Journal of the ACM (JACM)

Latest Articles

Engineering with Logic: Rigorous Test-Oracle Specification and Validation for TCP/IP and the Sockets API

Conventional computer engineering relies on test-and-debug development processes, with the behavior of common interfaces described (at best) with prose specification documents. But prose specifications cannot be used in test-and-debug development in any automated way, and prose is a poor medium for... (more)

The PCL Theorem: Transactions cannot be Parallel, Consistent, and Live

We establish a theorem called the PCL theorem, which states that it is impossible to design a transactional memory algorithm that ensures (1) parallelism, i.e., transactions do not need to synchronize unless they access the same application objects, (2) very little consistency, i.e., a consistency condition, called weak adaptive consistency,... (more)

Fast Learning Requires Good Memory: A Time-Space Lower Bound for Parity Learning

We prove that any algorithm for learning parities requires either a memory of quadratic size or an exponential number of samples. This proves a recent conjecture of  Steinhardt et al. (2016) and shows that for some learning problems, a large storage space is crucial. More formally, in the problem of parity learning, an unknown string x... (more)

Deterministic Edge Connectivity in Near-Linear Time

We present a deterministic algorithm that computes the edge-connectivity of a graph in near-linear time. This is for a simple undirected unweighted... (more)

Computing the Homology of Basic Semialgebraic Sets in Weak Exponential Time

We describe and analyze an algorithm for computing the homology (Betti numbers and torsion coefficients) of basic semialgebraic sets that works in... (more)

Scaling Exponential Backoff: Constant Throughput, Polylogarithmic Channel-Access Attempts, and Robustness

Randomized exponential backoff is a widely deployed technique for coordinating access to a shared resource. A good backoff protocol should, arguably, satisfy three natural properties: (1) it should provide constant throughput, wasting as little time as possible; (2) it should require few failed access attempts, minimizing the amount of wasted... (more)


About JACM

The Journal of the ACM (JACM) provides coverage of the most significant work on principles of computer science, broadly construed. The scope of research we cover encompasses contributions of lasting value to any area of computer science. To be accepted, a paper must be judged to be truly outstanding in its field.  JACM is interested  in work in core computer science and at the boundaries, both the boundaries of subdisciplines of computer science and the boundaries between computer science and other fields.  READ MORE

Editorial Process

The Journal of the ACM begins the refereeing process with a "quick review", to check whether the manuscript has a plausible chance of meeting JACM's high standards, even if all the claimed results are correct. JACM tries to cover a broad spectrum of areas, and can only accept 4-5 papers in any given area every year. Thus, we try to focus on the most significant papers in each area, that would be of interest to the broad community, and reject many papers that would be accepted by other journals. READ MORE

Important Note on P/NP

Some submissions purport to solve a long-standing open problem in complexity theory, such as the P/NP problem. Many of these turn out to be mistaken, and such submissions tax JACM volunteer editors and reviewers.  READ MORE

Pseudorandomness from Shrinkage

One powerful theme in complexity theory and pseudorandomness in the past few decades has been the use of lower bounds to give pseudorandom generators (PRGs). However, the general results using this hardness vs. randomness paradigm suffer a quantitative loss in parameters, and hence do not give nontrivial implications for models where we dont know super-polynomial lower bounds but do know lower bounds of a fixed polynomial. We show that when such lower bounds are proved using random restrictions, we can construct PRGs which are essentially best possible without in turn improving the lower bounds. More specifically, say that a circuit family has shrinkage exponent “ if a random restriction leaving a p fraction of variables unset shrinks the size of any circuit in the family by a factor of p“+o(1). Our PRG uses a seed of length s1/(“+1)+o(1) to fool circuits in the family of size s. By using this generic construction, we get PRGs with polynomially small error for the following classes of circuits of size s and with the following seed lengths: 1. For de Morgan formulas, seed length s1/3+o(1); 2. For formulas over an arbitrary basis, seed length s1/2+o(1); 3. For read-once de Morgan formulas, seed length s.234...; 4. For branching programs of size s, seed length s1/2+o(1). The previous best PRGs known for these classes used seeds of length bigger than n/2 to output n bits, and worked only for size s = O(n) (Bogdanov, Papakonstantinou, & Wan).

