(d1)+o(n), matching a spectral upper bound. Informally, ErdosRenyi graphs appear to behave similarly to random regular graphs for semidefinite programming. We next consider the sparse, twogroups, symmetric community detection problem (also known as planted partition). We establish that SDP achieves the informationtheoretically optimal detection threshold for large (bounded) degree. Namely, under this model, the vertex set is partitioned into subsets of size n/2, with edge probability a/n (within group) and b/n (across). We prove that SDP detects the partition with high probability provided (ab)^2/(4d)> 1+o_d(1), with d= (a+b)/2. By comparison, the information theoretic threshold for detecting the hidden partition is (ab)^2/(4d)> 1: SDP is nearly optimal for large bounded average degree. Our proof is based on tools from different research areas: (i) A new `higherrank' Grothendieck inequality for symmetric matrices; (ii) An interpolation method inspired from statistical physics; (iii) An analysis of the eigenvectors of deformed Gaussian random matrices.
@InProceedings{STOC16p814,
author = {Andrea Montanari and Subhabrata Sen},
title = {Semidefinite Programs on Sparse Random Graphs and Their Application to Community Detection},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {814827},
doi = {10.1145/2897518.2897548},
year = {2016},
}
Publisher's Version
Article Search


Morgenstern, Jamie 
STOC'16: "Do Prices Coordinate ..."
Do Prices Coordinate Markets?
Justin Hsu, Jamie Morgenstern, Ryan Rogers, Aaron Roth, and Rakesh Vohra (University of Pennsylvania, USA)
Walrasian equilibrium prices have a remarkable property: they allow each buyer to purchase a bundle of goods that she finds the most desirable, while guaranteeing that the induced allocation over all buyers will globally maximize social welfare. However, this clean story has two caveats. * First, the prices may induce indifferences. In fact, the minimal equilibrium prices necessarily induce indifferences. Accordingly, buyers may need to coordinate with one another to arrive at a socially optimal outcomethe prices alone are not sufficient to coordinate the market. * Second, although natural procedures converge to Walrasian equilibrium prices on a fixed population, in practice buyers typically observe prices without participating in a price computation process. These prices cannot be perfect Walrasian equilibrium prices, but instead somehow reflect distributional information about the market. To better understand the performance of Walrasian prices when facing these two problems, we give two results. First, we propose a mild genericity condition on valuations under which the minimal Walrasian equilibrium prices induce allocations which result in low overdemand, no matter how the buyers break ties. In fact, under genericity the overdemand of any good can be bounded by 1, which is the best possible at the minimal prices. We demonstrate our results for unit demand valuations and give an extension to matroid based valuations (MBV), conjectured to be equivalent to gross substitute valuations (GS). Second, we use techniques from learning theory to argue that the overdemand and welfare induced by a price vector converge to their expectations uniformly over the class of all price vectors, with respective sample complexity linear and quadratic in the number of goods in the market. These results make no assumption on the form of the valuation functions. These two results imply that under a mild genericity condition, the exact Walrasian equilibrium prices computed in a market are guaranteed to induce both low overdemand and high welfare when used in a new market where agents are sampled independently from the same distribution, whenever the number of agents is larger than the number of commodities in the market.
@InProceedings{STOC16p440,
author = {Justin Hsu and Jamie Morgenstern and Ryan Rogers and Aaron Roth and Rakesh Vohra},
title = {Do Prices Coordinate Markets?},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {440453},
doi = {10.1145/2897518.2897559},
year = {2016},
}
Publisher's Version
Article Search
Video


Moshkovitz, Dana 
STOC'16: "Candidate Hard Unique ..."
Candidate Hard Unique Game
Subhash Khot and Dana Moshkovitz (New York University, USA; Massachusetts Institute of Technology, USA)
We propose a candidate reduction for ruling out polynomialtime algorithms for unique games, either under plausible complexity assumptions, or unconditionally for Lasserre semidefinite programs with a constant number of rounds. We analyze the completeness and Lasserre solution of our construction, and provide a soundness analysis in a certain setting of interest. Addressing general settings is tightly connected to a question on Gaussian isoperimetry. Our construction is based on our previous work on the complexity of approximately solving a system of linear equations over reals, which we suggested as an avenue towards a (positive) resolution of the Unique Games Conjecture. The construction employs a new encoding scheme that we call the real code. The real code has two useful properties: like the long code, it has a unique local test, and like the Hadamard code, it has the socalled subcode covering property.
@InProceedings{STOC16p63,
author = {Subhash Khot and Dana Moshkovitz},
title = {Candidate Hard Unique Game},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {6376},
doi = {10.1145/2897518.2897531},
year = {2016},
}
Publisher's Version
Article Search


Nanongkai, Danupon 
STOC'16: "A Deterministic ..."
A Deterministic AlmostTight Distributed Algorithm for Approximating SingleSource Shortest Paths
Monika Henzinger, Sebastian Krinninger, and Danupon Nanongkai (University of Vienna, Austria; Max Planck Institute for Informatics, Germany; KTH, Sweden) We present a deterministic (1+o(1))approximation O(n^{1/2+o(1)}+D^{1+o(1)})time algorithm for solving the singlesource shortest paths problem on distributed weighted networks (the CONGEST model); here n is the number of nodes in the network and D is its (hop) diameter. This is the first nontrivial deterministic algorithm for this problem. It also improves (i) the running time of the randomized (1+o(1))approximation Õ(n^{1/2}D^{1/4}+D)time algorithm of Nanongkai [STOC 2014] by a factor of as large as n^{1/8}, and (ii) the O(T^{−1}logT^{−1})approximation factor of Lenzen and PattShamir’s Õ(n^{1/2+T}+D)time algorithm [STOC 2013] within the same running time. Our running time matches the known time lower bound of Ω(n^{1/2}/logn + D) [Das Sarma et al. STOC 2011] modulo some lowerorder terms, thus essentially settling the status of this problem which was raised at least a decade ago [Elkin SIGACT News 2004]. It also implies a (2+o(1))approximation O(n^{1/2+o(1)}+D^{1+o(1)})time algorithm for approximating a network’s weighted diameter which almost matches the lower bound by Holzer et al. [PODC 2012]. In achieving this result, we develop two techniques which might be of independent interest and useful in other settings: (i) a deterministic process that replaces the “hitting set argument” commonly used for shortest paths computation in various settings, and (ii) a simple, deterministic, construction of an (n^{o(1)}, o(1))hop set of size O(n^{1+o(1)}). We combine these techniques with many distributed algorithmic techniques, some of which from problems that are not directly related to shortest paths, e.g. ruling sets [Goldberg et al. STOC 1987], source detection [Lenzen, Peleg PODC 2013], and partial distance estimation [Lenzen, PattShamir PODC 2015]. Our hop set construction also leads to singlesource shortest paths algorithms in two other settings: (i) a (1+o(1))approximation O(n^{o(1)})time algorithm on congested cliques, and (ii) a (1+o(1))approximation O(n^{o(1)}logW)pass O(n^{1+o(1)}logW)space streaming algorithm, when edge weights are in {1, 2, …, W}. The first result answers an open problem in [Nanongkai, STOC 2014]. The second result partially answers an open problem raised by McGregor in 2006 [sublinear.info, Problem 14]. Sayan Bhattacharya, Monika Henzinger, and Danupon Nanongkai (Institute of Mathematical Sciences at Chennai, India; University of Vienna, Austria; KTH, Sweden) We present two deterministic dynamic algorithms for the maximum matching problem. (1) An algorithm that maintains a (2+T)approximate maximum matching in general graphs with O(poly(logn, 1/T)) update time. (2) An algorithm that maintains an α_{K} approximation of the value of the maximum matching with O(n^{2/K}) update time in bipartite graphs, for every sufficiently large constant positive integer K. Here, 1≤ α_{K} < 2 is a constant determined by the value of K. Result (1) is the first deterministic algorithm that can maintain an o(logn)approximate maximum matching with polylogarithmic update time, improving the seminal result of Onak et al. [STOC 2010]. Its approximation guarantee almost matches the guarantee of the best randomized polylogarithmic update time algorithm [Baswana et al. FOCS 2011]. Result (2) achieves a betterthantwo approximation with arbitrarily small polynomial update time on bipartite graphs. Previously the best update time for this problem was O(m^{1/4}) [Bernstein et al. ICALP 2015], where m is the current number of edges in the graph. 

Naor, Moni 
STOC'16: "Searchable Symmetric ..."
Searchable Symmetric Encryption: Optimal Locality in Linear Space via TwoDimensional Balanced Allocations
Gilad Asharov, Moni Naor, Gil Segev, and Ido Shahaf (IBM Research, USA; Weizmann Institute of Science, Israel; Hebrew University of Jerusalem, Israel) Searchable symmetric encryption (SSE) enables a client to store a database on an untrusted server while supporting keyword search in a secure manner. Despite the rapidly increasing interest in SSE technology, experiments indicate that the performance of the known schemes scales badly to large databases. Somewhat surprisingly, this is not due to their usage of cryptographic tools, but rather due to their poor locality (where locality is defined as the number of noncontiguous memory locations the server accesses with each query). The only known schemes that do not suffer from poor locality suffer either from an impractical space overhead or from an impractical read efficiency (where read efficiency is defined as the ratio between the number of bits the server reads with each query and the actual size of the answer). We construct the first SSE schemes that simultaneously enjoy optimal locality, optimal space overhead, and nearlyoptimal read efficiency. Specifically, for a database of size N, under the modest assumption that no keyword appears in more than N^{1 − 1/loglogN} documents, we construct a scheme with read efficiency Õ(loglogN). This essentially matches the lower bound of Cash and Tessaro (EUROCRYPT ’14) showing that any SSE scheme must be suboptimal in either its locality, its space overhead, or its read efficiency. In addition, even without making any assumptions on the structure of the database, we construct a scheme with read efficiency Õ(logN). Our schemes are obtained via a twodimensional generalization of the classic balanced allocations (“balls and bins”) problem that we put forward. We construct nearlyoptimal twodimensional balanced allocation schemes, and then combine their algorithmic structure with subtle cryptographic techniques. 

Nguyen, Huy L. 
STOC'16: "Communication Lower ..."
Communication Lower Bounds for Statistical Estimation Problems via a Distributed Data Processing Inequality
Mark Braverman, Ankit Garg, Tengyu Ma, Huy L. Nguyen, and David P. Woodruff (Princeton University, USA; Toyota Technological Institute at Chicago, USA; IBM Research, USA) We study the tradeoff between the statistical error and communication cost of distributed statistical estimation problems in high dimensions. In the distributed sparse Gaussian mean estimation problem, each of the m machines receives n data points from a ddimensional Gaussian distribution with unknown mean θ which is promised to be ksparse. The machines communicate by message passing and aim to estimate the mean θ. We provide a tight (up to logarithmic factors) tradeoff between the estimation error and the number of bits communicated between the machines. This directly leads to a lower bound for the distributed sparse linear regression problem: to achieve the statistical minimax error, the total communication is at least Ω(minn,dm), where n is the number of observations that each machine receives and d is the ambient dimension. These lower results improve upon Shamir (NIPS’14) and SteinhardtDuchi (COLT’15) by allowing multiround iterative communication model. We also give the first optimal simultaneous protocol in the dense case for mean estimation. As our main technique, we prove a distributed data processing inequality, as a generalization of usual data processing inequalities, which might be of independent interest and useful for other problems. 

Nikolov, Aleksandar 
STOC'16: "Maximizing Determinants ..."
Maximizing Determinants under Partition Constraints
Aleksandar Nikolov and Mohit Singh (University of Toronto, Canada; Microsoft Research, USA) Given a positive semidefinte matrix L whose columns and rows are indexed by a set U, and a partition matroid M=(U, I), we study the problem of selecting a basis B of M such that the determinant of the submatrix of L induced by the rows and columns in B is maximized. This problem appears in many areas including determinantal point processes in machine learning, experimental design, geographical placement problems, discrepancy theory and computational geometry to model subset selection problems that incorporate diversity. Our main result is to give a geometric concave program for the problem which approximates the optimum value within a factor of e^{r}+o(r), where r denotes the rank of the partition matroid M. We bound the integrality gap of the geometric concave program by giving a polynomial time randomized rounding algorithm. To analyze the rounding algorithm, we relate the solution of our algorithm as well the objective value of the relaxation to a certain stable polynomial. To prove the approximation guarantee, we utilize a general inequality about stable polynomials proved by Gurvits in the context of estimating the permanent of a doubly stochastic matrix. 

Nishimaki, Ryo 
STOC'16: "Watermarking Cryptographic ..."
Watermarking Cryptographic Capabilities
Aloni Cohen, Justin Holmgren, Ryo Nishimaki, Vinod Vaikuntanathan, and Daniel Wichs (Massachusetts Institute of Technology, USA; NTT, Japan; Northeastern University, USA)
A watermarking scheme for programs embeds some information called a mark into a program while preserving its functionality. No adversary can remove the mark without damaging the functionality of the program. In this work, we study the problem of watermarking various cryptographic programs such as pseudorandom function (PRF) evaluation, decryption, and signing. For example, given a PRF key K, we create a marked program C that evaluates the PRF F(K,). An adversary that gets C cannot come up with any program C* in which the mark is removed but which still evaluates the PRF correctly on even a small fraction of the inputs. The work of Barak, Goldreich, Impagliazzo, Rudich, Sahai, Vadhan, and Yang (CRYPTO'01 and Journal of ACM 59(2)) shows that, assuming indistinguishability obfuscation (iO), such watermarking is impossible if the marked program C evaluates the original program with perfect correctness. In this work we show that, assuming iO, such watermarking is possible if the marked program C is allowed to err with even a negligible probability, which would be undetectable to the user. Our watermarking schemes are public key, namely we use a secret marking key to embed marks in programs, and a public detection key that allows anyone to detect marks in programs. Our schemes are secure against chosen program attacks, that is even if the adversary is given oracle access to the marking functionality. We emphasize that our security notion of watermark nonremovability considers arbitrary adversarial strategies to modify the marked program, in contrast to the prior works (Nishimaki, EUROCRYPT '13).
@InProceedings{STOC16p1115,
author = {Aloni Cohen and Justin Holmgren and Ryo Nishimaki and Vinod Vaikuntanathan and Daniel Wichs},
title = {Watermarking Cryptographic Capabilities},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11151127},
doi = {10.1145/2897518.2897651},
year = {2016},
}
Publisher's Version
Article Search


Nissim, Kobbi 
STOC'16: "Algorithmic Stability ..."
Algorithmic Stability for Adaptive Data Analysis
Raef Bassily, Kobbi Nissim, Adam Smith, Thomas Steinke, Uri Stemmer, and Jonathan Ullman (University of California at San Diego, USA; BenGurion University of the Negev, Israel; Harvard University, USA; Pennsylvania State University, USA; Northeastern University, USA)
Adaptivity is an important feature of data analysis  the choice of questions to ask about a dataset often depends on previous interactions with the same dataset. However, statistical validity is typically studied in a nonadaptive model, where all questions are specified before the dataset is drawn. Recent work by Dwork et al. (STOC, 2015) and Hardt and Ullman (FOCS, 2014) initiated a general formal study of this problem, and gave the first upper and lower bounds on the achievable generalization error for adaptive data analysis.
Specifically, suppose there is an unknown distribution P and a set of n independent samples x is drawn from P. We seek an algorithm that, given x as input, accurately answers a sequence of adaptively chosen ``queries'' about the unknown distribution P. How many samples n must we draw from the distribution, as a function of the type of queries, the number of queries, and the desired level of accuracy?
In this work we make two new contributions towards resolving this question:
@InProceedings{STOC16p1046,
author = {Raef Bassily and Kobbi Nissim and Adam Smith and Thomas Steinke and Uri Stemmer and Jonathan Ullman},
title = {Algorithmic Stability for Adaptive Data Analysis},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {10461059},
doi = {10.1145/2897518.2897566},
year = {2016},
}
Publisher's Version
Article Search


O'Donnell, Ryan 
STOC'16: "Efficient Quantum ..."
Efficient Quantum Tomography
Ryan O'Donnell and John Wright (Carnegie Mellon University, USA) In the quantum state tomography problem, one wishes to estimate an unknown ddimensional mixed quantum state ρ, given few copies. We show that O(d/ε) copies suffice to obtain an estimate ρ that satisfies ρ − ρ_{F}^{2} ≤ ε (with high probability). An immediate consequence is that O((ρ) · d/ε^{2}) ≤ O(d^{2}/ε^{2}) copies suffice to obtain an εaccurate estimate in the standard trace distance. This improves on the best known prior result of O(d^{3}/ε^{2}) copies for full tomography, and even on the best known prior result of O(d^{2}log(d/ε)/ε^{2}) copies for spectrum estimation. Our result is the first to show that nontrivial tomography can be obtained using a number of copies that is just linear in the dimension. Next, we generalize these results to show that one can perform efficient principal component analysis on ρ. Our main result is that O(k d/ε^{2}) copies suffice to output a rankk approximation ρ whose tracedistance error is at most ε more than that of the best rankk approximator to ρ. This subsumes our above trace distance tomography result and generalizes it to the case when ρ is not guaranteed to be of low rank. A key part of the proof is the analogous generalization of our spectrumlearning results: we show that the largest k eigenvalues of ρ can be estimated to tracedistance error ε using O(k^{2}/ε^{2}) copies. In turn, this result relies on a new coupling theorem concerning the Robinson–Schensted–Knuth algorithm that should be of independent combinatorial interest. 

