On the expressive power of monadic least fixed point logic

  • Authors:
  • Nicole Schweikardt

  • Affiliations:
  • Institut für Informatik, Humboldt-Universität Berlin, Unter den Linden, Berlin, Germany

  • Venue:
  • Theoretical Computer Science - Automata, languages and programming: Logic and semantics (ICALP-B 2004)
  • Year:
  • 2006

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Abstract

Monadic least fixed point logic MLFP is a natural logic whose expressiveness lies between that of first-order logic FO and monadic second-order logic MSO. In this paper, we take a closer look at the expressive power of MLFP. Our results are: (1) MLFP can describe graph properties beyond any fixed level of the monadic second-order quantifier alternation hierarchy. (2) On strings with built-in addition, MLFP can describe at least all languages that belong to the linear time complexity class DLIN. (3) Settling the question whether addition-invariant MLFP = addition-invariant MSO on finite strings or, equivalently, settling the question whether MLFP = MSO on finite strings with addition would solve open problems in complexity theory: "=" would imply that PH = PTIME whereas "≠" would imply that DLIN ≠ LINH. Apart from this we give a self-contained proof of the previously known result that MLFP is strictly less expressive than MSO on the class of finite graphs.