Complexity Classifications for Propositional Abduction in Post’s Framework∗ Nadia Creignou1, Johannes Schmidt1, Michael Thomas2 1 LIF, UMR CNRS 6166, Aix-Marseille Universit´e 163, Avenue de Luminy, 13288 Marseille Cedex 9, France
[email protected] [email protected] 2 Institut f¨ur Theoretische Informatik, Gottfried Wilhelm Leibniz Universit¨at Appelstr. 4, 30167 Hannover, Germany
[email protected] Abstract. In this paper we investigate the complexity of abduction, a fundamental and important form of non-monotonic reasoning. Given a knowledge base explaining the world’s behavior it aims at finding an explanation for some observed manifestation. In this paper we consider propositional abduction, where the knowledge base and the manifesta- tion are represented by propositional formulae. The problem of deciding p whether there exists an explanation has been shown to be Σ2 -complete in general. We focus on formulae in which the allowed connectives are taken from certain sets of Boolean functions. We consider different vari- ants of the abduction problem in restricting both the manifestations and the hypotheses. For all these variants we obtain a complexity classifica- tion for all possible sets of Boolean functions. In this way, we identify easier cases, namely NP-complete, coNP-complete and polynomial cases. Thus, we get a detailed picture of the complexity of the propositional abduction problem, hence highlighting sources of intractability. Further, we address the problem of counting the explanations and draw a com- plete picture for the counting complexity. Keywords: abduction, computational complexity, Post’s lattice, propo- sitional logic, boolean connective arXiv:1006.4923v1 [cs.CC] 25 Jun 2010 ∗ Supported by ANR Algorithms and complexity 07-BLAN-0327-04 and DFG grant VO 630/6-1.