Approximate counting, the Lovasz local lemma, and inference in graphical models

In this paper we introduce a new approach for approximately counting in bounded degree systems with higher-order constraints. Our main result is an algorithm to approximately count the number of solutions to a CNF formula ¦ when the width is logarithmic in the maximum degree. This closes an exponential gap between the known upper and lower bounds. Moreover our algorithm extends straightforwardly to approximate sampling, which shows that under Lovasz Local Lemma-like conditions it is not only possible to find a satisfying assignment, it is also possible to generate one approximately uniformly at random from the set of all satisfying assignments. Our approach is a significant departure from earlier techniques in approximate counting, and is based on a framework to bootstrap an oracle for computing marginal probabilities on individual variables. Finally, we give an application of our results to show that it is algorithmically possible to sample from the posterior distribution in an interesting class of graphical models.

Nonhomogeneous Place-Dependent Markov Chains, Unsynchronized AIMD, and Network Utility Maximization

We present a solution of a class of network utility maximization (NUM) problems using minimal communication. The constraints of the problem are inspired less by TCP-like congestion control but by problems in the area of internet of things and related areas in which the need arises to bring the behavior of a large group of agents to a social optimum. The approach uses only intermittent feedback, no inter-agent communication, and no common clock. The proposed algorithm is a combination of the classical AIMD algorithm in conjunction with a simple probabilistic rule for the agents to respond to a capacity signal. This leads to a nonhomogeneous Markov chain and we show almost sure convergence of this chain to the social optimum.

Approaching 3/2 for the s-t-path TSP

We show that there is a polynomial-time algorithm with approximation guarantee 3/2 + ? for the s-t path TSP, for any fixed ? > 0. It is well known that Wolsey's analysis of Christofides' algorithm also works for the s-t path TSP with its natural LP relaxation except for the narrow cuts (in which the LP solution has value less than two). A fixed optimum tour has either a single edge in a narrow cut (then call the edge and the cut lonely) or at least three (then call the cut busy). Our algorithm ?guesses? (by dynamic programming) lonely cuts and edges. Then we partition the instance into smaller instances and strengthen the LP, requiring value at least three for busy cuts. By setting up a k -stage recursive dynamic program, we can compute a spanning tree (V , S ) and an LP solution y such that (1/2 + O(2?k))y is in the T -join polyhedron, where T is the set of vertices whose degree in S has the wrong parity.

Going Higher in First-Order Quantifier Alternation Hierarchies on Words

We investigate quantifier alternation hierarchies in first-order logic on finite words. Levels in these hierarchies are defined by counting the number of quantifier alternations in formulas. We prove that one can decide membership of a regular language in the levels B?2 (finite boolean combinations of formulas having only one alternation) and ?3 (formulas having only two alternations and beginning with an existential block). Our proofs work by considering a deeper problem, called separation, which, once solved for lower levels, allows us to solve membership for higher levels.

Near-optimal linear decision trees for k-SUM and related problems

We construct near optimal linear decision trees for a variety of decision problems in combinatorics and discrete geometry. For example, for any constant $k$, we construct linear decision trees that solve the $k$-SUM problem on $n$ elements using $O(n \log^2 n)$ linear queries. Moreover, the queries we use are comparison queries, which compare the sums of two $k$-subsets; when viewed as linear queries, comparison queries are $2k$-sparse and have only $\{-1,0,1\}$ coefficients. We give similar constructions for sorting sumsets $A+B$ and for solving the SUBSET-SUM problem, both with optimal number of queries, up to poly-logarithmic terms. Our constructions are based on the notion of ``inference dimension", recently introduced by the authors in the context of active classification with comparison queries. This can be viewed as another contribution to the fruitful link between machine learning and discrete geometry, which goes back to the discovery of the VC dimension.

