The modal µ-calculus caught off guard

  • Authors:
  • Oliver Friedmann;Martin Lange

  • Affiliations:
  • Dept. of Computer Science, University of Munich, Germany;Dept. of Elect. Eng. and Computer Science, University of Kassel, Germany

  • Venue:
  • TABLEAUX'11 Proceedings of the 20th international conference on Automated reasoning with analytic tableaux and related methods
  • Year:
  • 2011

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Abstract

The modal µ-calculus extends basic modal logic with second-order quantification in terms of arbitrarily nested fixpoint operators. Its satisfiability problem is EXPTIME-complete. Decision procedures for the modal µ-calculus are not easy to obtain though since the arbitrary nesting of fixpoint constructs requires some combinatorial arguments for showing the well-foundedness of least fixpoint unfoldings. The tableau-based decision procedures so far also make assumptions on the unfoldings of fixpoint formulas, e.g. explicitly require formulas to be in guarded normal form. In this paper we present a tableau calculus for deciding satisfiability of arbitrary, i.e. not necessarily guarded µ-calculus formulas. The novel contribution is a new unfolding rule for greatest fixpoint formulas which shows how to handle unguardedness without an explicit transformation into guarded form, thus avoiding a (seemingly) exponential blow-up in formula size. We prove soundness and completeness of the calculus, and discuss its advantages over existing approaches.