Journal of the ACM (JACM)


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

Certain Answers Meet Zero-One Laws

Query answering over incomplete data invariably relies on the standard notion of certain answers which gives a very coarse classification of query answers into those that are certain and those that are not. Our goal is to refine it by measuring how close an answer is to certainty. This measure is defined as the probability that the query is true under a random interpretation of missing information in a database. Since there are infinitely many such interpretations, to pick one at random we adopt the approach used in the study of asymptotic properties and 0--1 laws for logical sentences, and define the measure as the limit of a sequence. We prove that without any restrictions imposed, the standard model of missing data admits the 0--1 law. That is, the limit always exists and can be only 0 or 1 for a very large class of queries. In other words, query answers are either almost certainly true, or almost certainly false. We show that almost certainly true answers are precisely those returned by the naive evaluation of the query. When restrictions are imposed and databases are required to satisfy constraints, the measure is the conditional probability of the query being true if the constraints are true. This too is defined as a limit; we prove that it always exists, can be an arbitrary rational number, and is computable. For some constraints, such as functional dependencies, the 0-1 law continues to hold. We also look at evaluation procedures based on many-valued logics, as used in relational database systems that handle incomplete information. We identify conditions when such evaluation procedures return almost certainly true answers, and explain reasons why real-life DBMSs break such conditions and can thus return arbitrarily bad answers. As another refinement of the notion of certainty, we introduce a comparison of query answers: an answer with a larger set of interpretations that make it true is better. We identify the precise complexity of such comparisons, and of finding sets of best answers, for first-order queries.

Deterministic Factorization of Sparse Polynomials with Bounded Individual Degree

In this paper we study the problem of deterministic factorization of sparse polynomials. We show that if f ? F[x1, x2, ... , xn] is a polynomial with s monomials, with individual degrees of its variables bounded by d, then f can be deterministically factored in time spoly(d)·log (n). Prior to our work, the only efficient factoring algorithms known for this class of polynomials were randomized, and other than for the cases of d=1 and d=2, only exponential time deterministic factoring algorithms were known. A crucial ingredient in our proof is a quasi-polynomial sparsity bound for factors of sparse polynomials of bounded individual degree. In particular we show if f is an s-sparse polynomial in n variables, with individual degrees of its variables bounded by d, then the sparsity of each factor of f is bounded by sO(d^2·log(n) ). This is the first nontrivial bound on factor sparsity for d>2. Our sparsity bound uses techniques from convex geometry, such as the theory of Newton polytopes and an approximate version of the classical Carathéodory's Theorem. Our work addresses and partially answers a question of von zur Gathen and Kaltofen (JCSS 1985) who asked whether a quasi-polynomial bound holds for the sparsity of factors of sparse polynomials.

Matrix Multiplication, a Little Faster

Strassen's algorithm (1969) was the first sub-cubic matrix multiplication algorithm. Winograd (1971) improved the leading coefficient of its complexity from 6 to 7. Many asymptotic improvements followed. Unfortunately, most of them have done so at the cost of very large, often gigantic, hidden constants. Consequently, Strassen-Winograd's $O\left(n^{\log_{2}7}\right)$ algorithm often outperforms other fast matrix multiplication algorithms for all feasible matrix dimensions. The leading coefficient of Strassen-Winograd's algorithm was believed to be optimal for matrix multiplication algorithms with $2\times2$ base case, due to a lower bound by Probert (1976). Surprisingly, we obtain a faster matrix multiplication algorithm, with the same base case size and asymptotic complexity as Strassen-Winograd's algorithm, but with the leading coefficient reduced from 6 to 5. To this end, we extend Bodrato's (2010) method for matrix squaring, and transform matrices to an alternative basis. We prove a generalization of Probert's lower bound that holds under change of basis, showing that for matrix multiplication algorithms with a $2\times2$ base case, the leading coefficient of our algorithm cannot be further reduced, hence optimal. We apply our method to other fast matrix multiplication algorithms, improving their arithmetic and communication costs by significant constant factors.

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