Capacity Upper Bounds for Deletion-Type Channels

We develop a systematic approach, based on convex programming and real analysis, for obtaining upper bounds on the capacity of the binary deletion channel and, more generally, channels with i.i.d. insertions and deletions. Other than the classical deletion channel, we give a special attention to the Poisson-repeat channel introduced by Mitzenmacher and Drinea (IEEE Transactions on Information Theory, 2006). Our framework can be applied to obtain capacity upper bounds for any repetition distribution (the deletion and Poisson-repeat channels corresponding to the special cases of Bernoulli and Poisson distributions). Our techniques essentially reduce the task of proving capacity upper bounds to maximizing a univariate, real-valued, and often concave function over a bounded interval. The corresponding univariate function is carefully designed according to the underlying distribution of repetitions and the choices vary depending on the desired strength of the upper bounds as well as the desired simplicity of the function (e.g., being only efficiently computable versus having an explicit closed-form expression in terms of elementary, or common special, functions). Among our results, we show the following: 1. The capacity of the binary deletion channel with deletion probability $d$ is at most $(1-d) \log \varphi$ for $d \geq 1/2$, and, assuming the capacity function is convex, is at most $1-d \log(4/\varphi)$ for $d<1/2$, where $\varphi=(1+\sqrt{5})/2$ is the golden ratio. This is the first nontrivial capacity upper bound for any value of $d$ outside the limiting case $d \to 0$ that is fully explicit and proved without computer assistance. 2. We derive the first set of capacity upper bounds for the Poisson-repeat channel. Our results uncover further striking connections between this channel and the deletion channel, and suggest, somewhat counter-intuitively, that the Poisson-repeat channel is actually analytically simpler than the deletion channel and may be of key importance to a complete understanding of the deletion channel. 3. We derive several novel upper bounds on the capacity of the deletion channel. All upper bounds are maximums of efficiently computable, and concave, univariate real functions over a bounded domain. In turn, we upper bound these functions in terms of explicit elementary and standard special functions, whose maximums can be found even more efficiently (and sometimes, analytically, for example for $d=1/2$). Along the way, we develop several new techniques of potentially independent interest. For example, we develop systematic techniques to study channels with mean constraints over the reals. Furthermore, we motivate the study of novel probability distributions over non-negative integers, as well as novel special functions which could be of interest to mathematical analysis.

On the decidability of membership in matrix-exponential semigroups

Given k+1 square matrices A_1, ..., A_k, C, all of the same dimension and with real algebraic coordinates, we examine the problem of deciding whether there exist matrices of the form exp(A_i t_j) whose product equals C. Our results have applications to reachability problems for linear hybrid automata. Our decidability proof relies on a number of theorems from algebraic and transcendental number theory, most notably those of Baker, Lindemann, and Masser, as well as some useful geometric and linear-algebraic results, including the Minkowski-Weyl theorem and a new (to the best of our knowledge) result about the uniqueness of strictly upper triangular matrix logarithms of upper unitriangular matrices. On the other hand, our undecidability results are shown by reduction from Hilbert's Tenth Problem, through a series of matrix constructions.

Bar Induction is Compatible with Constructive Type Theory

Powerful yet effective induction principles play an important role in computing, being a paramount component of programming languages, automated reasoning and program verification systems. The Bar Induction principle is a fundamental concept of intuitionism, which is equivalent to the standard principle of transfinite induction. In this work we investigate the compatibility of several variants of Bar Induction with Constructive Type Theory (CTT), a dependent type theory in the spirit of Martin-Lof's extensional theory. We first show that CTT is compatible with a Bar Induction principle for sequences of numbers. Then, we establish the compatibility of CTT with a more general Bar Induction principle for sequences of name-free closed terms. The formalization of the latter principle within the theory involved enriching CTT's term syntax with a limit constructor and showing that consistency is preserved. Furthermore, we provide novel insights regarding Bar Induction, such as the non-truncated version of Bar Induction on monotone bars being intuitionistically false. These enhancements are carried out formally using the Nuprl proof assistant which implements CTT, and the formalization of CTT within the Coq proof assistant presented in previous works.