Oliveira, Igor C. 
STOC'16: "NearOptimal SmallDepth ..."
NearOptimal SmallDepth Lower Bounds for Small Distance Connectivity
Xi Chen, Igor C. Oliveira, Rocco A. Servedio, and LiYang Tan (Columbia University, USA; Charles University in Prague, Czechia; Toyota Technological Institute at Chicago, USA) We show that any depthd circuit for determining whether an nnode graph has an stot path of length at most k must have size n^{Ω(k1/d/d)} when k(n) ≤ n^{1/5}, and n^{Ω(k1/5d/d)} when k(n)≤ n. The previous best circuit size lower bounds were n^{kexp(−O(d))} (by Beame, Impagliazzo, and Pitassi (Computational Complexity 1998)) and n^{Ω((logk)/d)} (following from a recent formula size lower bound of Rossman (STOC 2014)). Our lower bound is quite close to optimal, as a simple construction gives depthd circuits of size n^{O(k2/d)} for this problem (and strengthening our bound even to n^{kΩ(1/d)} would require proving that undirected connectivity is not in NC^{1}). Our proof is by reduction to a new lower bound on the size of smalldepth circuits computing a skewed variant of the “Sipser functions” that have played an important role in classical circuit lower bounds. A key ingredient in our proof of the required lower bound for these Sipserlike functions is the use of random projections, an extension of random restrictions which were recently employed by Rossman, Servedio, and Tan (FOCS 2015). Random projections allow us to obtain sharper quantitative bounds while employing simpler arguments, both conceptually and technically, than in the previous works. 

Pachocki, Jakub 
STOC'16: "Geometric Median ..."
Geometric Median in Nearly Linear Time
Michael B. Cohen, Yin Tat Lee, Gary Miller, Jakub Pachocki, and Aaron Sidford (Massachusetts Institute of Technology, USA; Carnegie Mellon University, USA; Microsoft Research, USA) In this paper we provide faster algorithms for solving the geometric median problem: given n points in ^{d} compute a point that minimizes the sum of Euclidean distances to the points. This is one of the oldest nontrivial problems in computational geometry yet despite a long history of research the previous fastest running times for computing a (1+T)approximate geometric median were O(d· n^{4/3}T^{−8/3}) by Chin et. al, Õ(dexpT^{−4}logT^{−1}) by Badoiu et. al, O(nd+poly(d,T^{−1})) by Feldman and Langberg, and the polynomial running time of O((nd)^{O(1)}log1/T) by Parrilo and Sturmfels and Xue and Ye. In this paper we show how to compute such an approximate geometric median in time O(ndlog^{3}n/T) and O(dT^{−2}). While our O(dT^{−2}) is a fairly straightforward application of stochastic subgradient descent, our O(ndlog^{3}n/T) time algorithm is a novel long step interior point method. We start with a simple O((nd)^{O(1)}log1/T) time interior point method and show how to improve it, ultimately building an algorithm that is quite nonstandard from the perspective of interior point literature. Our result is one of few cases of outperforming standard interior point theory. Furthermore, it is the only case we know of where interior point methods yield a nearly linear time algorithm for a canonical optimization problem that traditionally requires superlinear time. Alina Ene, Gary Miller, Jakub Pachocki, and Aaron Sidford (University of Warwick, UK; Carnegie Mellon University, USA; Microsoft Research, USA) We introduce the notion of balance for directed graphs: a weighted directed graph is αbalanced if for every cut S ⊆ V, the total weight of edges going from S to V∖ S is within factor α of the total weight of edges going from V∖ S to S. Several important families of graphs are nearly balanced, in particular, Eulerian graphs (with α = 1) and residual graphs of (1+T)approximate undirected maximum flows (with α=O(1/T)). We use the notion of balance to give a more finegrained understanding of several wellstudied routing questions that are considerably harder in directed graphs. We first revisit oblivious routings in directed graphs. Our main algorithmic result is an oblivious routing scheme for singlesource instances that achieve an O(α · log^{3} n / loglogn) competitive ratio. In the process, we make several technical contributions which may be of independent interest. In particular, we give an efficient algorithm for computing lowradius decompositions of directed graphs parameterized by balance. We also define and construct lowstretch arborescences, a generalization of lowstretch spanning trees to directed graphs. On the negative side, we present new lower bounds for oblivious routing problems on directed graphs. We show that the competitive ratio of oblivious routing algorithms for directed graphs is Ω(n) in general; this result improves upon the longstanding best known lower bound of Ω(√n) by Hajiaghayi et al. We also show that our restriction to singlesource instances is necessary by showing an Ω(√n) lower bound for multiplesource oblivious routing in Eulerian graphs. We also study the maximum flow problem in balanced directed graphs with arbitrary capacities. We develop an efficient algorithm that finds an (1+T)approximate maximum flows in αbalanced graphs in time O(m α^{2} / T^{2}). We show that, using our approximate maximum flow algorithm, we can efficiently determine whether a given directed graph is αbalanced. Additionally, we give an application to the directed sparsest cut problem. 

Pandey, Omkant 
STOC'16: "Textbook Nonmalleable ..."
Textbook Nonmalleable Commitments
Vipul Goyal, Omkant Pandey, and Silas Richelson (Microsoft Research, India; Drexel University, USA; Massachusetts Institute of Technology, USA)
We present a new nonmalleable commitment protocol. Our protocol has the following features: itemize The protocol has only three rounds of interaction. Pass (TCC 2013) showed an impossibility result for a tworound nonmalleable commitment scheme w.r.t. a blackbox reduction to any ``standard" intractability reduction. Thus, this resolves the round complexity of nonmalleable commitment at least w.r.t. blackbox security reductions. Our construction is secure as per the standard notion of nonmalleability w.r.t. commitment. Our protocol is truly efficient. In our basic protocol, the entire computation of the committer is dominated by just three invocations of a noninteractive statically binding commitment scheme, while, the receiver computation (in the commitment stage) is limited to just sampling a random string. Unlike many previous works, we directly construct a protocol for large tags and hence avoid any nonmalleability amplification steps. Our protocol is based on a blackbox use of any noninteractive statistically binding commitment scheme. Such schemes, in turn, can be based on any onetoone oneway function (or any oneway function at the cost of an extra initialization round). Previously, the best known blackbox construction of nonmalleable commitments required a larger (constant) number of rounds. Our construction is publiccoin and makes use of only blackbox simulation. Prior to our work, no publiccoin constant round nonmalleable commitment schemes were known based on blackbox simulation. itemize Our techniques depart significantly from the techniques used previously to construct nonmalleable commitment schemes. As a main technical tool, we rely on nonmalleable codes in the split state model. Our proofs of security are purely combinatorial in nature. In addition, we also present a simple construction of constant round nonmalleable commitments from any oneway function. While this result is not new, the main feature is its simplicity compared to any previous construction of nonmalleable commitments (in any number of rounds). We believe the construction is simple enough to be covered in a graduate level course on cryptography. The construction uses nonmalleable codes in the split state model in a blackbox way.
@InProceedings{STOC16p1128,
author = {Vipul Goyal and Omkant Pandey and Silas Richelson},
title = {Textbook Nonmalleable Commitments},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11281141},
doi = {10.1145/2897518.2897657},
year = {2016},
}
Publisher's Version
Article Search


Pegden, Wesley 
STOC'16: "Separating Subadditive ..."
Separating Subadditive Euclidean Functionals
Alan Frieze and Wesley Pegden (Carnegie Mellon University, USA)
The classical BeardwoodHaltonHammersly theorem (1959) asserts the existence of an asymptotic formula of the form constant times square root n for the minimum length of a Traveling Salesperson Tour through n random points in the unit square, and in the decades since it was proved, the existence of such formulas has been shown for other such Euclidean functionals on random points in the unit square as well. Despite more than 50 years of attention, however, it remained unknown whether the minimum length TSP through n random points in the unit square was asymptotically distinct from its natural lower bounds, such as the minimum length spanning tree, the minimum length 2factor, or, as raised by Goemans and Bertsimas, from its linear programming relaxation. We prove that the TSP on random points in Euclidean space is indeed asymptotically distinct from these and other natural lower bounds, and show that this separation implies that branchandbound algorithms based on these natural lower bounds must take nearly exponential time to solve the TSP to optimality, even in average case. This is the first averagecase superpolynomial lower bound for these branchandbound algorithms.
@InProceedings{STOC16p22,
author = {Alan Frieze and Wesley Pegden},
title = {Separating Subadditive Euclidean Functionals},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {2235},
doi = {10.1145/2897518.2897571},
year = {2016},
}
Publisher's Version
Article Search


Peng, Pan 
STOC'16: "Relating Two Property ..."
Relating Two Property Testing Models for Bounded Degree Directed Graphs
Artur Czumaj, Pan Peng, and Christian Sohler (University of Warwick, UK; TU Dortmund, Germany; Institute of Software at Chinese Academy of Sciences, China) We study property testing algorithms in directed graphs (digraphs) with maximum indegree and maximum outdegree upper bounded by d. For directed graphs with bounded degree, there are two different models in property testing introduced by Bender and Ron (2002). In the bidirectional model, one can access both incoming and outgoing edges while in the unidirectional model one can only access outgoing edges. In our paper we provide a new relation between the two models: we prove that if a property can be tested with constant query complexity in the bidirectional model, then it can be tested with sublinear query complexity in the unidirectional model. A corollary of this result is that in the unidirectional model (the model allowing only queries to the outgoing neighbors), every property in hyperfinite digraphs is testable with sublinear query complexity. 

Peng, Richard 
STOC'16: "Sparsified Cholesky ..."
Sparsified Cholesky and Multigrid Solvers for Connection Laplacians
Rasmus Kyng, Yin Tat Lee, Richard Peng, Sushant Sachdeva, and Daniel A. Spielman (Yale University, USA; Massachusetts Institute of Technology, USA; Georgia Tech, USA)
We introduce the sparsified Cholesky and sparsified multigrid algorithms for solving systems of linear equations. These algorithms accelerate Gaussian elimination by sparsifying the nonzero matrix entries created by the elimination process. We use these new algorithms to derive the first nearly linear time algorithms for solving systems of equations in connection Laplaciansa generalization of Laplacian matrices that arise in many problems in image and signal processing. We also prove that every connection Laplacian has a linear sized approximate inverse. This is an LU factorization with a linear number of nonzero entries that is a strong approximation of the original matrix. Using such a factorization one can solve systems of equations in a connection Laplacian in linear time. Such a factorization was unknown even for ordinary graph Laplacians.
@InProceedings{STOC16p842,
author = {Rasmus Kyng and Yin Tat Lee and Richard Peng and Sushant Sachdeva and Daniel A. Spielman},
title = {Sparsified Cholesky and Multigrid Solvers for Connection Laplacians},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {842850},
doi = {10.1145/2897518.2897640},
year = {2016},
}
Publisher's Version
Article Search


Perry, William 
STOC'16: "How Robust Are Reconstruction ..."
How Robust Are Reconstruction Thresholds for Community Detection?
Ankur Moitra, William Perry, and Alexander S. Wein (Massachusetts Institute of Technology, USA)
The stochastic block model is one of the oldest and most ubiquitous models for studying clustering and community detection. In an exciting sequence of developments, motivated by deep but nonrigorous ideas from statistical physics, Decelle et al. conjectured a sharp threshold for when community detection is possible in the sparse regime. Mossel, Neeman and Sly and Massoulie proved the conjecture and gave matching algorithms and lower bounds.
Here we revisit the stochastic block model from the perspective of semirandom models where we allow an adversary to make `helpful' changes that strengthen ties within each community and break ties between them. We show a surprising result that these `helpful' changes can shift the informationtheoretic threshold, making the community detection problem strictly harder. We complement this by showing that an algorithm based on semidefinite programming (which was known to get close to the threshold) continues to work in the semirandom model (even for partial recovery). This suggests that algorithms based on semidefinite programming are robust in ways that any algorithm meeting the informationtheoretic threshold cannot be.
These results point to an interesting new direction: Can we find robust, semirandom analogues to some of the classical, averagecase thresholds in statistics? We also explore this question in the broadcast tree model, and we show that the viewpoint of semirandom models can help explain why some algorithms are preferred to others in practice, in spite of the gaps in their statistical performance on random models.
@InProceedings{STOC16p828,
author = {Ankur Moitra and William Perry and Alexander S. Wein},
title = {How Robust Are Reconstruction Thresholds for Community Detection?},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {828841},
doi = {10.1145/2897518.2897573},
year = {2016},
}
Publisher's Version
Article Search


Pettie, Seth 
STOC'16: "Contention Resolution ..."
Contention Resolution with LogLogstar Channel Accesses
Michael A. Bender, Tsvi Kopelowitz, Seth Pettie, and Maxwell Young (Stony Brook University, USA; University of Michigan, USA; Mississippi State University, USA)
For decades, randomized exponential backoff has provided a critical
algorithmic building block in situations where multiple devices seek
access to a shared resource. Surprisingly, despite this history,
the performance of standard backoff is poor under worstcase
scheduling of demands on the resource: (i) subconstant throughput
can occur under plausible scenarios, and (ii) each of N devices
requires Omega(log N) access attempts before obtaining the
resource.
In this paper, we address these shortcomings by offering a new
backoff protocol for a shared communications channel that guarantees
expected constant throughput with only O(log(log* N)) access
attempts in expectation. Central to this result are new algorithms
for approximate counting and leader election with the
same performance guarantees.
@InProceedings{STOC16p499,
author = {Michael A. Bender and Tsvi Kopelowitz and Seth Pettie and Maxwell Young},
title = {Contention Resolution with LogLogstar Channel Accesses},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {499508},
doi = {10.1145/2897518.2897655},
year = {2016},
}
Publisher's Version
Article Search


Pfister, Henry D. 
STOC'16: "ReedMuller Codes ..."
ReedMuller Codes Achieve Capacity on Erasure Channels
Shrinivas Kudekar, Santhosh Kumar, Marco Mondelli, Henry D. Pfister, Eren Şaşoğlu, and Rüdiger Urbanke (Qualcomm Research, USA; Texas A&M University, USA; EPFL, Switzerland; Duke University, USA; Intel, USA)
We introduce a new approach to proving that a sequence of deterministic linear codes achieves capacity on an erasure channel under maximum a posteriori decoding. Rather than relying on the precise structure of the codes, our method exploits code symmetry. In particular, the technique applies to any sequence of linear codes where the block lengths are strictly increasing, the code rates converge, and the permutation group of each code is doubly transitive. In a nutshell, we show that symmetry alone implies nearoptimal performance.
An important consequence of this result is that a sequence of ReedMuller codes with increasing block length and converging rate achieves capacity. This possibility has been suggested previously in the literature, but it has only been proven for cases where the limiting code rate is 0 or 1. Moreover, these results extend naturally to affineinvariant codes and, thus, to all extended primitive narrowsense BCH codes. This is used to resolve, in the affirmative, the existence question for capacityachieving sequences of binary cyclic codes. The primary tools used in the proofs are the sharp threshold property for symmetric monotone boolean functions and the area theorem for extrinsic information transfer (EXIT) functions.
@InProceedings{STOC16p658,
author = {Shrinivas Kudekar and Santhosh Kumar and Marco Mondelli and Henry D. Pfister and Eren Şaşoğlu and Rüdiger Urbanke},
title = {ReedMuller Codes Achieve Capacity on Erasure Channels},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {658669},
doi = {10.1145/2897518.2897584},
year = {2016},
}
Publisher's Version
Article Search


Pitassi, Toniann 
STOC'16: "Polylogarithmic ..."
Polylogarithmic Frege Depth Lower Bounds via an Expander Switching Lemma
Toniann Pitassi, Benjamin Rossman, Rocco A. Servedio, and LiYang Tan (University of Toronto, Canada; National Institute of Informatics, Japan; Columbia University, USA; Toyota Technological Institute at Chicago, USA) We show that any polynomialsize Frege refutation of a certain linearsize unsatisfiable 3CNF formula over n variables must have depth Ω(√logn). This is an exponential improvement over the previous best results (Pitassi et al. 1993, Krajíček et al. 1995, BenSasson 2002) which give Ω(loglogn) lower bounds. The 3CNF formulas which we use to establish this result are Tseitin contradictions on 3regular expander graphs. In more detail, our main result is a proof that for every d, any depthd Frege refutation of the Tseitin contradiction over these nnode graphs must have size n^{Ω((logn)/d2)}. A key ingredient of our approach is a new switching lemma for a carefully designed random restriction process over these expanders. These random restrictions reduce a Tseitin instance on a 3regular nnode expander to a Tseitin instance on a random subgraph which is a topological embedding of a 3regular n′node expander, for some n′ which is not too much less than n. Our result involves Ω(√logn) iterative applications of this type of random restriction. 

