On the succinctness of some modal logics

  • Authors:
  • Tim French;Wiebe Van Der Hoek;Petar Iliev;Barteld Kooi

  • Affiliations:
  • School of Computer Science and Software Engineering, University of Western Australia, Australia;Department of Computer Science, University of Liverpool, UK;Department of Computer Science, University of Liverpool, UK;Faculty of Philosophy, University of Groningen, The Netherlands

  • Venue:
  • Artificial Intelligence
  • Year:
  • 2013

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Abstract

One way of comparing knowledge representation formalisms that has attracted attention recently is in terms of representational succinctness, i.e., we can ask whether one of the formalisms allows for a more 'economical' encoding of information than the other. Proving that one logic is more succinct than another becomes harder when the underlying semantics is stronger. We propose to use Formula Size Games (as put forward by Adler and Immerman (2003) [1], but we present them as games for one player, called Spoiler), games that are played on two sets of models, and that directly link the length of a play in which Spoiler wins the game with the size of a formula, i.e., a formula that is true in the first set of models but false in all models of the second set. Using formula size games, we prove the following succinctness results for m-dimensional modal logic, where one has a set I={i"1,...,i"m} of indices for m modalities: (1) on general Kripke models (and also on binary trees), a definition [@?"@C]@f=@?"i"@?"@C[i]@f (with @C@?I) makes the resulting logic exponentially more succinct for m1; (2) several modal logics use such abbreviations [@?"@C]@f, e.g., in description logics the construct corresponds to adding role disjunctions, and an epistemic interpretation of it is 'everybody in @C knows'. Indeed, we show that on epistemic models (i.e., S"5-models), the logic with [@?"@C]@f becomes more succinct for m3; (3) the results for the logic with 'everybody knows' also hold for a logic with 'somebody knows', and (4) on epistemic models, Public Announcement Logic is exponentially more succinct than epistemic logic, if m3. The latter settles an open problem raised by Lutz (2006) [18].