Miki Hermann

Complexity of generalized satisfiability counting problems

Abstract The class of generalized satisfiability problems, introduced in 1978 by Schaefer, presents a uniform way of studying the complexity of satisfiability problems with special conditions. The complexity of each decision and counting problem in this class depends on the involved logical relations. In 1979, Valiant defined the class #P, the class of functions definable as the number of accepting computations of a polynomial-time nondeterministic Turing machine. Clearly, all satisfiability counting problems belong to this class #P. We prove a Dichotomy Theorem for generalized satisfiability counting problems. That is, if all logical relations involved in a generalized satisfiability counting problem are affine then the number of satisfying assignments of this problem can be computed in polynomial time, otherwise this function is #P-complete. This gives us a comparison between decision and counting generalized satisfiability problems. We can determine exactly the polynomial satisfiability decision problems whose number of solutions can be computed in polynomial time and also the polynomial satisfiability decision problems whose counting counterparts are already #P-complete. Moreover, taking advantage of a similar dichotomy result proved in 1978 by Schaefer for generalized satisfiability decision problems, we get as a corollary the implication that the counting counterpart of each NP-complete generalized satisfiability decision problem is #P-complete.

Unification of infinite sets of terms schematized by primal grammars

Abstract Infinite sets of terms appear frequently at different places in computer science. On the other hand, several practically oriented parts of logic and computer science require the manipulated objects to be finite or finitely representable. Schematizations present a suitable formalism to manipulate finitely infinite sets of terms. Since schematizations provide a different approach to solve the same kind of problems as constraints do, they can be viewed as a new type of constraints. The paper presents a new recurrent schematization called primal grammars. The main idea behind the primal grammars is to use primitive recursion as the generating engine of infinite sets. The evaluation of primal grammars is based on substitution and rewriting, hence no particular semantics for them is necessary. This fact allows also a natural integration of primal grammars into Prolog, into functional languages or into other rewrite-based applications. Primal grammars have a decidable unification problem and the paper presents a unification algorithm for them that produces finite results. This unification algorithm is proved sound and complete, and it terminates for every input.

The Inference Problem for Propositional Circumscription of Affine Formulas Is coNP-Complete

We prove that the inference problem of propositional circumscription for affine formulas is coNP-complete, settling this way a longstanding open question in the complexity of nonmonotonic reasoning. We also show that the considered problem becomes polynomial-time decidable if only a single literal has to be inferred from an affine formula.

On the complexity of computing generators of closed sets

We investigate the computational complexity of some decision and counting problems related to generators of closed sets fundamental in Formal Concept Analysis. We recall results from the literature about the problem of checking the existence of a generator with a specified cardinality, and about the problem of determining the number of minimal generators. Moreover, we show that the problem of counting minimum cardinality generators is #ċcoNP-complete. We also present an incremental-polynomial time algorithm from relational database theory that can be used for computing all minimal generators of an implication-closed set.

An algebraic approach to the complexity of generalized conjunctive queries

Conjunctive-query containment is considered as a fundamental problem in database query evaluation and optimization. Kolaitis and Vardi pointed out that constraint satisfaction and conjunctive query containment are essentially the same problem. We study the Boolean conjunctive queries under a more detailed scope, where we investigate their counting problem by means of the algebraic approach through Galois theory, taking advantage of Post’s lattice. We prove a trichotomy theorem for the generalized conjunctive query counting problem, showing this way that, contrary to the corresponding decision problems, constraint satisfaction and conjunctive-query containment differ for other computational goals. We also study the audit problem for conjunctive queries asking whether there exists a frozen variable in a given query. This problem is important in databases supporting statistical queries. We derive a dichotomy theorem for this audit problem that sheds more light on audit applicability within database systems.

Chain properties of rule closures

This article introduces a generalisation of the crossed rule approach to the detection of Knuth-Bendix completion procedure divergence. It introduces closure chains, which are special rule closures constructed by means of particular substitution operations and operators, as a suitable formalism for progress in this direction. Supporting substitution algebra is developed first, followed by considerations concerning rule closures in general, concluding with an investigation of closure chain properties. Issues concerning the narrowing process are not discussed here.

The complexity of counting problems in equational matching

We introduce a class of counting problems that arise naturally in equational matching and study their computational complexity. If E is an equational theory, then #E-Matching is the problem of counting the number of complete minimal E-matchers of two given terms. #E-Matching is a well-defined algorithmic problem for every finitary equational theory. Moreover, it captures more accurately the computational difficulties associated with finding complete sets of minimal E-matchers than the corresponding decision problem for E-matching does.

Implementations of term rewriting systems

Two main applications of term rewriting systems are equational reasoning in theorem provers and equational computation in programming languages. The present paper examines a number of term rewriting systems in terms of how rewriting is used in different implementations of theorem provers, the use of rewriting techniques in logic programming languages and the problem of efficiency. In addition a non-exhaustive catalogue of distributed implementations is presented

On the counting complexity of propositional circumscription

Propositional circumscription, asking for the minimal models of a Boolean formula, is an important problem in artificial intelligence, in data mining, in coding theory, and in the model checking based procedures in automated reasoning. We consider the counting problems of propositional circumscription for several subclasses with respect to the structure of the formula. We prove that the counting problem of propositional circumscription for dual Horn, bijunctive, and affine formulas is #P-complete for a particular case of Turing reduction, whereas for Horn and 2affine formulas it is in FP. As a corollary, we obtain also the #P-completeness result for the counting problem of hypergraph transversal.

On the Relation Between Primitive Recursion, Schematization and Divergence

The paper presents a new schematization of infinite families of terms called the primal grammars, based on the notion of primitive recursive rewrite systems. This schematization is presented by a generating term and a canonical rewrite system. It is proved that the class of primal grammars covers completely the class of crossed rewrite systems. This proof contains a construction of a primal grammar from a crossed rewrite system.