Psomas, ChristosAlexandros 
STOC'16: "The Sample Complexity ..."
The Sample Complexity of Auctions with Side Information
Nikhil R. Devanur, Zhiyi Huang, and ChristosAlexandros Psomas (Microsoft Research, USA; University of Hong Kong, China; University of California at Berkeley, USA)
Traditionally, the Bayesian optimal auction design problem has been considered either when the bidder values are i.i.d, or when each bidder is individually identifiable via her value distribution. The latter is a reasonable approach when the bidders can be classified into a few categories, but there are many instances where the classification of bidders is a continuum. For example, the classification of the bidders may be based on their annual income, their propensity to buy an item based on past behavior, or in the case of ad auctions, the click through rate of their ads. We introduce an alternate model that captures this aspect, where bidders are a priori identical, but can be distinguished based (only) on some side information the auctioneer obtains at the time of the auction. We extend the sample complexity approach of Dhangwatnotai et al. and Cole and Roughgarden to this model and obtain almost matching upper and lower bounds. As an aside, we obtain a revenue monotonicity lemma which may be of independent interest. We also show how to use Empirical Risk Minimization techniques to improve the sample complexity bound of Cole and Roughgarden for the nonidentical but independent value distribution case.
@InProceedings{STOC16p426,
author = {Nikhil R. Devanur and Zhiyi Huang and ChristosAlexandros Psomas},
title = {The Sample Complexity of Auctions with Side Information},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {426439},
doi = {10.1145/2897518.2897553},
year = {2016},
}
Publisher's Version
Article Search


Puder, Doron 
STOC'16: "Ramanujan Coverings ..."
Ramanujan Coverings of Graphs
Chris Hall, Doron Puder, and William F. Sawin (University of Wyoming, USA; Institute for Advanced Study at Princeton, USA; Princeton University, USA) Let G be a finite connected graph, and let ρ be the spectral radius of its universal cover. For example, if G is kregular then ρ=2√k−1. We show that for every r, there is an rcovering (a.k.a. an rlift) of G where all the new eigenvalues are bounded from above by ρ. It follows that a bipartite Ramanujan graph has a Ramanujan rcovering for every r. This generalizes the r=2 case due to Marcus, Spielman and Srivastava (2013). Every rcovering of G corresponds to a labeling of the edges of G by elements of the symmetric group S_{r}. We generalize this notion to labeling the edges by elements of various groups and present a broader scenario where Ramanujan coverings are guaranteed to exist. In particular, this shows the existence of richer families of bipartite Ramanujan graphs than was known before. Inspired by MarcusSpielmanSrivastava, a crucial component of our proof is the existence of interlacing families of polynomials for complex reflection groups. The core argument of this component is taken from MarcusSpielmanSrivastava (2015). Another important ingredient of our proof is a new generalization of the matching polynomial of a graph. We define the rth matching polynomial of G to be the average matching polynomial of all rcoverings of G. We show this polynomial shares many properties with the original matching polynomial. For example, it is real rooted with all its roots inside [−ρ,ρ]. 

Raz, Ran 
STOC'16: "Exponential Separation ..."
Exponential Separation of Communication and External Information
Anat Ganor, Gillat Kol, and Ran Raz (Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) We show an exponential gap between communication complexity and external information complexity, by analyzing a communication task suggested as a candidate by Braverman. Previously, only a separation of communication complexity and internal information complexity was known. More precisely, we obtain an explicit example of a search problem with external information complexity ≤ O(k), with respect to any input distribution, and distributional communication complexity ≥ 2^{k}, with respect to some input distribution. In particular, this shows that a communication protocol cannot always be compressed to its external information. By a result of Braverman, our gap is the largest possible. Moreover, since the upper bound of O(k) on the external information complexity of the problem is obtained with respect to any input distribution, our result implies an exponential gap between communication complexity and information complexity (both internal and external) in the nondistributional setting of Braverman. In this setting, no gap was previously known, even for internal information complexity. 

Razenshteyn, Ilya 
STOC'16: "Weighted Low Rank ..."
Weighted Low Rank Approximations with Provable Guarantees
Ilya Razenshteyn, Zhao Song, and David P. Woodruff (Massachusetts Institute of Technology, USA; University of Texas at Austin, USA; IBM Research, USA) The classical low rank approximation problem is: given a matrix A, find a rankk matrix B such that the Frobenius norm of A − B is minimized. It can be solved efficiently using, for instance, the Singular Value Decomposition (SVD). If one allows randomization and approximation, it can be solved in time proportional to the number of nonzero entries of A with high probability. Inspired by practical applications, we consider a weighted version of low rank approximation: for a nonnegative weight matrix W we seek to minimize ∑_{i, j} (W_{i, j} · (A_{i,j} − B_{i,j}))^{2}. The classical problem is a special case of this problem when all weights are 1. Weighted low rank approximation is known to be NPhard, so we are interested in a meaningful parametrization that would allow efficient algorithms. In this paper we present several efficient algorithms for the case of small k and under the assumption that the weight matrix W is of low rank, or has a small number of distinct columns. An important feature of our algorithms is that they do not assume anything about the matrix A. We also obtain lower bounds that show that our algorithms are nearly optimal in these parameters. We give several applications in which these parameters are small. To the best of our knowledge, the present paper is the first to provide algorithms for the weighted low rank approximation problem with provable guarantees. Perhaps even more importantly, our algorithms proceed via a new technique, which we call “guess the sketch”. The technique turns out to be general enough to give solutions to several other fundamental problems: adversarial matrix completion, weighted nonnegative matrix factorization and tensor completion. 

Reingold, Omer 
STOC'16: "ConstantRound Interactive ..."
ConstantRound Interactive Proofs for Delegating Computation
Omer Reingold, Guy N. Rothblum, and Ron D. Rothblum (Samsung Research, USA; Massachusetts Institute of Technology, USA)
The celebrated IP=PSPACE Theorem of Lund etal. (J.ACM 1992) and
Shamir (J.ACM 1992), allows an allpowerful but untrusted prover
to convince a polynomialtime verifier of the validity of
extremely complicated statements (as long as they can be
evaluated using polynomial space). The interactive proof system
designed for this purpose requires a polynomial number of
communication rounds and an exponentialtime (polynomialspace
complete) prover. In this paper, we study the power of more
efficient interactive proof systems.
Our main result is that for every statement that can be evaluated
in polynomial time and boundedpolynomial space there exists an
interactive proof that satisfies the following strict efficiency
requirements: (1) the honest prover runs in polynomial time, (2)
the verifier is almost linear time (and under some conditions
even sub linear), and (3) the interaction consists of only a
constant number of communication rounds. Prior to this work,
very little was known about the power of efficient,
constantround interactive proofs (rather than arguments). This
result represents significant progress on the round complexity of
interactive proofs (even if we ignore the running time of the
honest prover), and on the expressive power of interactive proofs
with polynomialtime honest prover (even if we ignore the round
complexity). This result has several applications, and in
particular it can be used for verifiable delegation of
computation.
Our construction leverages several new notions of interactive
proofs, which may be of independent interest. One of these
notions is that of unambiguous interactive proofs where the
prover has a unique successful strategy. Another notion is that
of probabilistically checkable interactive proofs (PCIPs) where
the verifier only reads a few bits of the transcript in checking
the proof (this could be viewed as an interactive extension of
PCPs).
@InProceedings{STOC16p49,
author = {Omer Reingold and Guy N. Rothblum and Ron D. Rothblum},
title = {ConstantRound Interactive Proofs for Delegating Computation},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {4962},
doi = {10.1145/2897518.2897652},
year = {2016},
}
Publisher's Version
Article Search


Richelson, Silas 
STOC'16: "Textbook Nonmalleable ..."
Textbook Nonmalleable Commitments
Vipul Goyal, Omkant Pandey, and Silas Richelson (Microsoft Research, India; Drexel University, USA; Massachusetts Institute of Technology, USA)
We present a new nonmalleable commitment protocol. Our protocol has the following features: itemize The protocol has only three rounds of interaction. Pass (TCC 2013) showed an impossibility result for a tworound nonmalleable commitment scheme w.r.t. a blackbox reduction to any ``standard" intractability reduction. Thus, this resolves the round complexity of nonmalleable commitment at least w.r.t. blackbox security reductions. Our construction is secure as per the standard notion of nonmalleability w.r.t. commitment. Our protocol is truly efficient. In our basic protocol, the entire computation of the committer is dominated by just three invocations of a noninteractive statically binding commitment scheme, while, the receiver computation (in the commitment stage) is limited to just sampling a random string. Unlike many previous works, we directly construct a protocol for large tags and hence avoid any nonmalleability amplification steps. Our protocol is based on a blackbox use of any noninteractive statistically binding commitment scheme. Such schemes, in turn, can be based on any onetoone oneway function (or any oneway function at the cost of an extra initialization round). Previously, the best known blackbox construction of nonmalleable commitments required a larger (constant) number of rounds. Our construction is publiccoin and makes use of only blackbox simulation. Prior to our work, no publiccoin constant round nonmalleable commitment schemes were known based on blackbox simulation. itemize Our techniques depart significantly from the techniques used previously to construct nonmalleable commitment schemes. As a main technical tool, we rely on nonmalleable codes in the split state model. Our proofs of security are purely combinatorial in nature. In addition, we also present a simple construction of constant round nonmalleable commitments from any oneway function. While this result is not new, the main feature is its simplicity compared to any previous construction of nonmalleable commitments (in any number of rounds). We believe the construction is simple enough to be covered in a graduate level course on cryptography. The construction uses nonmalleable codes in the split state model in a blackbox way.
@InProceedings{STOC16p1128,
author = {Vipul Goyal and Omkant Pandey and Silas Richelson},
title = {Textbook Nonmalleable Commitments},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11281141},
doi = {10.1145/2897518.2897657},
year = {2016},
}
Publisher's Version
Article Search


Rodeh, Yoav 
STOC'16: "Parallel Exhaustive ..."
Parallel Exhaustive Search without Coordination
Pierre Fraigniaud, Amos Korman, and Yoav Rodeh (CNRS, France; University of Paris Diderot, France; Weizmann Institute of Science, Israel) We analyse parallel algorithms in the context of exhaustive search over totally ordered sets. Imagine an infinite list of “boxes”, with a “treasure” hidden in one of them, where the boxes’ order reflects the importance of finding the treasure in a given box. At each time step, a search protocol executed by a searcher has the ability to peek into one box, and see whether the treasure is present or not. Clearly, the best strategy of a single searcher would be to open the boxes one by one, in increasing order. Moreover, by equally dividing the workload between them, k searchers can trivially find the treasure k times faster than one searcher. However, this straightforward strategy is very sensitive to failures (e.g., crashes of processors), and overcoming this issue seems to require a large amount of communication. We therefore address the question of designing parallel search algorithms maximizing their speedup and maintaining high levels of robustness, while minimizing the amount of resources for coordination. Based on the observation that algorithms that avoid communication are inherently robust, we focus our attention on identifying the best running time performance of noncoordinating algorithms. Specifically, we devise noncoordinating algorithms that achieve a speedup of 9/8 for two searchers, a speedup of 4/3 for three searchers, and in general, a speedup of k/4(1+1/k)^{2} for any k≥ 1 searchers. Thus, asymptotically, the speedup is only four times worse compared to the case of full coordination. Moreover, these bounds are tight in a strong sense as no noncoordinating search algorithm can achieve better speedups. Our algorithms are surprisingly simple and hence applicable. However they are memory intensive and so we suggest a practical, memory efficient version, with a speedup of (k^{2} − 1)/4k. That is, it is only a factor of (k+1)/(k−1) slower than the optimal algorithm. Overall, we highlight that, in faulty contexts in which coordination between the searchers is technically difficult to implement, intrusive with respect to privacy, and/or costly in term of resources, it might well be worth giving up on coordination, and simply run our noncoordinating exhaustive search algorithms. 

Roditty, Liam 
STOC'16: "Fault Tolerant Subgraph ..."
Fault Tolerant Subgraph for Single Source Reachability: Generic and Optimal
Surender Baswana, Keerti Choudhary, and Liam Roditty (IIT Kanpur, India; BarIlan University, Israel) Let G=(V,E) be an nvertices medges directed graph. Let s∈ V be any designated source vertex. We address the problem of single source reachability (SSR) from s in presence of failures of vertices/edges. We show that for every k≥ 1, there is a subgraph H of G with at most 2^{k} n edges that preserves the reachability from s even after the failure of any k edges. Formally, given a set F of k edges, a vertex u∈ V is reachable from s in G∖ F if and only if u is reachable from s in H∖ F. We call H a kFault Tolerant Reachability Subgraph (kFTRS). We prove also a matching lower bound of Ω(2^{k}n) for such subgraphs. Our results extend to vertex failures without any extra overhead. The general construction of kFTRS is interesting from several different perspectives. From the Graph theory perspective it reveals a separation between SSR and single source shortest paths (SSSP) in directed graphs. More specifically, in the case of SSSP in weighted directed graphs, there is a lower bound of Ω(m) even for a single edge failure. In the case of unweighted graphs there is a lower bound of Ω(n^{3/2}) edges, again, even for a single edge failure. There is also a matching upper bound but nothing is known for two or more failures in the directed graphs. From the Algorithms perspective it implies fault tolerant solutions to other interesting problems, namely, (i) verifying if the strong connectivity of a graph is preserved after k edge or vertex failures, (ii) computing a dominator tree of a graph after kfailures. From the perspective of Techniques it makes an interesting usage of the concept of farthest mincut which was already introduced by Ford and Fulkerson in their pioneering work on flows and cuts. We show that there is a close relationship between the farthest mincut and the kFTRS. We believe that our new technique is of independent interest. 

Rogers, Ryan 
STOC'16: "Do Prices Coordinate ..."
Do Prices Coordinate Markets?
Justin Hsu, Jamie Morgenstern, Ryan Rogers, Aaron Roth, and Rakesh Vohra (University of Pennsylvania, USA)
Walrasian equilibrium prices have a remarkable property: they allow each buyer to purchase a bundle of goods that she finds the most desirable, while guaranteeing that the induced allocation over all buyers will globally maximize social welfare. However, this clean story has two caveats. * First, the prices may induce indifferences. In fact, the minimal equilibrium prices necessarily induce indifferences. Accordingly, buyers may need to coordinate with one another to arrive at a socially optimal outcomethe prices alone are not sufficient to coordinate the market. * Second, although natural procedures converge to Walrasian equilibrium prices on a fixed population, in practice buyers typically observe prices without participating in a price computation process. These prices cannot be perfect Walrasian equilibrium prices, but instead somehow reflect distributional information about the market. To better understand the performance of Walrasian prices when facing these two problems, we give two results. First, we propose a mild genericity condition on valuations under which the minimal Walrasian equilibrium prices induce allocations which result in low overdemand, no matter how the buyers break ties. In fact, under genericity the overdemand of any good can be bounded by 1, which is the best possible at the minimal prices. We demonstrate our results for unit demand valuations and give an extension to matroid based valuations (MBV), conjectured to be equivalent to gross substitute valuations (GS). Second, we use techniques from learning theory to argue that the overdemand and welfare induced by a price vector converge to their expectations uniformly over the class of all price vectors, with respective sample complexity linear and quadratic in the number of goods in the market. These results make no assumption on the form of the valuation functions. These two results imply that under a mild genericity condition, the exact Walrasian equilibrium prices computed in a market are guaranteed to induce both low overdemand and high welfare when used in a new market where agents are sampled independently from the same distribution, whenever the number of agents is larger than the number of commodities in the market.
@InProceedings{STOC16p440,
author = {Justin Hsu and Jamie Morgenstern and Ryan Rogers and Aaron Roth and Rakesh Vohra},
title = {Do Prices Coordinate Markets?},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {440453},
doi = {10.1145/2897518.2897559},
year = {2016},
}
Publisher's Version
Article Search
Video


RonZewi, Noga 
STOC'16: "HighRate LocallyCorrectable ..."
HighRate LocallyCorrectable and LocallyTestable Codes with Subpolynomial Query Complexity
Swastik Kopparty, Or Meir, Noga RonZewi, and Shubhangi Saraf (Rutgers University, USA; Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) In this work, we construct the first locallycorrectable codes (LCCs), and locallytestable codes (LTCs) with constant rate, constant relative distance, and subpolynomial query complexity. Specifically, we show that there exist LCCs and LTCs with block length n, constant rate (which can even be taken arbitrarily close to 1) and constant relative distance, whose query complexity is exp(Õ(√logn)) (for LCCs) and (logn)^{O(loglogn)} (for LTCs). Previously such codes were known to exist only with Ω(n^{β}) query complexity (for constant β>0). In addition to having small query complexity, our codes also achieve better tradeoffs between the rate and the relative distance than were previously known to be achievable by LCCs or LTCs. Specifically, over large (but constant size) alphabet, our codes approach the Singleton bound, that is, they have almost the bestpossible relationship between their rate and distance. This has the surprising consequence that asking for a largealphabet errorcorrecting code to further be an LCC or LTC with subpolynomial query complexity does not require any sacrifice in terms of rate and distance! Over the binary alphabet, our codes meet the Zyablov bound. Such tradeoffs between the rate and the relative distance were previously not known for any o(n) query complexity. Our results on LCCs also immediately give locallydecodable codes (LDCs) with the same parameters. Our codes are based on a technique of Alon, Edmonds and Luby. We observe that this technique can be used as a general distanceamplification method, and show that it interacts well with local correctors and testers. We obtain our main results by applying this method to suitably constructed LCCs and LTCs in the nonstandard regime of subconstant relative distance. 

