Abstract
We establish the complexity of decision problems associated with the nonmonotonic modal logic S4. We prove that the problem of existence of an S4-expansion for a given set A of premises is Σ P2 -complete. Similarly, we show that for a given formula ϕ and a set A of premises, it is Σ P2 -complete to decide whether ϕ belongs to at least one S4-expansion for A, and it is Π P2 -complete to decide whether ϕ belongs to all S4-expansions for A. An interesting aspect of these results is that reasoning (testing satisfiability and provability) in the monotonic modal logic S4 is PSPACE-complete. To the best of our knowledge, the nonmonotonic logic S4 is the first example of a nonmonotonic formalism which is computationally easier than the monotonic logic that underlies it (assuming PSPACE does not collapse to Σ 2)P .
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© 1993 Springer-Verlag Berlin Heidelberg
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Schwarz, G., Truszczyński, M. (1993). Nonmonotonic reasoning is sometimes simpler. In: Gottlob, G., Leitsch, A., Mundici, D. (eds) Computational Logic and Proof Theory. KGC 1993. Lecture Notes in Computer Science, vol 713. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0022579
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DOI: https://doi.org/10.1007/BFb0022579
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