Topological Complexity of omega-Powers: Extended Abstract

Authors Olivier Finkel, Dominique Lecomte



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Olivier Finkel
Dominique Lecomte

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Olivier Finkel and Dominique Lecomte. Topological Complexity of omega-Powers: Extended Abstract. In Topological and Game-Theoretic Aspects of Infinite Computations. Dagstuhl Seminar Proceedings, Volume 8271, pp. 1-9, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2008)
https://doi.org/10.4230/DagSemProc.08271.7

Abstract

The operation of taking the omega-power $V^omega$ of a language $V$ is a fundamental operation over finitary languages leading to omega-languages. Since the set $X^omega$ of infinite words over a finite alphabet $X$ can be equipped with the usual Cantor topology, the question of the topological complexity of omega-powers of finitary languages naturally arises and has been posed by Damian Niwinski (1990), Pierre Simonnet (1992), and Ludwig Staiger (1997). We investigate the topological complexity of omega-powers. We prove the following very surprising results which show that omega-powers exhibit a great opological complexity: for each non-null countable ordinal $xi$, there exist some $Sigma^0_xi$-complete omega-powers, and some $Pi^0_xi$-complete omega-powers. On the other hand, the Wadge hierarchy is a great refinement of the Borel hierarchy, determined by Bill Wadge. We show that, for each ordinal $xi$ greater than or equal to 3, there are uncountably many Wadge degrees of omega-powers of Borel rank $xi +1$. Using tools of effective descriptive set theory, we prove some effective versions of the above results.
Keywords
  • Infinite words
  • omega-languages
  • omega-powers
  • Cantor topology
  • topological complexity
  • Borel sets
  • Borel ranks
  • complete sets
  • Wadge hierarchy
  • Wadge

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