Computability

About: Computability is a research topic. Over the lifetime, 2829 publications have been published within this topic receiving 85162 citations.

On Confluence in the pi-Calculus

Abstract: An account of the basic theory of confluence in the π-calculus is presented, techniques for showing confluence of mobile systems are given, and the utility of some of the theory presented is illustrated via an analysis of a distributed algorithm

Complexity analysis by graph rewriting

Abstract: Recently, many techniques have been introduced that allow the (automated) classification of the runtime complexity of term rewrite systems (TRSs for short). In this paper we show that polynomial (innermost) runtime complexity of TRSs induces polytime computability of the functions defined. In this way we show a tight correspondence between the number of steps performed in a given rewrite system and the computational complexity of an implementation of rewriting. The result uses graph rewriting as a first step towards the implementation of term rewriting. In particular, we prove the adequacy of (innermost) graph rewriting for (innermost) term rewriting.

Sequences close to periodic

Abstract: This paper is a survey of concepts and results connected with generalizations of the notion of a periodic sequence, both classical and new. The topics discussed relate to almost periodicity in such areas as combinatorics on words, symbolic dynamics, expressibility in logical theories, computability, Kolmogorov complexity, and number theory. Bibliography: 124 titles.

A generalized asynchronous computability theorem

Abstract: We consider the models of distributed computation defined as subsets of the runs of the iterated immediate snapshot model. Given a task $T$ and a model $M$, we provide topological conditions for $T$ to be solvable in $M$. When applied to the wait-free model, our conditions result in the celebrated Asynchronous Computability Theorem (ACT) of Herlihy and Shavit. To demonstrate the utility of our characterization, we consider a task that has been shown earlier to admit only a very complex $t$-resilient solution. In contrast, our generalized computability theorem confirms its $t$-resilient solvability in a straightforward manner.

Dichotomies in equilibrium computation, and complementary pivot algorithms for a new class of non-separable utility functions

Abstract: After more than a decade of work in TCS on the computability of market equilibria, complementary pivot algorithms have emerged as the best hope of obtaining practical algorithms. So far they have been used for markets under separable, piecewise-linear concave (SPLC) utility functions [23] and SPLC production sets [25]. Can his approach extend to non-separable utility functions and production sets? A major impediment is rationality, i.e., if all parameters are set to rational numbers, there should be a rational equilibrium. Recently, [35] introduced classes of non-separable utility functions and production sets, called Leontief-free, which are applicable when goods are substitutes. For markets with these utility functions and production sets, and satisfying mild sufficiency conditions, we obtain the following results: • Proof of rationality. • Complementary pivot algorithms based on a suitable adaptation of Lemke's classic algorithm. • A strongly polynomial bound on the running time of our algorithms if the number of goods is a constant, despite the fact that the set of solutions is disconnected. • Experimental verification, which confirms that our algorithms are practical. • Proof of PPAD-completeness. Next we give a proof of membership in FIXP for markets under piecewise-linear concave (PLC) utility functions and PLC production sets by capturing equilibria as fixed points of a continuous function via a nonlinear complementarity problem (NCP) formulation. Finally we provide, for the first time, dichotomies for equilibrium computation problems, both Nash and market; in particular, the results stated above play a central role in arriving at the dichotomies for exchange markets and for markets with production. We note that in the past, dichotomies have played a key role in bringing clarity to the complexity of decision and counting problems.