Rossman, Benjamin 
STOC'16: "Polylogarithmic ..."
Polylogarithmic Frege Depth Lower Bounds via an Expander Switching Lemma
Toniann Pitassi, Benjamin Rossman, Rocco A. Servedio, and LiYang Tan (University of Toronto, Canada; National Institute of Informatics, Japan; Columbia University, USA; Toyota Technological Institute at Chicago, USA) We show that any polynomialsize Frege refutation of a certain linearsize unsatisfiable 3CNF formula over n variables must have depth Ω(√logn). This is an exponential improvement over the previous best results (Pitassi et al. 1993, Krajíček et al. 1995, BenSasson 2002) which give Ω(loglogn) lower bounds. The 3CNF formulas which we use to establish this result are Tseitin contradictions on 3regular expander graphs. In more detail, our main result is a proof that for every d, any depthd Frege refutation of the Tseitin contradiction over these nnode graphs must have size n^{Ω((logn)/d2)}. A key ingredient of our approach is a new switching lemma for a carefully designed random restriction process over these expanders. These random restrictions reduce a Tseitin instance on a 3regular nnode expander to a Tseitin instance on a random subgraph which is a topological embedding of a 3regular n′node expander, for some n′ which is not too much less than n. Our result involves Ω(√logn) iterative applications of this type of random restriction. 

Roth, Aaron 
STOC'16: "Do Prices Coordinate ..."
Do Prices Coordinate Markets?
Justin Hsu, Jamie Morgenstern, Ryan Rogers, Aaron Roth, and Rakesh Vohra (University of Pennsylvania, USA)
Walrasian equilibrium prices have a remarkable property: they allow each buyer to purchase a bundle of goods that she finds the most desirable, while guaranteeing that the induced allocation over all buyers will globally maximize social welfare. However, this clean story has two caveats. * First, the prices may induce indifferences. In fact, the minimal equilibrium prices necessarily induce indifferences. Accordingly, buyers may need to coordinate with one another to arrive at a socially optimal outcomethe prices alone are not sufficient to coordinate the market. * Second, although natural procedures converge to Walrasian equilibrium prices on a fixed population, in practice buyers typically observe prices without participating in a price computation process. These prices cannot be perfect Walrasian equilibrium prices, but instead somehow reflect distributional information about the market. To better understand the performance of Walrasian prices when facing these two problems, we give two results. First, we propose a mild genericity condition on valuations under which the minimal Walrasian equilibrium prices induce allocations which result in low overdemand, no matter how the buyers break ties. In fact, under genericity the overdemand of any good can be bounded by 1, which is the best possible at the minimal prices. We demonstrate our results for unit demand valuations and give an extension to matroid based valuations (MBV), conjectured to be equivalent to gross substitute valuations (GS). Second, we use techniques from learning theory to argue that the overdemand and welfare induced by a price vector converge to their expectations uniformly over the class of all price vectors, with respective sample complexity linear and quadratic in the number of goods in the market. These results make no assumption on the form of the valuation functions. These two results imply that under a mild genericity condition, the exact Walrasian equilibrium prices computed in a market are guaranteed to induce both low overdemand and high welfare when used in a new market where agents are sampled independently from the same distribution, whenever the number of agents is larger than the number of commodities in the market.
@InProceedings{STOC16p440,
author = {Justin Hsu and Jamie Morgenstern and Ryan Rogers and Aaron Roth and Rakesh Vohra},
title = {Do Prices Coordinate Markets?},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {440453},
doi = {10.1145/2897518.2897559},
year = {2016},
}
Publisher's Version
Article Search
Video
STOC'16: "Watch and Learn: ..."
Watch and Learn: Optimizing from Revealed Preferences Feedback
Aaron Roth, Jonathan Ullman, and Zhiwei Steven Wu (University of Pennsylvania, USA; Northeastern University, USA)
A Stackelberg game is played between a leader and a follower. The leader first chooses an action, then the follower plays his best response. The goal of the leader is to pick the action that will maximize his payoff given the follower’s best response. In this paper we present an approach to solving for the leader’s optimal strategy in certain Stackelberg games where the follower’s utility function (and thus the subsequent best response of the follower) is unknown. Stackelberg games capture, for example, the following interaction between a producer and a consumer. The producer chooses the prices of the goods he produces, and then a consumer chooses to buy a utility maximizing bundle of goods. The goal of the seller here is to set prices to maximize his profit—his revenue, minus the production cost of the purchased bundle. It is quite natural that the seller in this example should not know the buyer’s utility function. However, he does have access to revealed preference feedbackhe can set prices, and then observe the purchased bundle and his own profit. We give algorithms for efficiently solving, in terms of both computational and query complexity, a broad class of Stackelberg games in which the follower’s utility function is unknown, using only “revealed preference” access to it. This class includes in particular the profit maximization problem, as well as the optimal tolling problem in nonatomic congestion games, when the latency functions are unknown. Surprisingly, we are able to solve these problems even though the optimization problems are nonconvex in the leader’s actions.
@InProceedings{STOC16p949,
author = {Aaron Roth and Jonathan Ullman and Zhiwei Steven Wu},
title = {Watch and Learn: Optimizing from Revealed Preferences Feedback},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {949962},
doi = {10.1145/2897518.2897579},
year = {2016},
}
Publisher's Version
Article Search


Rothblum, Guy N. 
STOC'16: "ConstantRound Interactive ..."
ConstantRound Interactive Proofs for Delegating Computation
Omer Reingold, Guy N. Rothblum, and Ron D. Rothblum (Samsung Research, USA; Massachusetts Institute of Technology, USA)
The celebrated IP=PSPACE Theorem of Lund etal. (J.ACM 1992) and
Shamir (J.ACM 1992), allows an allpowerful but untrusted prover
to convince a polynomialtime verifier of the validity of
extremely complicated statements (as long as they can be
evaluated using polynomial space). The interactive proof system
designed for this purpose requires a polynomial number of
communication rounds and an exponentialtime (polynomialspace
complete) prover. In this paper, we study the power of more
efficient interactive proof systems.
Our main result is that for every statement that can be evaluated
in polynomial time and boundedpolynomial space there exists an
interactive proof that satisfies the following strict efficiency
requirements: (1) the honest prover runs in polynomial time, (2)
the verifier is almost linear time (and under some conditions
even sub linear), and (3) the interaction consists of only a
constant number of communication rounds. Prior to this work,
very little was known about the power of efficient,
constantround interactive proofs (rather than arguments). This
result represents significant progress on the round complexity of
interactive proofs (even if we ignore the running time of the
honest prover), and on the expressive power of interactive proofs
with polynomialtime honest prover (even if we ignore the round
complexity). This result has several applications, and in
particular it can be used for verifiable delegation of
computation.
Our construction leverages several new notions of interactive
proofs, which may be of independent interest. One of these
notions is that of unambiguous interactive proofs where the
prover has a unique successful strategy. Another notion is that
of probabilistically checkable interactive proofs (PCIPs) where
the verifier only reads a few bits of the transcript in checking
the proof (this could be viewed as an interactive extension of
PCPs).
@InProceedings{STOC16p49,
author = {Omer Reingold and Guy N. Rothblum and Ron D. Rothblum},
title = {ConstantRound Interactive Proofs for Delegating Computation},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {4962},
doi = {10.1145/2897518.2897652},
year = {2016},
}
Publisher's Version
Article Search


Rothblum, Ron D. 
STOC'16: "ConstantRound Interactive ..."
ConstantRound Interactive Proofs for Delegating Computation
Omer Reingold, Guy N. Rothblum, and Ron D. Rothblum (Samsung Research, USA; Massachusetts Institute of Technology, USA)
The celebrated IP=PSPACE Theorem of Lund etal. (J.ACM 1992) and
Shamir (J.ACM 1992), allows an allpowerful but untrusted prover
to convince a polynomialtime verifier of the validity of
extremely complicated statements (as long as they can be
evaluated using polynomial space). The interactive proof system
designed for this purpose requires a polynomial number of
communication rounds and an exponentialtime (polynomialspace
complete) prover. In this paper, we study the power of more
efficient interactive proof systems.
Our main result is that for every statement that can be evaluated
in polynomial time and boundedpolynomial space there exists an
interactive proof that satisfies the following strict efficiency
requirements: (1) the honest prover runs in polynomial time, (2)
the verifier is almost linear time (and under some conditions
even sub linear), and (3) the interaction consists of only a
constant number of communication rounds. Prior to this work,
very little was known about the power of efficient,
constantround interactive proofs (rather than arguments). This
result represents significant progress on the round complexity of
interactive proofs (even if we ignore the running time of the
honest prover), and on the expressive power of interactive proofs
with polynomialtime honest prover (even if we ignore the round
complexity). This result has several applications, and in
particular it can be used for verifiable delegation of
computation.
Our construction leverages several new notions of interactive
proofs, which may be of independent interest. One of these
notions is that of unambiguous interactive proofs where the
prover has a unique successful strategy. Another notion is that
of probabilistically checkable interactive proofs (PCIPs) where
the verifier only reads a few bits of the transcript in checking
the proof (this could be viewed as an interactive extension of
PCPs).
@InProceedings{STOC16p49,
author = {Omer Reingold and Guy N. Rothblum and Ron D. Rothblum},
title = {ConstantRound Interactive Proofs for Delegating Computation},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {4962},
doi = {10.1145/2897518.2897652},
year = {2016},
}
Publisher's Version
Article Search


Rothvoss, Thomas 
STOC'16: "A (1+epsilon)Approximation ..."
A (1+epsilon)Approximation for Makespan Scheduling with Precedence Constraints using LP Hierarchies
Elaine Levey and Thomas Rothvoss (University of Washington, USA) In a classical problem in scheduling, one has n unit size jobs with a precedence order and the goal is to find a schedule of those jobs on m identical machines as to minimize the makespan. It is one of the remaining four open problems from the book of Garey & Johnson whether or not this problem is NPhard for m=3. We prove that for any fixed ε and m, an LPhierarchy lift of the timeindex LP with a slightly super polylogarithmic number of r = (log(n))^{Θ(loglogn)} rounds provides a (1 + ε)approximation. For example SheraliAdams suffices as hierarchy. This implies an algorithm that yields a (1+ε)approximation in time n^{O(r)}. The previously best approximation algorithms guarantee a 2 − 7/3m+1approximation in polynomial time for m ≥ 4 and 4/3 for m=3. Our algorithm is based on a recursive scheduling approach where in each step we reduce the correlation in form of long chains. Our method adds to the rather short list of examples where hierarchies are actually useful to obtain better approximation algorithms. 

Roughgarden, Tim 
STOC'16: "The Price of Anarchy ..."
The Price of Anarchy in Large Games
Michal Feldman, Nicole Immorlica, Brendan Lucier, Tim Roughgarden, and Vasilis Syrgkanis (Tel Aviv University, Israel; Microsoft Research, USA; Stanford University, USA)
We present an analysis framework for bounding the price of anarchy (POA) in games that have many players, as in many of the games most pertinent to computer science applications. We use this framework to demonstrate that, in many of the models in which the POA has been studied, the POA in large games is much smaller than the worstcase bound. Our framework also differentiates between mechanisms with similar worstcase performance, such as simultaneous uniformprice auctions and greedy combinatorial auctions, thereby providing new insights about which mechanisms are likely to perform well in realistic settings.
@InProceedings{STOC16p963,
author = {Michal Feldman and Nicole Immorlica and Brendan Lucier and Tim Roughgarden and Vasilis Syrgkanis},
title = {The Price of Anarchy in Large Games},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {963976},
doi = {10.1145/2897518.2897580},
year = {2016},
}
Publisher's Version
Article Search


Rubinstein, Aviad 
STOC'16: "Beyond Matroids: ..."
Beyond Matroids: Secretary Problem and Prophet Inequality with General Constraints
Aviad Rubinstein (University of California at Berkeley, USA)
We study generalizations of the ``Prophet Inequality'' and ``Secretary Problem'', where the algorithm is restricted to an arbitrary downwardclosed set system. For 0,1 values, we give O(n)competitive algorithms for both problems. This is close to the Omega(n/log n) lower bound due to Babaioff, Immorlica, and Kleinberg. For general values, our results translate to O(log(n) log(r))competitive algorithms, where r is the cardinality of the largest feasible set. This resolves (up to the O(loglog(n) log(r)) factor) an open question posed to us by Bobby Kleinberg.
@InProceedings{STOC16p324,
author = {Aviad Rubinstein},
title = {Beyond Matroids: Secretary Problem and Prophet Inequality with General Constraints},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {324332},
doi = {10.1145/2897518.2897540},
year = {2016},
}
Publisher's Version
Article Search


Rybicki, Joel 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Sachdeva, Sushant 
STOC'16: "Sparsified Cholesky ..."
Sparsified Cholesky and Multigrid Solvers for Connection Laplacians
Rasmus Kyng, Yin Tat Lee, Richard Peng, Sushant Sachdeva, and Daniel A. Spielman (Yale University, USA; Massachusetts Institute of Technology, USA; Georgia Tech, USA)
We introduce the sparsified Cholesky and sparsified multigrid algorithms for solving systems of linear equations. These algorithms accelerate Gaussian elimination by sparsifying the nonzero matrix entries created by the elimination process. We use these new algorithms to derive the first nearly linear time algorithms for solving systems of equations in connection Laplaciansa generalization of Laplacian matrices that arise in many problems in image and signal processing. We also prove that every connection Laplacian has a linear sized approximate inverse. This is an LU factorization with a linear number of nonzero entries that is a strong approximation of the original matrix. Using such a factorization one can solve systems of equations in a connection Laplacian in linear time. Such a factorization was unknown even for ordinary graph Laplacians.
@InProceedings{STOC16p842,
author = {Rasmus Kyng and Yin Tat Lee and Richard Peng and Sushant Sachdeva and Daniel A. Spielman},
title = {Sparsified Cholesky and Multigrid Solvers for Connection Laplacians},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {842850},
doi = {10.1145/2897518.2897640},
year = {2016},
}
Publisher's Version
Article Search


Saha, Chandan 
STOC'16: "On the Size of Homogeneous ..."
On the Size of Homogeneous and of Depth Four Formulas with Low Individual Degree
Neeraj Kayal, Chandan Saha, and Sébastien Tavenas (Microsoft Research, India; Indian Institute of Science, India)
Let r be an integer. Let us call a polynomial f as a multiric polynomial if the degree of f with respect to any variable is at most r (this generalizes the notion of multilinear polynomials). We investigate arithmetic circuits in which the output is syntactically forced to be a multiric polynomial and refer to these as multiric circuits.
Specifically, first define the formal degree of a node a with respect to a variable x inductively as follows. For a leaf it is 1 if a is labelled with x and zero otherwise; for an internal node labelled with * (respectively +) it is the sum of (respectively the maximum of) the formal degrees of the children with respect to x. We call an arithmetic circuit as a multiric circuit if the formal degree of the output node with respect to any variable is at most r. We prove lower bounds for various subclasses of multiric circuits.
@InProceedings{STOC16p626,
author = {Neeraj Kayal and Chandan Saha and Sébastien Tavenas},
title = {On the Size of Homogeneous and of Depth Four Formulas with Low Individual Degree},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {626632},
doi = {10.1145/2897518.2897550},
year = {2016},
}
Publisher's Version
Article Search


Santha, Miklos 
STOC'16: "Separations in Query ..."
Separations in Query Complexity Based on Pointer Functions
Andris Ambainis, Kaspars Balodis, Aleksandrs Belovs, Troy Lee, Miklos Santha, and Juris Smotrovs (University of Latvia, Latvia; CWI, Netherlands; Nanyang Technological University, Singapore; National University of Singapore, Singapore; CNRS, France) In 1986, Saks and Wigderson conjectured that the largest separation between deterministic and zeroerror randomized query complexity for a total boolean function is given by the function f on n=2^{k} bits defined by a complete binary tree of NAND gates of depth k, which achieves R_{0}(f) = O(D(f)^{0.7537…}). We show this is false by giving an example of a total boolean function f on n bits whose deterministic query complexity is Ω(n/log(n)) while its zeroerror randomized query complexity is Õ(√n). We further show that the quantum query complexity of the same function is Õ(n^{1/4}), giving the first example of a total function with a superquadratic gap between its quantum and deterministic query complexities. We also construct a total boolean function g on n variables that has zeroerror randomized query complexity Ω(n/log(n)) and boundederror randomized query complexity R(g) = Õ(√n). This is the first superlinear separation between these two complexity measures. The exact quantum query complexity of the same function is Q_{E}(g) = Õ(√n). These functions show that the relations D(f) = O(R_{1}(f)^{2}) and R_{0}(f) = Õ(R(f)^{2}) are optimal, up to polylogarithmic factors. Further variations of these functions give additional separations between other query complexity measures: a cubic separation between Q and R_{0}, a 3/2power separation between Q_{E} and R, and a 4th power separation between approximate degree and boundederror randomized query complexity. All of these examples are variants of a function recently introduced by Goos, Pitassi, and Watson which they used to separate the unambiguous 1certificate complexity from deterministic query complexity and to resolve the famous Clique versus Independent Set problem in communication complexity. 

