Characterising equilibrium logic and nested logic programs: Reductions and complexity

TL;DR

This paper presents polynomial reductions of reasoning tasks in equilibrium logic and nested logic programs into quantified propositional logic, analyzing their computational complexity and providing encodings for various reasoning and equivalence testing tasks.

Contribution

It introduces polynomial reductions for key reasoning tasks in equilibrium and nested logic programs into quantified propositional logic, along with complexity analysis.

Findings

Reductions for consistency, brave, and skeptical reasoning tasks.

Encodings for theory and program equivalence testing.

Complexity bounds for all reasoning tasks.

Abstract

Equilibrium logic is an approach to nonmonotonic reasoning that extends the stable-model and answer-set semantics for logic programs. In particular, it includes the general case of nested logic programs, where arbitrary Boolean combinations are permitted in heads and bodies of rules, as special kinds of theories. In this paper, we present polynomial reductions of the main reasoning tasks associated with equilibrium logic and nested logic programs into quantified propositional logic, an extension of classical propositional logic where quantifications over atomic formulas are permitted. We provide reductions not only for decision problems, but also for the central semantical concepts of equilibrium logic and nested logic programs. In particular, our encodings map a given decision problem into some formula such that the latter is valid precisely in case the former holds. The basic tasks we deal with here are the consistency problem, brave reasoning, and skeptical reasoning. Additionally, we also provide encodings for testing equivalence of theories or programs under different notions of equivalence, viz. ordinary, strong, and uniform equivalence. For all considered reasoning tasks, we analyse their computational complexity and give strict complexity bounds.