First-order and counting theories of ω-automatic structures

  • Authors:
  • Dietrich Kuske;Markus Lohrey

  • Affiliations:
  • Institut für Informatik, Universität Leipzig, Germany;Universität Stuttgart, FMI, Germany

  • Venue:
  • FOSSACS'06 Proceedings of the 9th European joint conference on Foundations of Software Science and Computation Structures
  • Year:
  • 2006

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Abstract

The logic ${\mathcal L}({\mathcal Q}_u)$ extends first-order logic by a generalized form of counting quantifiers (“the number of elements satisfying ... belongs to the set C”). This logic is investigated for structures with an injective ω-automatic presentation. If first-order logic is extended by an infinity-quantifier, the resulting theory of any such structure is known to be decidable [4]. It is shown that, as in the case of automatic structures [13], also modulo-counting quantifiers as well as infinite cardinality quantifiers (“there are $\varkappa$ many elements satisfying ...”) lead to decidable theories. For a structure of bounded degree with injective ω-automatic presentation, the fragment of ${\mathcal L}({\mathcal Q}_u)$ that contains only effective quantifiers is shown to be decidable and an elementary algorithm for this decision is presented. Both assumptions (ω-automaticity and bounded degree) are necessary for this result to hold.