Saptharishi, Ramprasad 
STOC'16: "Efficiently Decoding ..."
Efficiently Decoding ReedMuller Codes from Random Errors
Ramprasad Saptharishi, Amir Shpilka, and Ben Lee Volk (Tel Aviv University, Israel) ReedMuller codes encode an mvariate polynomial of degree r by evaluating it on all points in {0,1}^{m}. We denote this code by RM(m,r). The minimal distance of RM(m,r) is 2^{m−r} and so it cannot correct more than half that number of errors in the worst case. For random errors one may hope for a better result. In this work we give an efficient algorithm (in the block length n=2^{m}) for decoding random errors in ReedMuller codes far beyond the minimal distance. Specifically, for low rate codes (of degree r=o(√m)) we can correct a random set of (1/2−o(1))n errors with high probability. For high rate codes (of degree m−r for r=o(√m/logm)), we can correct roughly m^{r/2} errors. More generally, for any integer r, our algorithm can correct any error pattern in RM(m,m−(2r+2)) for which the same erasure pattern can be corrected in RM(m,m−(r+1)). The results above are obtained by applying recent results of Abbe, Shpilka and Wigderson (STOC, 2015), Kumar and Pfister (2015) and Kudekar et al. (2015) regarding the ability of ReedMuller codes to correct random erasures. The algorithm is based on solving a carefully defined set of linear equations and thus it is significantly different than other algorithms for decoding ReedMuller codes that are based on the recursive structure of the code. It can be seen as a more explicit proof of a result of Abbe et al. that shows a reduction from correcting erasures to correcting errors, and it also bares some similarities with the famous BerlekampWelch algorithm for decoding ReedSolomon codes. 

Saraf, Shubhangi 
STOC'16: "HighRate LocallyCorrectable ..."
HighRate LocallyCorrectable and LocallyTestable Codes with Subpolynomial Query Complexity
Swastik Kopparty, Or Meir, Noga RonZewi, and Shubhangi Saraf (Rutgers University, USA; Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) In this work, we construct the first locallycorrectable codes (LCCs), and locallytestable codes (LTCs) with constant rate, constant relative distance, and subpolynomial query complexity. Specifically, we show that there exist LCCs and LTCs with block length n, constant rate (which can even be taken arbitrarily close to 1) and constant relative distance, whose query complexity is exp(Õ(√logn)) (for LCCs) and (logn)^{O(loglogn)} (for LTCs). Previously such codes were known to exist only with Ω(n^{β}) query complexity (for constant β>0). In addition to having small query complexity, our codes also achieve better tradeoffs between the rate and the relative distance than were previously known to be achievable by LCCs or LTCs. Specifically, over large (but constant size) alphabet, our codes approach the Singleton bound, that is, they have almost the bestpossible relationship between their rate and distance. This has the surprising consequence that asking for a largealphabet errorcorrecting code to further be an LCC or LTC with subpolynomial query complexity does not require any sacrifice in terms of rate and distance! Over the binary alphabet, our codes meet the Zyablov bound. Such tradeoffs between the rate and the relative distance were previously not known for any o(n) query complexity. Our results on LCCs also immediately give locallydecodable codes (LDCs) with the same parameters. Our codes are based on a technique of Alon, Edmonds and Luby. We observe that this technique can be used as a general distanceamplification method, and show that it interacts well with local correctors and testers. We obtain our main results by applying this method to suitably constructed LCCs and LTCs in the nonstandard regime of subconstant relative distance. 

Şaşoğlu, Eren 
STOC'16: "ReedMuller Codes ..."
ReedMuller Codes Achieve Capacity on Erasure Channels
Shrinivas Kudekar, Santhosh Kumar, Marco Mondelli, Henry D. Pfister, Eren Şaşoğlu, and Rüdiger Urbanke (Qualcomm Research, USA; Texas A&M University, USA; EPFL, Switzerland; Duke University, USA; Intel, USA)
We introduce a new approach to proving that a sequence of deterministic linear codes achieves capacity on an erasure channel under maximum a posteriori decoding. Rather than relying on the precise structure of the codes, our method exploits code symmetry. In particular, the technique applies to any sequence of linear codes where the block lengths are strictly increasing, the code rates converge, and the permutation group of each code is doubly transitive. In a nutshell, we show that symmetry alone implies nearoptimal performance.
An important consequence of this result is that a sequence of ReedMuller codes with increasing block length and converging rate achieves capacity. This possibility has been suggested previously in the literature, but it has only been proven for cases where the limiting code rate is 0 or 1. Moreover, these results extend naturally to affineinvariant codes and, thus, to all extended primitive narrowsense BCH codes. This is used to resolve, in the affirmative, the existence question for capacityachieving sequences of binary cyclic codes. The primary tools used in the proofs are the sharp threshold property for symmetric monotone boolean functions and the area theorem for extrinsic information transfer (EXIT) functions.
@InProceedings{STOC16p658,
author = {Shrinivas Kudekar and Santhosh Kumar and Marco Mondelli and Henry D. Pfister and Eren Şaşoğlu and Rüdiger Urbanke},
title = {ReedMuller Codes Achieve Capacity on Erasure Channels},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {658669},
doi = {10.1145/2897518.2897584},
year = {2016},
}
Publisher's Version
Article Search


Saurabh, Saket 
STOC'16: "Exact Algorithms ..."
Exact Algorithms via Monotone Local Search
Fedor V. Fomin, Serge Gaspers, Daniel Lokshtanov, and Saket Saurabh (University of Bergen, Norway; UNSW, Australia; Data61, Australia; Institute of Mathematical Sciences at Chennai, India) We give a new general approach for designing exact exponentialtime algorithms for subset problems. In a subset problem the input implicitly describes a family of sets over a universe of size n and the task is to determine whether the family contains at least one set. A typical example of a subset problem is Weighted dSAT. Here, the input is a CNFformula with clauses of size at most d, and an integer W. The universe is the set of variables and the variables have integer weights. The family contains all the subsets S of variables such that the total weight of the variables in S does not exceed W, and setting the variables in S to 1 and the remaining variables to 0 satisfies the formula. Our approach is based on “monotone local search”, where the goal is to extend a partial solution to a solution by adding as few elements as possible. More formally, in the extension problem we are also given as input a subset X of the universe and an integer k. The task is to determine whether one can add at most k elements to X to obtain a set in the (implicitly defined) family. Our main result is that a c^{k}n^{O(1)} time algorithm for the extension problem immediately yields a randomized algorithm for finding a solution of any size with running time O((2−1/c)^{n}). In many cases, the extension problem can be reduced to simply finding a solution of size at most k. Furthermore, efficient algorithms for finding small solutions have been extensively studied in the field of parameterized algorithms. Directly applying these algorithms, our theorem yields in one stroke significant improvements over the best known exponentialtime algorithms for several wellstudied problems, including dHitting Set, Feedback Vertex Set, Node Unique Label Cover, and Weighted dSAT. Our results demonstrate an interesting and very concrete connection between parameterized algorithms and exact exponentialtime algorithms. We also show how to derandomize our algorithms at the cost of a subexponential multiplicative factor in the running time. Our derandomization is based on an efficient construction of a new pseudorandom object that might be of independent interest. Finally, we extend our methods to establish new combinatorial upper bounds and develop enumeration algorithms. 

Sawin, William F. 
STOC'16: "Ramanujan Coverings ..."
Ramanujan Coverings of Graphs
Chris Hall, Doron Puder, and William F. Sawin (University of Wyoming, USA; Institute for Advanced Study at Princeton, USA; Princeton University, USA) Let G be a finite connected graph, and let ρ be the spectral radius of its universal cover. For example, if G is kregular then ρ=2√k−1. We show that for every r, there is an rcovering (a.k.a. an rlift) of G where all the new eigenvalues are bounded from above by ρ. It follows that a bipartite Ramanujan graph has a Ramanujan rcovering for every r. This generalizes the r=2 case due to Marcus, Spielman and Srivastava (2013). Every rcovering of G corresponds to a labeling of the edges of G by elements of the symmetric group S_{r}. We generalize this notion to labeling the edges by elements of various groups and present a broader scenario where Ramanujan coverings are guaranteed to exist. In particular, this shows the existence of richer families of bipartite Ramanujan graphs than was known before. Inspired by MarcusSpielmanSrivastava, a crucial component of our proof is the existence of interlacing families of polynomials for complex reflection groups. The core argument of this component is taken from MarcusSpielmanSrivastava (2015). Another important ingredient of our proof is a new generalization of the matching polynomial of a graph. We define the rth matching polynomial of G to be the average matching polynomial of all rcoverings of G. We show this polynomial shares many properties with the original matching polynomial. For example, it is real rooted with all its roots inside [−ρ,ρ]. 

Schneider, Johannes 
STOC'16: "Distributed (Δ+1)Coloring ..."
Distributed (Δ+1)Coloring in Sublogarithmic Rounds
David G. Harris, Johannes Schneider, and HsinHao Su (University of Maryland, USA; ABB Corporate Research, USA; Massachusetts Institute of Technology, USA)
The (∆+1)coloring problem is a fundamental symmetry breaking problem in distributed computing. We give a new randomized coloring algorithm for (∆+1)coloring running in O(√log ∆)+ 2^O(√log log n) rounds with probability 11/n^Ω(1) in a graph with n nodes and maximum degree ∆. This implies that the (∆+1)coloring problem is easier than the maximal independent set problem and the maximal matching problem, due to their lower bounds by Kuhn, Moscibroda, and Wattenhofer [PODC'04]. Our algorithm also extends to the listcoloring problem where the palette of each node contains ∆+1 colors.
@InProceedings{STOC16p465,
author = {David G. Harris and Johannes Schneider and HsinHao Su},
title = {Distributed (Δ+1)Coloring in Sublogarithmic Rounds},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {465478},
doi = {10.1145/2897518.2897533},
year = {2016},
}
Publisher's Version
Article Search


Schramm, Tselil 
STOC'16: "Fast Spectral Algorithms ..."
Fast Spectral Algorithms from SumofSquares Proofs: Tensor Decomposition and Planted Sparse Vectors
Samuel B. Hopkins, Tselil Schramm, Jonathan Shi, and David Steurer (Cornell University, USA; University of California at Berkeley, USA) We consider two problems that arise in machine learning applications: the problem of recovering a planted sparse vector in a random linear subspace and the problem of decomposing a random lowrank overcomplete 3tensor. For both problems, the best known guarantees are based on the sumofsquares method. We develop new algorithms inspired by analyses of the sumofsquares method. Our algorithms achieve the same or similar guarantees as sumofsquares for these problems but the running time is significantly faster. For the planted sparse vector problem, we give an algorithm with running time nearly linear in the input size that approximately recovers a planted sparse vector with up to constant relative sparsity in a random subspace of ℝ^{n} of dimension up to Ω(√n). These recovery guarantees match the best known ones of Barak, Kelner, and Steurer (STOC 2014) up to logarithmic factors. For tensor decomposition, we give an algorithm with running time close to linear in the input size (with exponent ≈ 1.125) that approximately recovers a component of a random 3tensor over ℝ^{n} of rank up to Ω(n^{4/3}). The best previous algorithm for this problem due to Ge and Ma (RANDOM 2015) works up to rank Ω(n^{3/2}) but requires quasipolynomial time. 

Segev, Gil 
STOC'16: "Searchable Symmetric ..."
Searchable Symmetric Encryption: Optimal Locality in Linear Space via TwoDimensional Balanced Allocations
Gilad Asharov, Moni Naor, Gil Segev, and Ido Shahaf (IBM Research, USA; Weizmann Institute of Science, Israel; Hebrew University of Jerusalem, Israel) Searchable symmetric encryption (SSE) enables a client to store a database on an untrusted server while supporting keyword search in a secure manner. Despite the rapidly increasing interest in SSE technology, experiments indicate that the performance of the known schemes scales badly to large databases. Somewhat surprisingly, this is not due to their usage of cryptographic tools, but rather due to their poor locality (where locality is defined as the number of noncontiguous memory locations the server accesses with each query). The only known schemes that do not suffer from poor locality suffer either from an impractical space overhead or from an impractical read efficiency (where read efficiency is defined as the ratio between the number of bits the server reads with each query and the actual size of the answer). We construct the first SSE schemes that simultaneously enjoy optimal locality, optimal space overhead, and nearlyoptimal read efficiency. Specifically, for a database of size N, under the modest assumption that no keyword appears in more than N^{1 − 1/loglogN} documents, we construct a scheme with read efficiency Õ(loglogN). This essentially matches the lower bound of Cash and Tessaro (EUROCRYPT ’14) showing that any SSE scheme must be suboptimal in either its locality, its space overhead, or its read efficiency. In addition, even without making any assumptions on the structure of the database, we construct a scheme with read efficiency Õ(logN). Our schemes are obtained via a twodimensional generalization of the classic balanced allocations (“balls and bins”) problem that we put forward. We construct nearlyoptimal twodimensional balanced allocation schemes, and then combine their algorithmic structure with subtle cryptographic techniques. 

Sen, Subhabrata 
STOC'16: "Semidefinite Programs ..."
Semidefinite Programs on Sparse Random Graphs and Their Application to Community Detection
Andrea Montanari and Subhabrata Sen (Stanford University, USA)
Denote by A the adjacency matrix of an ErdosRenyi graph with bounded average degree. We consider the problem of maximizing <AEA,X> over the set of positive semidefinite matrices X with diagonal entries X_ii=1. We prove that for large (bounded) average degree d, the value of this semidefinite program (SDP) is with high probability 2n*sqrt(d) + n, o(sqrt(d))+o(n). For a random regular graph of degree d, we prove that the SDP value is 2n*sqrt(d1)+o(n), matching a spectral upper bound. Informally, ErdosRenyi graphs appear to behave similarly to random regular graphs for semidefinite programming. We next consider the sparse, twogroups, symmetric community detection problem (also known as planted partition). We establish that SDP achieves the informationtheoretically optimal detection threshold for large (bounded) degree. Namely, under this model, the vertex set is partitioned into subsets of size n/2, with edge probability a/n (within group) and b/n (across). We prove that SDP detects the partition with high probability provided (ab)^2/(4d)> 1+o_d(1), with d= (a+b)/2. By comparison, the information theoretic threshold for detecting the hidden partition is (ab)^2/(4d)> 1: SDP is nearly optimal for large bounded average degree. Our proof is based on tools from different research areas: (i) A new `higherrank' Grothendieck inequality for symmetric matrices; (ii) An interpolation method inspired from statistical physics; (iii) An analysis of the eigenvectors of deformed Gaussian random matrices.
@InProceedings{STOC16p814,
author = {Andrea Montanari and Subhabrata Sen},
title = {Semidefinite Programs on Sparse Random Graphs and Their Application to Community Detection},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {814827},
doi = {10.1145/2897518.2897548},
year = {2016},
}
Publisher's Version
Article Search


Servedio, Rocco A. 
STOC'16: "NearOptimal SmallDepth ..."
NearOptimal SmallDepth Lower Bounds for Small Distance Connectivity
Xi Chen, Igor C. Oliveira, Rocco A. Servedio, and LiYang Tan (Columbia University, USA; Charles University in Prague, Czechia; Toyota Technological Institute at Chicago, USA) We show that any depthd circuit for determining whether an nnode graph has an stot path of length at most k must have size n^{Ω(k1/d/d)} when k(n) ≤ n^{1/5}, and n^{Ω(k1/5d/d)} when k(n)≤ n. The previous best circuit size lower bounds were n^{kexp(−O(d))} (by Beame, Impagliazzo, and Pitassi (Computational Complexity 1998)) and n^{Ω((logk)/d)} (following from a recent formula size lower bound of Rossman (STOC 2014)). Our lower bound is quite close to optimal, as a simple construction gives depthd circuits of size n^{O(k2/d)} for this problem (and strengthening our bound even to n^{kΩ(1/d)} would require proving that undirected connectivity is not in NC^{1}). Our proof is by reduction to a new lower bound on the size of smalldepth circuits computing a skewed variant of the “Sipser functions” that have played an important role in classical circuit lower bounds. A key ingredient in our proof of the required lower bound for these Sipserlike functions is the use of random projections, an extension of random restrictions which were recently employed by Rossman, Servedio, and Tan (FOCS 2015). Random projections allow us to obtain sharper quantitative bounds while employing simpler arguments, both conceptually and technically, than in the previous works. Toniann Pitassi, Benjamin Rossman, Rocco A. Servedio, and LiYang Tan (University of Toronto, Canada; National Institute of Informatics, Japan; Columbia University, USA; Toyota Technological Institute at Chicago, USA) We show that any polynomialsize Frege refutation of a certain linearsize unsatisfiable 3CNF formula over n variables must have depth Ω(√logn). This is an exponential improvement over the previous best results (Pitassi et al. 1993, Krajíček et al. 1995, BenSasson 2002) which give Ω(loglogn) lower bounds. The 3CNF formulas which we use to establish this result are Tseitin contradictions on 3regular expander graphs. In more detail, our main result is a proof that for every d, any depthd Frege refutation of the Tseitin contradiction over these nnode graphs must have size n^{Ω((logn)/d2)}. A key ingredient of our approach is a new switching lemma for a carefully designed random restriction process over these expanders. These random restrictions reduce a Tseitin instance on a 3regular nnode expander to a Tseitin instance on a random subgraph which is a topological embedding of a 3regular n′node expander, for some n′ which is not too much less than n. Our result involves Ω(√logn) iterative applications of this type of random restriction. 