Parallel Bayesian Search with no Coordination

Coordinating the actions of agents (e.g., volunteers analyzing radio signals in [email protected]) yields efficient search algorithms. However, such an efficiency is often at the cost of implementing complex coordination mechanisms which may be expensive in term of communication and/or computation overheads. Instead, non-coordinating algorithms, in which each agent operates independently from the others, are typically very simple, and easy to implement. They are also inherently robust to slight misbehaviors, or even crashes of agents. In this paper, we investigate the ``price of non-coordinating'', in term of search performances, and we show that this price is actually quite small. Specifically, we consider a parallel version of a classical Bayesian search problem, where set of k e 1 searchers are looking for a treasure placed in one of the boxes indexed by positive integers, according to some distribution p. Each searcher can open a random box at each step, and the objective is to find the treasure in a minimum number of steps. We show that there is a very simple non-coordinating algorithm which has expected running time at most 4(1 - 1/(k+1))^2 * OPT + 10, where OPT is the expected running time of the best fully coordinated algorithm. Our algorithm does not even use the precise description of the distribution p, but only the relative likelihood of the boxes. We prove that, under this restriction, our algorithm has the best possible competitive ratio with respect to OPT. For the case where a complete description of the distribution p is given to the search algorithm, we describe an optimal non-coordinating algorithm for Bayesian search. This latter algorithm can be twice as fast as our former algorithm in practical scenarios such as uniform distributions. All these results provide a complete characterization of non-coordinating Bayesian search. The take-away message is that, for their simplicity and robustness, non-coordinating algorithms are viable alternatives to complex coordinating mechanisms subject to significant overheads. Most of these results apply as well to \emph{linear} search, in which the indices of the boxes reflect their relative importance, and where important boxes must be visited first.

Uniform Sampling through the Lovász Local Lemma

We propose a new algorithmic framework, called "partial rejection sampling'', to draw samples exactly from a product distribution, conditioned on none of a number of bad events occurring. Our framework builds new connections between the variable framework of the Lovász Local Lemma and some classical sampling algorithms such as the ``cycle-popping'' algorithm for rooted spanning trees by Wilson (1996). Among other applications, we discover new algorithms to sample satisfying assignments of k-CNF formulas with bounded variable occurrences.

Bandits and Experts in Metric Spaces

In a multi-armed bandit problem, an online algorithm chooses from a set of strategies in a sequence of trials so as to maximize the total payoff of the chosen strategies. While the performance of bandit algorithms with a small finite strategy set is quite well understood, bandit problems with large strategy sets are still a topic of very active investigation, motivated by practical applications such as online auctions and web advertisement. The goal of such research is to identify broad and natural classes of strategy sets and payoff functions which enable the design of efficient solutions. In this work we study a very general setting for the multi-armed bandit problem in which the strategies form a metric space, and the payoff function satisfies a Lipschitz condition with respect to the metric. We refer to this problem as the "Lipschitz MAB problem". We present a solution for the multi-armed bandit problem in this setting. That is, for every metric space we define an isometry invariant which bounds from below the performance of Lipschitz MAB algorithms for this metric space, and we present an algorithm which comes arbitrarily close to meeting this bound. Furthermore, our technique gives even better results for benign payoff functions. We also address the full-feedback (``best expert'') version of the problem, where after every round the payoffs from all arms are revealed.

Exact Algorithms via Monotone Local Search

We give a new general approach for designing exact exponential-time algorithms for subset prob- lems. 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 d-SAT. Here, the input is a CNF-formula 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 vari- ables 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^kn^{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). c 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 exponential-time algorithms for several well-studied problems, including d-Hitting Set, Feedback Vertex Set, Node Unique Label Cover, and Weighted d-SAT. Our results demonstrate an interesting and very concrete connection between parameterized algorithms and exact exponential-time 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 pseudo-random object that might be of independent interest. Finally, we extend our methods to establish new combinatorial upper bounds and develop enumeration algorithms.

From Real-Time Logic to Timed Automata

We show how to transform formulae written in the real-time temporal logic MITL into timed automata recognizing their satisfying models, taken to be continuous-time Boolean signals. This compositional construction is much simpler than previously known; it supports both past and future operators and can easily be extended.

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