Structure Theorem and Strict Alternation Hierarchy for FO² on Words

Authors Philipp Weis, Neil Immerman

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Philipp Weis
Neil Immerman

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Philipp Weis and Neil Immerman. Structure Theorem and Strict Alternation Hierarchy for FO² on Words. In Circuits, Logic, and Games. Dagstuhl Seminar Proceedings, Volume 6451, pp. 1-22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2007)


It is well-known that every first-order property on words is expressible using at most three variables. The subclass of properties expressible with only two variables is also quite interesting and well-studied. We prove precise structure theorems that characterize the exact expressive power of first-order logic with two variables on words. Our results apply to FO$^2[<]$ and FO$^2[<,suc]$, the latter of which includes the binary successor relation in addition to the linear ordering on string positions. For both languages, our structure theorems show exactly what is expressible using a given quantifier depth, $n$, and using $m$ blocks of alternating quantifiers, for any $mleq n$. Using these characterizations, we prove, among other results, that there is a strict hierarchy of alternating quantifiers for both languages. The question whether there was such a hierarchy had been completely open since it was asked in [Etessami, Vardi, and Wilke 1997].
  • Descriptive complexity
  • finite model theory
  • alternation hierarchy
  • Ehrenfeucht-Fraisse games


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