Shahaf, Ido 
STOC'16: "Searchable Symmetric ..."
Searchable Symmetric Encryption: Optimal Locality in Linear Space via TwoDimensional Balanced Allocations
Gilad Asharov, Moni Naor, Gil Segev, and Ido Shahaf (IBM Research, USA; Weizmann Institute of Science, Israel; Hebrew University of Jerusalem, Israel) Searchable symmetric encryption (SSE) enables a client to store a database on an untrusted server while supporting keyword search in a secure manner. Despite the rapidly increasing interest in SSE technology, experiments indicate that the performance of the known schemes scales badly to large databases. Somewhat surprisingly, this is not due to their usage of cryptographic tools, but rather due to their poor locality (where locality is defined as the number of noncontiguous memory locations the server accesses with each query). The only known schemes that do not suffer from poor locality suffer either from an impractical space overhead or from an impractical read efficiency (where read efficiency is defined as the ratio between the number of bits the server reads with each query and the actual size of the answer). We construct the first SSE schemes that simultaneously enjoy optimal locality, optimal space overhead, and nearlyoptimal read efficiency. Specifically, for a database of size N, under the modest assumption that no keyword appears in more than N^{1 − 1/loglogN} documents, we construct a scheme with read efficiency Õ(loglogN). This essentially matches the lower bound of Cash and Tessaro (EUROCRYPT ’14) showing that any SSE scheme must be suboptimal in either its locality, its space overhead, or its read efficiency. In addition, even without making any assumptions on the structure of the database, we construct a scheme with read efficiency Õ(logN). Our schemes are obtained via a twodimensional generalization of the classic balanced allocations (“balls and bins”) problem that we put forward. We construct nearlyoptimal twodimensional balanced allocation schemes, and then combine their algorithmic structure with subtle cryptographic techniques. 

Sharir, Micha 
STOC'16: "Almost Tight Bounds ..."
Almost Tight Bounds for Eliminating Depth Cycles in Three Dimensions
Boris Aronov and Micha Sharir (New York University, USA; Tel Aviv University, Israel) Given n nonvertical lines in 3space, their vertical depth (above/below) relation can contain cycles. We show that the lines can be cut into O(n^{3/2}polylog n) pieces, such that the depth relation among these pieces is now a proper partial order. This bound is nearly tight in the worst case. As a consequence, we deduce that the number of pairwise nonoverlapping cycles, namely, cycles whose xyprojections do not overlap, is O(n^{3/2}polylog n); this bound too is almost tight in the worst case. Previous results on this topic could only handle restricted cases of the problem (such as handling only triangular cycles, by Aronov, Koltun, and Sharir, or only cycles in gridlike patterns, by Chazelle et al.), and the bounds were considerably weaker—much closer to the trivial quadratic bound. Our proof uses a recent variant of the polynomial partitioning technique, due to Guth, and some simple tools from algebraic geometry. It is much more straightforward than the previous “purely combinatorial” methods. Our approach extends to eliminating all cycles in the depth relation among segments, and among constantdegree algebraic arcs. We hope that a suitable extension of this technique could be used to handle the much more difficult case of pairwisedisjoint triangles as well. Our results almost completely settle a longstanding (35 years old) open problem in computational geometry, motivated by hiddensurface removal in computer graphics. 

Shi, Jonathan 
STOC'16: "Fast Spectral Algorithms ..."
Fast Spectral Algorithms from SumofSquares Proofs: Tensor Decomposition and Planted Sparse Vectors
Samuel B. Hopkins, Tselil Schramm, Jonathan Shi, and David Steurer (Cornell University, USA; University of California at Berkeley, USA) We consider two problems that arise in machine learning applications: the problem of recovering a planted sparse vector in a random linear subspace and the problem of decomposing a random lowrank overcomplete 3tensor. For both problems, the best known guarantees are based on the sumofsquares method. We develop new algorithms inspired by analyses of the sumofsquares method. Our algorithms achieve the same or similar guarantees as sumofsquares for these problems but the running time is significantly faster. For the planted sparse vector problem, we give an algorithm with running time nearly linear in the input size that approximately recovers a planted sparse vector with up to constant relative sparsity in a random subspace of ℝ^{n} of dimension up to Ω(√n). These recovery guarantees match the best known ones of Barak, Kelner, and Steurer (STOC 2014) up to logarithmic factors. For tensor decomposition, we give an algorithm with running time close to linear in the input size (with exponent ≈ 1.125) that approximately recovers a component of a random 3tensor over ℝ^{n} of rank up to Ω(n^{4/3}). The best previous algorithm for this problem due to Ge and Ma (RANDOM 2015) works up to rank Ω(n^{3/2}) but requires quasipolynomial time. 

Shpilka, Amir 
STOC'16: "Efficiently Decoding ..."
Efficiently Decoding ReedMuller Codes from Random Errors
Ramprasad Saptharishi, Amir Shpilka, and Ben Lee Volk (Tel Aviv University, Israel) ReedMuller codes encode an mvariate polynomial of degree r by evaluating it on all points in {0,1}^{m}. We denote this code by RM(m,r). The minimal distance of RM(m,r) is 2^{m−r} and so it cannot correct more than half that number of errors in the worst case. For random errors one may hope for a better result. In this work we give an efficient algorithm (in the block length n=2^{m}) for decoding random errors in ReedMuller codes far beyond the minimal distance. Specifically, for low rate codes (of degree r=o(√m)) we can correct a random set of (1/2−o(1))n errors with high probability. For high rate codes (of degree m−r for r=o(√m/logm)), we can correct roughly m^{r/2} errors. More generally, for any integer r, our algorithm can correct any error pattern in RM(m,m−(2r+2)) for which the same erasure pattern can be corrected in RM(m,m−(r+1)). The results above are obtained by applying recent results of Abbe, Shpilka and Wigderson (STOC, 2015), Kumar and Pfister (2015) and Kudekar et al. (2015) regarding the ability of ReedMuller codes to correct random erasures. The algorithm is based on solving a carefully defined set of linear equations and thus it is significantly different than other algorithms for decoding ReedMuller codes that are based on the recursive structure of the code. It can be seen as a more explicit proof of a result of Abbe et al. that shows a reduction from correcting erasures to correcting errors, and it also bares some similarities with the famous BerlekampWelch algorithm for decoding ReedSolomon codes. 

Sidford, Aaron 
STOC'16: "Geometric Median ..."
Geometric Median in Nearly Linear Time
Michael B. Cohen, Yin Tat Lee, Gary Miller, Jakub Pachocki, and Aaron Sidford (Massachusetts Institute of Technology, USA; Carnegie Mellon University, USA; Microsoft Research, USA) In this paper we provide faster algorithms for solving the geometric median problem: given n points in ^{d} compute a point that minimizes the sum of Euclidean distances to the points. This is one of the oldest nontrivial problems in computational geometry yet despite a long history of research the previous fastest running times for computing a (1+T)approximate geometric median were O(d· n^{4/3}T^{−8/3}) by Chin et. al, Õ(dexpT^{−4}logT^{−1}) by Badoiu et. al, O(nd+poly(d,T^{−1})) by Feldman and Langberg, and the polynomial running time of O((nd)^{O(1)}log1/T) by Parrilo and Sturmfels and Xue and Ye. In this paper we show how to compute such an approximate geometric median in time O(ndlog^{3}n/T) and O(dT^{−2}). While our O(dT^{−2}) is a fairly straightforward application of stochastic subgradient descent, our O(ndlog^{3}n/T) time algorithm is a novel long step interior point method. We start with a simple O((nd)^{O(1)}log1/T) time interior point method and show how to improve it, ultimately building an algorithm that is quite nonstandard from the perspective of interior point literature. Our result is one of few cases of outperforming standard interior point theory. Furthermore, it is the only case we know of where interior point methods yield a nearly linear time algorithm for a canonical optimization problem that traditionally requires superlinear time. Alina Ene, Gary Miller, Jakub Pachocki, and Aaron Sidford (University of Warwick, UK; Carnegie Mellon University, USA; Microsoft Research, USA) We introduce the notion of balance for directed graphs: a weighted directed graph is αbalanced if for every cut S ⊆ V, the total weight of edges going from S to V∖ S is within factor α of the total weight of edges going from V∖ S to S. Several important families of graphs are nearly balanced, in particular, Eulerian graphs (with α = 1) and residual graphs of (1+T)approximate undirected maximum flows (with α=O(1/T)). We use the notion of balance to give a more finegrained understanding of several wellstudied routing questions that are considerably harder in directed graphs. We first revisit oblivious routings in directed graphs. Our main algorithmic result is an oblivious routing scheme for singlesource instances that achieve an O(α · log^{3} n / loglogn) competitive ratio. In the process, we make several technical contributions which may be of independent interest. In particular, we give an efficient algorithm for computing lowradius decompositions of directed graphs parameterized by balance. We also define and construct lowstretch arborescences, a generalization of lowstretch spanning trees to directed graphs. On the negative side, we present new lower bounds for oblivious routing problems on directed graphs. We show that the competitive ratio of oblivious routing algorithms for directed graphs is Ω(n) in general; this result improves upon the longstanding best known lower bound of Ω(√n) by Hajiaghayi et al. We also show that our restriction to singlesource instances is necessary by showing an Ω(√n) lower bound for multiplesource oblivious routing in Eulerian graphs. We also study the maximum flow problem in balanced directed graphs with arbitrary capacities. We develop an efficient algorithm that finds an (1+T)approximate maximum flows in αbalanced graphs in time O(m α^{2} / T^{2}). We show that, using our approximate maximum flow algorithm, we can efficiently determine whether a given directed graph is αbalanced. Additionally, we give an application to the directed sparsest cut problem. 

Singh, Mohit 
STOC'16: "Maximizing Determinants ..."
Maximizing Determinants under Partition Constraints
Aleksandar Nikolov and Mohit Singh (University of Toronto, Canada; Microsoft Research, USA) Given a positive semidefinte matrix L whose columns and rows are indexed by a set U, and a partition matroid M=(U, I), we study the problem of selecting a basis B of M such that the determinant of the submatrix of L induced by the rows and columns in B is maximized. This problem appears in many areas including determinantal point processes in machine learning, experimental design, geographical placement problems, discrepancy theory and computational geometry to model subset selection problems that incorporate diversity. Our main result is to give a geometric concave program for the problem which approximates the optimum value within a factor of e^{r}+o(r), where r denotes the rank of the partition matroid M. We bound the integrality gap of the geometric concave program by giving a polynomial time randomized rounding algorithm. To analyze the rounding algorithm, we relate the solution of our algorithm as well the objective value of the relaxation to a certain stable polynomial. To prove the approximation guarantee, we utilize a general inequality about stable polynomials proved by Gurvits in the context of estimating the permanent of a doubly stochastic matrix. 

Singhal, Vikrant 
STOC'16: "Deterministic and ..."
Deterministic and Probabilistic Binary Search in Graphs
Ehsan EmamjomehZadeh, David Kempe, and Vikrant Singhal (University of Southern California, USA) We consider the following natural generalization of Binary Search: in a given undirected, positively weighted graph, one vertex is a target. The algorithm’s task is to identify the target by adaptively querying vertices. In response to querying a node q, the algorithm learns either that q is the target, or is given an edge out of q that lies on a shortest path from q to the target. We study this problem in a general noisy model in which each query independently receives a correct answer with probability p > 1/2 (a known constant), and an (adversarial) incorrect one with probability 1 − p. Our main positive result is that when p = 1 (i.e., all answers are correct), log_{2} n queries are always sufficient. For general p, we give an (almost informationtheoretically optimal) algorithm that uses, in expectation, no more than (1 − δ) logn/1 − H(p) + o(logn) + O(log^{2} (1/δ)) queries, and identifies the target correctly with probability at leas 1 − δ. Here, H(p) = −(p logp + (1 − p) log(1 − p)) denotes the entropy. The first bound is achieved by the algorithm that iteratively queries a 1median of the nodes not ruled out yet; the second bound by careful repeated invocations of a multiplicative weights algorithm. Even for p = 1, we show several hardness results for the problem of determining whether a target can be found using K queries. Our upper bound of log_{2} n implies a quasipolynomialtime algorithm for undirected connected graphs; we show that this is bestpossible under the Strong Exponential Time Hypothesis (SETH). Furthermore, for directed graphs, or for undirected graphs with nonuniform node querying costs, the problem is PSPACEcomplete. For a semiadaptive version, in which one may query r nodes each in k rounds, we show membership in Σ_{2k−1} in the polynomial hierarchy, and hardness for Σ_{2k−5}. 

Smith, Adam 
STOC'16: "Algorithmic Stability ..."
Algorithmic Stability for Adaptive Data Analysis
Raef Bassily, Kobbi Nissim, Adam Smith, Thomas Steinke, Uri Stemmer, and Jonathan Ullman (University of California at San Diego, USA; BenGurion University of the Negev, Israel; Harvard University, USA; Pennsylvania State University, USA; Northeastern University, USA)
Adaptivity is an important feature of data analysis  the choice of questions to ask about a dataset often depends on previous interactions with the same dataset. However, statistical validity is typically studied in a nonadaptive model, where all questions are specified before the dataset is drawn. Recent work by Dwork et al. (STOC, 2015) and Hardt and Ullman (FOCS, 2014) initiated a general formal study of this problem, and gave the first upper and lower bounds on the achievable generalization error for adaptive data analysis.
Specifically, suppose there is an unknown distribution P and a set of n independent samples x is drawn from P. We seek an algorithm that, given x as input, accurately answers a sequence of adaptively chosen ``queries'' about the unknown distribution P. How many samples n must we draw from the distribution, as a function of the type of queries, the number of queries, and the desired level of accuracy?
In this work we make two new contributions towards resolving this question:
@InProceedings{STOC16p1046,
author = {Raef Bassily and Kobbi Nissim and Adam Smith and Thomas Steinke and Uri Stemmer and Jonathan Ullman},
title = {Algorithmic Stability for Adaptive Data Analysis},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {10461059},
doi = {10.1145/2897518.2897566},
year = {2016},
}
Publisher's Version
Article Search


Smotrovs, Juris 
STOC'16: "Separations in Query ..."
Separations in Query Complexity Based on Pointer Functions
Andris Ambainis, Kaspars Balodis, Aleksandrs Belovs, Troy Lee, Miklos Santha, and Juris Smotrovs (University of Latvia, Latvia; CWI, Netherlands; Nanyang Technological University, Singapore; National University of Singapore, Singapore; CNRS, France) In 1986, Saks and Wigderson conjectured that the largest separation between deterministic and zeroerror randomized query complexity for a total boolean function is given by the function f on n=2^{k} bits defined by a complete binary tree of NAND gates of depth k, which achieves R_{0}(f) = O(D(f)^{0.7537…}). We show this is false by giving an example of a total boolean function f on n bits whose deterministic query complexity is Ω(n/log(n)) while its zeroerror randomized query complexity is Õ(√n). We further show that the quantum query complexity of the same function is Õ(n^{1/4}), giving the first example of a total function with a superquadratic gap between its quantum and deterministic query complexities. We also construct a total boolean function g on n variables that has zeroerror randomized query complexity Ω(n/log(n)) and boundederror randomized query complexity R(g) = Õ(√n). This is the first superlinear separation between these two complexity measures. The exact quantum query complexity of the same function is Q_{E}(g) = Õ(√n). These functions show that the relations D(f) = O(R_{1}(f)^{2}) and R_{0}(f) = Õ(R(f)^{2}) are optimal, up to polylogarithmic factors. Further variations of these functions give additional separations between other query complexity measures: a cubic separation between Q and R_{0}, a 3/2power separation between Q_{E} and R, and a 4th power separation between approximate degree and boundederror randomized query complexity. All of these examples are variants of a function recently introduced by Goos, Pitassi, and Watson which they used to separate the unambiguous 1certificate complexity from deterministic query complexity and to resolve the famous Clique versus Independent Set problem in communication complexity. 

