An omega-Power of a Finitary Language Which is a Borel Set of Infinite Rank

Olivier Finkel (1)

1 ELM - Équipe de Logique Mathématique

Omega-powers of finitary languages are omega languages in the form V^omega, where V is a finitary language over a finite alphabet X. Since the set of infinite words over X can be equipped with the usual Cantor topology, the question of the topological complexity of omega-powers naturally arises and has been raised by Niwinski, by Simonnet, and by Staiger. It has been recently proved that for each integer n > 0 , there exist some omega-powers of context free languages which are Pi^0_n-complete Borel sets, and that there exists a context free language L such that L^omega is analytic but not Borel. But the question was still open whether there exists a finitary language V such that V^omega is a Borel set of infinite rank. We answer this question in this paper, giving an example of a finitary language whose omega-power is Borel of infinite rank.

Mots clés: Infinite words; omega-languages; omega-powers; Cantor topology; topological complexity; Borel sets; infinite rank

Domaines: Logique en informatique [cs.LO]; Logique [math.LO]

Olivier Finkel. An omega-Power of a Finitary Language Which is a Borel Set of Infinite Rank. Fundamenta Informaticae, 2004, 62 (3-4), pp.333-342. ⟨hal-00108219v2⟩