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Complexity of super-coherence problems in ASP*

Published online by Cambridge University Press:  09 August 2013

MARIO ALVIANO
Affiliation:
University of Calabria, Rende (CS) 87036, Italy (e-mail: alviano@mat.unical.it, faber@mat.unical.it)
WOLFGANG FABER
Affiliation:
University of Calabria, Rende (CS) 87036, Italy (e-mail: alviano@mat.unical.it, faber@mat.unical.it)
STEFAN WOLTRAN
Affiliation:
Vienna University of Technology, Vienna 1040, Austria (e-mail: woltran@dbai.tuwien.ac.at)

Abstract

Adapting techniques from database theory in order to optimize Answer Set Programming (ASP) systems, and in particular the grounding components of ASP systems, is an important topic in ASP. In recent years, the Magic Set method has received some interest in this setting, and a variant of it, called Dynamic Magic Set, has been proposed for ASP. However, this technique has a caveat, because it is not correct (in the sense of being query-equivalent) for all ASP programs. In a recent work, a large fragment of ASP programs, referred to as super-coherent programs, has been identified, for which Dynamic Magic Set is correct. The fragment contains all programs which possess at least one answer set, no matter which set of facts is added to them. Two open question remained: How complex is it to determine whether a given program is super-coherent? Does the restriction to super-coherent programs limit the problems that can be solved? Especially the first question turned out to be quite difficult to answer precisely. In this paper, we formally prove that deciding whether a propositional program is super-coherent is Π3P-complete in the disjunctive case, while it is Π2P-complete for normal programs. The hardness proofs are the difficult part in this endeavor: We proceed by characterizing the reductions by the models and reduct models which the ASP programs should have, and then provide instantiations that meet the given specifications. Concerning the second question, we show that all relevant ASP reasoning tasks can be transformed into tasks over super-coherent programs, although this transformation is more of theoretical than practical interest.

Type
Regular Papers
Copyright
Copyright © Cambridge University Press 2013 

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Footnotes

*

Preliminary versions of this paper have been presented at the International Conference on Logic Programming (ICLP) workshop on Answer Set Programming and Other Computing Paradigms (ASPOCP) and at the Convegno Italiano di Logica Computazionale (CILC).

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