Sohler, Christian 
STOC'16: "Relating Two Property ..."
Relating Two Property Testing Models for Bounded Degree Directed Graphs
Artur Czumaj, Pan Peng, and Christian Sohler (University of Warwick, UK; TU Dortmund, Germany; Institute of Software at Chinese Academy of Sciences, China) We study property testing algorithms in directed graphs (digraphs) with maximum indegree and maximum outdegree upper bounded by d. For directed graphs with bounded degree, there are two different models in property testing introduced by Bender and Ron (2002). In the bidirectional model, one can access both incoming and outgoing edges while in the unidirectional model one can only access outgoing edges. In our paper we provide a new relation between the two models: we prove that if a property can be tested with constant query complexity in the bidirectional model, then it can be tested with sublinear query complexity in the unidirectional model. A corollary of this result is that in the unidirectional model (the model allowing only queries to the outgoing neighbors), every property in hyperfinite digraphs is testable with sublinear query complexity. 

Song, Zhao 
STOC'16: "Weighted Low Rank ..."
Weighted Low Rank Approximations with Provable Guarantees
Ilya Razenshteyn, Zhao Song, and David P. Woodruff (Massachusetts Institute of Technology, USA; University of Texas at Austin, USA; IBM Research, USA) The classical low rank approximation problem is: given a matrix A, find a rankk matrix B such that the Frobenius norm of A − B is minimized. It can be solved efficiently using, for instance, the Singular Value Decomposition (SVD). If one allows randomization and approximation, it can be solved in time proportional to the number of nonzero entries of A with high probability. Inspired by practical applications, we consider a weighted version of low rank approximation: for a nonnegative weight matrix W we seek to minimize ∑_{i, j} (W_{i, j} · (A_{i,j} − B_{i,j}))^{2}. The classical problem is a special case of this problem when all weights are 1. Weighted low rank approximation is known to be NPhard, so we are interested in a meaningful parametrization that would allow efficient algorithms. In this paper we present several efficient algorithms for the case of small k and under the assumption that the weight matrix W is of low rank, or has a small number of distinct columns. An important feature of our algorithms is that they do not assume anything about the matrix A. We also obtain lower bounds that show that our algorithms are nearly optimal in these parameters. We give several applications in which these parameters are small. To the best of our knowledge, the present paper is the first to provide algorithms for the weighted low rank approximation problem with provable guarantees. Perhaps even more importantly, our algorithms proceed via a new technique, which we call “guess the sketch”. The technique turns out to be general enough to give solutions to several other fundamental problems: adversarial matrix completion, weighted nonnegative matrix factorization and tensor completion. 

Spielman, Daniel A. 
STOC'16: "Sparsified Cholesky ..."
Sparsified Cholesky and Multigrid Solvers for Connection Laplacians
Rasmus Kyng, Yin Tat Lee, Richard Peng, Sushant Sachdeva, and Daniel A. Spielman (Yale University, USA; Massachusetts Institute of Technology, USA; Georgia Tech, USA)
We introduce the sparsified Cholesky and sparsified multigrid algorithms for solving systems of linear equations. These algorithms accelerate Gaussian elimination by sparsifying the nonzero matrix entries created by the elimination process. We use these new algorithms to derive the first nearly linear time algorithms for solving systems of equations in connection Laplaciansa generalization of Laplacian matrices that arise in many problems in image and signal processing. We also prove that every connection Laplacian has a linear sized approximate inverse. This is an LU factorization with a linear number of nonzero entries that is a strong approximation of the original matrix. Using such a factorization one can solve systems of equations in a connection Laplacian in linear time. Such a factorization was unknown even for ordinary graph Laplacians.
@InProceedings{STOC16p842,
author = {Rasmus Kyng and Yin Tat Lee and Richard Peng and Sushant Sachdeva and Daniel A. Spielman},
title = {Sparsified Cholesky and Multigrid Solvers for Connection Laplacians},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {842850},
doi = {10.1145/2897518.2897640},
year = {2016},
}
Publisher's Version
Article Search


Srinivasan, Aravind 
STOC'16: "LiftandRound to ..."
LiftandRound to Improve Weighted Completion Time on Unrelated Machines
Nikhil Bansal, Aravind Srinivasan, and Ola Svensson (Eindhoven University of Technology, Netherlands; University of Maryland, USA; EPFL, Switzerland)
We consider the problem of scheduling jobs on unrelated machines so as to minimize the sum of weighted completion times. Our main result is a (3/2c)approximation algorithm for some fixed c>0, improving upon the longstanding bound of 3/2. To do this, we first introduce a new liftandproject based SDP relaxation for the problem. This is necessary as the previous convex programming relaxations have an integrality gap of 3/2. Second, we give a new general bipartiterounding procedure that produces an assignment with certain strong negative correlation properties.
@InProceedings{STOC16p156,
author = {Nikhil Bansal and Aravind Srinivasan and Ola Svensson},
title = {LiftandRound to Improve Weighted Completion Time on Unrelated Machines},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {156167},
doi = {10.1145/2897518.2897572},
year = {2016},
}
Publisher's Version
Article Search


Steinke, Thomas 
STOC'16: "Algorithmic Stability ..."
Algorithmic Stability for Adaptive Data Analysis
Raef Bassily, Kobbi Nissim, Adam Smith, Thomas Steinke, Uri Stemmer, and Jonathan Ullman (University of California at San Diego, USA; BenGurion University of the Negev, Israel; Harvard University, USA; Pennsylvania State University, USA; Northeastern University, USA)
Adaptivity is an important feature of data analysis  the choice of questions to ask about a dataset often depends on previous interactions with the same dataset. However, statistical validity is typically studied in a nonadaptive model, where all questions are specified before the dataset is drawn. Recent work by Dwork et al. (STOC, 2015) and Hardt and Ullman (FOCS, 2014) initiated a general formal study of this problem, and gave the first upper and lower bounds on the achievable generalization error for adaptive data analysis.
Specifically, suppose there is an unknown distribution P and a set of n independent samples x is drawn from P. We seek an algorithm that, given x as input, accurately answers a sequence of adaptively chosen ``queries'' about the unknown distribution P. How many samples n must we draw from the distribution, as a function of the type of queries, the number of queries, and the desired level of accuracy?
In this work we make two new contributions towards resolving this question:
@InProceedings{STOC16p1046,
author = {Raef Bassily and Kobbi Nissim and Adam Smith and Thomas Steinke and Uri Stemmer and Jonathan Ullman},
title = {Algorithmic Stability for Adaptive Data Analysis},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {10461059},
doi = {10.1145/2897518.2897566},
year = {2016},
}
Publisher's Version
Article Search


Stemmer, Uri 
STOC'16: "Algorithmic Stability ..."
Algorithmic Stability for Adaptive Data Analysis
Raef Bassily, Kobbi Nissim, Adam Smith, Thomas Steinke, Uri Stemmer, and Jonathan Ullman (University of California at San Diego, USA; BenGurion University of the Negev, Israel; Harvard University, USA; Pennsylvania State University, USA; Northeastern University, USA)
Adaptivity is an important feature of data analysis  the choice of questions to ask about a dataset often depends on previous interactions with the same dataset. However, statistical validity is typically studied in a nonadaptive model, where all questions are specified before the dataset is drawn. Recent work by Dwork et al. (STOC, 2015) and Hardt and Ullman (FOCS, 2014) initiated a general formal study of this problem, and gave the first upper and lower bounds on the achievable generalization error for adaptive data analysis.
Specifically, suppose there is an unknown distribution P and a set of n independent samples x is drawn from P. We seek an algorithm that, given x as input, accurately answers a sequence of adaptively chosen ``queries'' about the unknown distribution P. How many samples n must we draw from the distribution, as a function of the type of queries, the number of queries, and the desired level of accuracy?
In this work we make two new contributions towards resolving this question:
@InProceedings{STOC16p1046,
author = {Raef Bassily and Kobbi Nissim and Adam Smith and Thomas Steinke and Uri Stemmer and Jonathan Ullman},
title = {Algorithmic Stability for Adaptive Data Analysis},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {10461059},
doi = {10.1145/2897518.2897566},
year = {2016},
}
Publisher's Version
Article Search


Steurer, David 
STOC'16: "Fast Spectral Algorithms ..."
Fast Spectral Algorithms from SumofSquares Proofs: Tensor Decomposition and Planted Sparse Vectors
Samuel B. Hopkins, Tselil Schramm, Jonathan Shi, and David Steurer (Cornell University, USA; University of California at Berkeley, USA) We consider two problems that arise in machine learning applications: the problem of recovering a planted sparse vector in a random linear subspace and the problem of decomposing a random lowrank overcomplete 3tensor. For both problems, the best known guarantees are based on the sumofsquares method. We develop new algorithms inspired by analyses of the sumofsquares method. Our algorithms achieve the same or similar guarantees as sumofsquares for these problems but the running time is significantly faster. For the planted sparse vector problem, we give an algorithm with running time nearly linear in the input size that approximately recovers a planted sparse vector with up to constant relative sparsity in a random subspace of ℝ^{n} of dimension up to Ω(√n). These recovery guarantees match the best known ones of Barak, Kelner, and Steurer (STOC 2014) up to logarithmic factors. For tensor decomposition, we give an algorithm with running time close to linear in the input size (with exponent ≈ 1.125) that approximately recovers a component of a random 3tensor over ℝ^{n} of rank up to Ω(n^{4/3}). The best previous algorithm for this problem due to Ge and Ma (RANDOM 2015) works up to rank Ω(n^{3/2}) but requires quasipolynomial time. 

Stewart, Alistair 
STOC'16: "The Fourier Transform ..."
The Fourier Transform of Poisson Multinomial Distributions and Its Algorithmic Applications
Ilias Diakonikolas, Daniel M. Kane, and Alistair Stewart (University of Southern California, USA; University of California at San Diego, USA) An (n, k)Poisson Multinomial Distribution (PMD) is a random variable of the form X = ∑_{i=1}^{n} X_{i}, where the X_{i}’s are independent random vectors supported on the set of standard basis vectors in ^{k}. In this paper, we obtain a refined structural understanding of PMDs by analyzing their Fourier transform. As our core structural result, we prove that the Fourier transform of PMDs is approximately sparse, i.e., its L_{1}norm is small outside a small set. By building on this result, we obtain the following applications: Learning Theory. We give the first computationally efficient learning algorithm for PMDs under the total variation distance. Our algorithm learns an (n, k)PMD within variation distance ε using a nearoptimal sample size of O_{k}(1/ε^{2}), and runs in time O_{k}(1/ε^{2}) · logn. Previously, no algorithm with a (1/ε) runtime was known, even for k=3. Game Theory. We give the first efficient polynomialtime approximation scheme (EPTAS) for computing Nash equilibria in anonymous games. For normalized anonymous games with n players and k strategies, our algorithm computes a wellsupported εNash equilibrium in time n^{O(k3)} · (k/ε)^{O(k3log(k/ε)/loglog(k/ε))k−1}. The best previous algorithm for this problem [, ] had running time n^{(f(k)/ε)k}, where f(k) = Ω(k^{k2}), for any k>2. Statistics. We prove a multivariate central limit theorem (CLT) that relates an arbitrary PMD to a discretized multivariate Gaussian with the same mean and covariance, in total variation distance. Our new CLT strengthens the CLT of Valiant and Valiant [, ] by removing the dependence on n in the error bound. Along the way we prove several new structural results of independent interest about PMDs. These include: (i) a robust momentmatching lemma, roughly stating that two PMDs that approximately agree on their lowdegree parameter moments are close in variation distance; (ii) nearoptimal size proper εcovers for PMDs in total variation distance (constructive upper bound and nearlymatching lower bound). In addition to Fourier analysis, we employ a number of analytic tools, including the saddlepoint method from complex analysis, that may find other applications. 

Su, HsinHao 
STOC'16: "Distributed (Δ+1)Coloring ..."
Distributed (Δ+1)Coloring in Sublogarithmic Rounds
David G. Harris, Johannes Schneider, and HsinHao Su (University of Maryland, USA; ABB Corporate Research, USA; Massachusetts Institute of Technology, USA)
The (∆+1)coloring problem is a fundamental symmetry breaking problem in distributed computing. We give a new randomized coloring algorithm for (∆+1)coloring running in O(√log ∆)+ 2^O(√log log n) rounds with probability 11/n^Ω(1) in a graph with n nodes and maximum degree ∆. This implies that the (∆+1)coloring problem is easier than the maximal independent set problem and the maximal matching problem, due to their lower bounds by Kuhn, Moscibroda, and Wattenhofer [PODC'04]. Our algorithm also extends to the listcoloring problem where the palette of each node contains ∆+1 colors.
@InProceedings{STOC16p465,
author = {David G. Harris and Johannes Schneider and HsinHao Su},
title = {Distributed (Δ+1)Coloring in Sublogarithmic Rounds},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {465478},
doi = {10.1145/2897518.2897533},
year = {2016},
}
Publisher's Version
Article Search


Suomela, Jukka 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Svensson, Ola 
STOC'16: "LiftandRound to ..."
LiftandRound to Improve Weighted Completion Time on Unrelated Machines
Nikhil Bansal, Aravind Srinivasan, and Ola Svensson (Eindhoven University of Technology, Netherlands; University of Maryland, USA; EPFL, Switzerland)
We consider the problem of scheduling jobs on unrelated machines so as to minimize the sum of weighted completion times. Our main result is a (3/2c)approximation algorithm for some fixed c>0, improving upon the longstanding bound of 3/2. To do this, we first introduce a new liftandproject based SDP relaxation for the problem. This is necessary as the previous convex programming relaxations have an integrality gap of 3/2. Second, we give a new general bipartiterounding procedure that produces an assignment with certain strong negative correlation properties.
@InProceedings{STOC16p156,
author = {Nikhil Bansal and Aravind Srinivasan and Ola Svensson},
title = {LiftandRound to Improve Weighted Completion Time on Unrelated Machines},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {156167},
doi = {10.1145/2897518.2897572},
year = {2016},
}
Publisher's Version
Article Search


Syrgkanis, Vasilis 
STOC'16: "The Price of Anarchy ..."
The Price of Anarchy in Large Games
Michal Feldman, Nicole Immorlica, Brendan Lucier, Tim Roughgarden, and Vasilis Syrgkanis (Tel Aviv University, Israel; Microsoft Research, USA; Stanford University, USA)
We present an analysis framework for bounding the price of anarchy (POA) in games that have many players, as in many of the games most pertinent to computer science applications. We use this framework to demonstrate that, in many of the models in which the POA has been studied, the POA in large games is much smaller than the worstcase bound. Our framework also differentiates between mechanisms with similar worstcase performance, such as simultaneous uniformprice auctions and greedy combinatorial auctions, thereby providing new insights about which mechanisms are likely to perform well in realistic settings.
@InProceedings{STOC16p963,
author = {Michal Feldman and Nicole Immorlica and Brendan Lucier and Tim Roughgarden and Vasilis Syrgkanis},
title = {The Price of Anarchy in Large Games},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {963976},
doi = {10.1145/2897518.2897580},
year = {2016},
}
Publisher's Version
Article Search


Tal, Avishay 
STOC'16: "Matrix Rigidity ..."
Matrix Rigidity of Random Toeplitz Matrices
Oded Goldreich and Avishay Tal (Weizmann Institute of Science, Israel; Institute for Advanced Study at Princeton, USA) We prove that random nbyn Toeplitz (alternatively Hankel) matrices over F_{2} have rigidity Ω(n^{3}/r^{2}logn) for rank r ≥ √n, with high probability. For r = o(n/logn · loglogn), this improves over the Ω(n^{2}/r · log(n/r)) bound that is known for many explicit matrices. Our result implies that the explicit trilinear [n]× [n] × [2n] function defined by F(x,y,z) = ∑_{i,j}x_{i} y_{j} z_{i+j} has complexity Ω(n^{3/5}) in the multilinear circuit model suggested by Goldreich and Wigderson (ECCC, 2013), which yields an exp(n^{3/5}) lower bound on the size of the socalled canonical depththree circuits for F. We also prove that F has complexity Ω(n^{2/3}) if the multilinear circuits are further restricted to be of depth 2. In addition, we show that a matrix whose entries are sampled from a 2^{−n}biased distribution has complexity Ω(n^{2/3}), regardless of depth restrictions, almost matching the known O(n^{2/3}) upper bound for any matrix. We turn this randomized construction into an explicit 4linear construction with similar lower bounds, using the quadratic smallbiased construction of Mossel et al. (RS&A, 2006). 

Tan, LiYang 
STOC'16: "NearOptimal SmallDepth ..."
NearOptimal SmallDepth Lower Bounds for Small Distance Connectivity
Xi Chen, Igor C. Oliveira, Rocco A. Servedio, and LiYang Tan (Columbia University, USA; Charles University in Prague, Czechia; Toyota Technological Institute at Chicago, USA) We show that any depthd circuit for determining whether an nnode graph has an stot path of length at most k must have size n^{Ω(k1/d/d)} when k(n) ≤ n^{1/5}, and n^{Ω(k1/5d/d)} when k(n)≤ n. The previous best circuit size lower bounds were n^{kexp(−O(d))} (by Beame, Impagliazzo, and Pitassi (Computational Complexity 1998)) and n^{Ω((logk)/d)} (following from a recent formula size lower bound of Rossman (STOC 2014)). Our lower bound is quite close to optimal, as a simple construction gives depthd circuits of size n^{O(k2/d)} for this problem (and strengthening our bound even to n^{kΩ(1/d)} would require proving that undirected connectivity is not in NC^{1}). Our proof is by reduction to a new lower bound on the size of smalldepth circuits computing a skewed variant of the “Sipser functions” that have played an important role in classical circuit lower bounds. A key ingredient in our proof of the required lower bound for these Sipserlike functions is the use of random projections, an extension of random restrictions which were recently employed by Rossman, Servedio, and Tan (FOCS 2015). Random projections allow us to obtain sharper quantitative bounds while employing simpler arguments, both conceptually and technically, than in the previous works. Toniann Pitassi, Benjamin Rossman, Rocco A. Servedio, and LiYang Tan (University of Toronto, Canada; National Institute of Informatics, Japan; Columbia University, USA; Toyota Technological Institute at Chicago, USA) We show that any polynomialsize Frege refutation of a certain linearsize unsatisfiable 3CNF formula over n variables must have depth Ω(√logn). This is an exponential improvement over the previous best results (Pitassi et al. 1993, Krajíček et al. 1995, BenSasson 2002) which give Ω(loglogn) lower bounds. The 3CNF formulas which we use to establish this result are Tseitin contradictions on 3regular expander graphs. In more detail, our main result is a proof that for every d, any depthd Frege refutation of the Tseitin contradiction over these nnode graphs must have size n^{Ω((logn)/d2)}. A key ingredient of our approach is a new switching lemma for a carefully designed random restriction process over these expanders. These random restrictions reduce a Tseitin instance on a 3regular nnode expander to a Tseitin instance on a random subgraph which is a topological embedding of a 3regular n′node expander, for some n′ which is not too much less than n. Our result involves Ω(√logn) iterative applications of this type of random restriction. 

Tavenas, Sébastien 
STOC'16: "On the Size of Homogeneous ..."
On the Size of Homogeneous and of Depth Four Formulas with Low Individual Degree
Neeraj Kayal, Chandan Saha, and Sébastien Tavenas (Microsoft Research, India; Indian Institute of Science, India)
Let r be an integer. Let us call a polynomial f as a multiric polynomial if the degree of f with respect to any variable is at most r (this generalizes the notion of multilinear polynomials). We investigate arithmetic circuits in which the output is syntactically forced to be a multiric polynomial and refer to these as multiric circuits.
Specifically, first define the formal degree of a node a with respect to a variable x inductively as follows. For a leaf it is 1 if a is labelled with x and zero otherwise; for an internal node labelled with * (respectively +) it is the sum of (respectively the maximum of) the formal degrees of the children with respect to x. We call an arithmetic circuit as a multiric circuit if the formal degree of the output node with respect to any variable is at most r. We prove lower bounds for various subclasses of multiric circuits.
@InProceedings{STOC16p626,
author = {Neeraj Kayal and Chandan Saha and Sébastien Tavenas},
title = {On the Size of Homogeneous and of Depth Four Formulas with Low Individual Degree},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {626632},
doi = {10.1145/2897518.2897550},
year = {2016},
}
Publisher's Version
Article Search


Thierauf, Thomas 
STOC'16: "Bipartite Perfect ..."
Bipartite Perfect Matching Is in QuasiNC
Stephen Fenner, Rohit Gurjar, and Thomas Thierauf (University of South Carolina, USA; Aalen University, Germany) We show that the bipartite perfect matching problem is in quasi NC^{2}. That is, it has uniform circuits of quasipolynomial size n^{O(logn)}, and O(log^{2} n) depth. Previously, only an exponential upper bound was known on the size of such circuits with polylogarithmic depth. We obtain our result by an almost complete derandomization of the famous Isolation Lemma when applied to yield an efficient randomized parallel algorithm for the bipartite perfect matching problem. 

Tzamos, Christos 
STOC'16: "A SizeFree CLT ..."
A SizeFree CLT for Poisson Multinomials and Its Applications
Constantinos Daskalakis, Anindya De, Gautam Kamath, and Christos Tzamos (Massachusetts Institute of Technology, USA; Northwestern University, USA) An (n,k)Poisson Multinomial Distribution (PMD) is the distribution of the sum of n independent random vectors supported on the set B_{k}={e_{1},…,e_{k}} of standard basis vectors in ℝ^{k}. We show that any (n,k)PMD is poly(k/σ)close in total variation distance to the (appropriately discretized) multidimensional Gaussian with the same first two moments, removing the dependence on n from the Central Limit Theorem of Valiant and Valiant. Interestingly, our CLT is obtained by bootstrapping the ValiantValiant CLT itself through the structural characterization of PMDs shown in recent work by Daskalakis, Kamath and Tzamos. In turn, our stronger CLT can be leveraged to obtain an efficient PTAS for approximate Nash equilibria in anonymous games, significantly improving the state of the art, and matching qualitatively the running time dependence on n and 1/T of the best known algorithm for twostrategy anonymous games. Our new CLT also enables the construction of covers for the set of (n,k)PMDs, which are proper and whose size is shown to be essentially optimal. Our cover construction combines our CLT with the ShapleyFolkman theorem and recent sparsification results for Laplacian matrices by Batson, Spielman, and Srivastava. Our cover size lower bound is based on an algebraic geometric construction. Finally, leveraging the structural properties of the Fourier spectrum of PMDs we show that these distributions can be learned from O_{k}(1/T^{2}) samples in poly_{k}(1/T)time, removing the quasipolynomial dependence of the running time on 1/T from prior work. 

Uitto, Jara 
STOC'16: "A Lower Bound for ..."
A Lower Bound for the Distributed Lovász Local Lemma
Sebastian Brandt, Orr Fischer, Juho Hirvonen, Barbara Keller, Tuomo Lempiäinen, Joel Rybicki, Jukka Suomela, and Jara Uitto (ETH Zurich, Switzerland; Tel Aviv University, Israel; Aalto University, Finland; Bitsplitters, Switzerland)
We show that any randomised Monte Carlo distributed algorithm for the Lovász local lemma requires Omega(log log n) communication rounds, assuming that it finds a correct assignment with high probability. Our result holds even in the special case of d = O(1), where d is the maximum degree of the dependency graph. By prior work, there are distributed algorithms for the Lovász local lemma with a running time of O(log n) rounds in boundeddegree graphs, and the best lower bound before our work was Omega(log* n) rounds [Chung et al. 2014].
@InProceedings{STOC16p479,
author = {Sebastian Brandt and Orr Fischer and Juho Hirvonen and Barbara Keller and Tuomo Lempiäinen and Joel Rybicki and Jukka Suomela and Jara Uitto},
title = {A Lower Bound for the Distributed Lovász Local Lemma},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {479488},
doi = {10.1145/2897518.2897570},
year = {2016},
}
Publisher's Version
Article Search


Ullman, Jonathan 
STOC'16: "Watch and Learn: ..."
Watch and Learn: Optimizing from Revealed Preferences Feedback
Aaron Roth, Jonathan Ullman, and Zhiwei Steven Wu (University of Pennsylvania, USA; Northeastern University, USA)
A Stackelberg game is played between a leader and a follower. The leader first chooses an action, then the follower plays his best response. The goal of the leader is to pick the action that will maximize his payoff given the follower’s best response. In this paper we present an approach to solving for the leader’s optimal strategy in certain Stackelberg games where the follower’s utility function (and thus the subsequent best response of the follower) is unknown. Stackelberg games capture, for example, the following interaction between a producer and a consumer. The producer chooses the prices of the goods he produces, and then a consumer chooses to buy a utility maximizing bundle of goods. The goal of the seller here is to set prices to maximize his profit—his revenue, minus the production cost of the purchased bundle. It is quite natural that the seller in this example should not know the buyer’s utility function. However, he does have access to revealed preference feedbackhe can set prices, and then observe the purchased bundle and his own profit. We give algorithms for efficiently solving, in terms of both computational and query complexity, a broad class of Stackelberg games in which the follower’s utility function is unknown, using only “revealed preference” access to it. This class includes in particular the profit maximization problem, as well as the optimal tolling problem in nonatomic congestion games, when the latency functions are unknown. Surprisingly, we are able to solve these problems even though the optimization problems are nonconvex in the leader’s actions.
@InProceedings{STOC16p949,
author = {Aaron Roth and Jonathan Ullman and Zhiwei Steven Wu},
title = {Watch and Learn: Optimizing from Revealed Preferences Feedback},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {949962},
doi = {10.1145/2897518.2897579},
year = {2016},
}
Publisher's Version
Article Search
STOC'16: "Algorithmic Stability ..."
Algorithmic Stability for Adaptive Data Analysis
Raef Bassily, Kobbi Nissim, Adam Smith, Thomas Steinke, Uri Stemmer, and Jonathan Ullman (University of California at San Diego, USA; BenGurion University of the Negev, Israel; Harvard University, USA; Pennsylvania State University, USA; Northeastern University, USA)
Adaptivity is an important feature of data analysis  the choice of questions to ask about a dataset often depends on previous interactions with the same dataset. However, statistical validity is typically studied in a nonadaptive model, where all questions are specified before the dataset is drawn. Recent work by Dwork et al. (STOC, 2015) and Hardt and Ullman (FOCS, 2014) initiated a general formal study of this problem, and gave the first upper and lower bounds on the achievable generalization error for adaptive data analysis.
Specifically, suppose there is an unknown distribution P and a set of n independent samples x is drawn from P. We seek an algorithm that, given x as input, accurately answers a sequence of adaptively chosen ``queries'' about the unknown distribution P. How many samples n must we draw from the distribution, as a function of the type of queries, the number of queries, and the desired level of accuracy?
In this work we make two new contributions towards resolving this question:
@InProceedings{STOC16p1046,
author = {Raef Bassily and Kobbi Nissim and Adam Smith and Thomas Steinke and Uri Stemmer and Jonathan Ullman},
title = {Algorithmic Stability for Adaptive Data Analysis},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {10461059},
doi = {10.1145/2897518.2897566},
year = {2016},
}
Publisher's Version
Article Search


Urbanke, Rüdiger 
STOC'16: "ReedMuller Codes ..."
ReedMuller Codes Achieve Capacity on Erasure Channels
Shrinivas Kudekar, Santhosh Kumar, Marco Mondelli, Henry D. Pfister, Eren Şaşoğlu, and Rüdiger Urbanke (Qualcomm Research, USA; Texas A&M University, USA; EPFL, Switzerland; Duke University, USA; Intel, USA)
We introduce a new approach to proving that a sequence of deterministic linear codes achieves capacity on an erasure channel under maximum a posteriori decoding. Rather than relying on the precise structure of the codes, our method exploits code symmetry. In particular, the technique applies to any sequence of linear codes where the block lengths are strictly increasing, the code rates converge, and the permutation group of each code is doubly transitive. In a nutshell, we show that symmetry alone implies nearoptimal performance.
An important consequence of this result is that a sequence of ReedMuller codes with increasing block length and converging rate achieves capacity. This possibility has been suggested previously in the literature, but it has only been proven for cases where the limiting code rate is 0 or 1. Moreover, these results extend naturally to affineinvariant codes and, thus, to all extended primitive narrowsense BCH codes. This is used to resolve, in the affirmative, the existence question for capacityachieving sequences of binary cyclic codes. The primary tools used in the proofs are the sharp threshold property for symmetric monotone boolean functions and the area theorem for extrinsic information transfer (EXIT) functions.
@InProceedings{STOC16p658,
author = {Shrinivas Kudekar and Santhosh Kumar and Marco Mondelli and Henry D. Pfister and Eren Şaşoğlu and Rüdiger Urbanke},
title = {ReedMuller Codes Achieve Capacity on Erasure Channels},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {658669},
doi = {10.1145/2897518.2897584},
year = {2016},
}
Publisher's Version
Article Search


Vaikuntanathan, Vinod 
STOC'16: "Watermarking Cryptographic ..."
Watermarking Cryptographic Capabilities
Aloni Cohen, Justin Holmgren, Ryo Nishimaki, Vinod Vaikuntanathan, and Daniel Wichs (Massachusetts Institute of Technology, USA; NTT, Japan; Northeastern University, USA)
A watermarking scheme for programs embeds some information called a mark into a program while preserving its functionality. No adversary can remove the mark without damaging the functionality of the program. In this work, we study the problem of watermarking various cryptographic programs such as pseudorandom function (PRF) evaluation, decryption, and signing. For example, given a PRF key K, we create a marked program C that evaluates the PRF F(K,). An adversary that gets C cannot come up with any program C* in which the mark is removed but which still evaluates the PRF correctly on even a small fraction of the inputs. The work of Barak, Goldreich, Impagliazzo, Rudich, Sahai, Vadhan, and Yang (CRYPTO'01 and Journal of ACM 59(2)) shows that, assuming indistinguishability obfuscation (iO), such watermarking is impossible if the marked program C evaluates the original program with perfect correctness. In this work we show that, assuming iO, such watermarking is possible if the marked program C is allowed to err with even a negligible probability, which would be undetectable to the user. Our watermarking schemes are public key, namely we use a secret marking key to embed marks in programs, and a public detection key that allows anyone to detect marks in programs. Our schemes are secure against chosen program attacks, that is even if the adversary is given oracle access to the marking functionality. We emphasize that our security notion of watermark nonremovability considers arbitrary adversarial strategies to modify the marked program, in contrast to the prior works (Nishimaki, EUROCRYPT '13).
@InProceedings{STOC16p1115,
author = {Aloni Cohen and Justin Holmgren and Ryo Nishimaki and Vinod Vaikuntanathan and Daniel Wichs},
title = {Watermarking Cryptographic Capabilities},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {11151127},
doi = {10.1145/2897518.2897651},
year = {2016},
}
Publisher's Version
Article Search


Valiant, Gregory 
STOC'16: "Instance Optimal ..."
Instance Optimal Learning of Discrete Distributions
Gregory Valiant and Paul Valiant (Stanford University, USA; Brown University, USA)
We consider the following basic learning task: given independent draws from an unknown distribution over a discrete support, output an approximation of the distribution that is as accurate as possible in L1 distance (equivalently, total variation distance, or "statistical distance"). Perhaps surprisingly, it is often possible to "denoise" the empirical distribution of the samples to return an approximation of the true distribution that is significantly more accurate than the empirical distribution, without relying on any prior assumptions on the distribution. We present an instance optimal learning algorithm which optimally performs this denoising for every distribution for which such a denoising is possible. More formally, given n independent draws from a distribution p, our algorithm returns a labelled vector whose expected distance from p is equal to the minimum possible expected error that could be obtained by any algorithm, even one that is given the true unlabeled vector of probabilities of distribution p and simply needs to assign labelsup to an additive subconstant term that is independent of p and goes to zero as n gets large. This somewhat surprising result has several conceptual implications, including the fact that, for any large sample from a distribution over discrete support, prior knowledge of the rates of decay of the tails of the distribution (e.g. powerlaw type assumptions) is not significantly helpful for the task of learning the distribution. As a consequence of our techniques, we also show that given a set of n samples from an arbitrary distribution, one can accurately estimate the expected number of distinct elements that will be observed in a sample of any size up to n log n. This sort of extrapolation is practically relevant, particularly to domains such as genomics where it is important to understand how much more might be discovered given larger sample sizes, and we are optimistic that our approach is practically viable.
@InProceedings{STOC16p142,
author = {Gregory Valiant and Paul Valiant},
title = {Instance Optimal Learning of Discrete Distributions},
booktitle = {Proc.\ STOC},
publisher = {ACM},
pages = {142155},
doi = {10.1145/2897518.2897641},
year = {2016},
}
Publisher's Version
Article Search


Valiant, Paul 
STOC'16: "Instance Optimal ..."
Instance Optimal Learning of Discrete Distributions
Gregory Valiant and Paul Valiant (Stanford University, USA; Brown University, USA)
We consider the following basic learning task: given independent draws from an unknown distribution over a discrete support, output an approximation of the distr 