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Level Two of the Quantifier Alternation Hierarchy Over Infinite Words

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Abstract

The study of various decision problems for logic fragments has a long history in computer science. This paper is on the membership problem for a fragment of first-order logic over infinite words; the membership problem asks for a given language whether it is definable in some fixed fragment. The alphabetic topology was introduced as part of an effective characterization of the fragment Σ2 over infinite words. Here, Σ2 consists of the first-order formulas with two blocks of quantifiers, starting with an existential quantifier. Its Boolean closure is \(\mathbb {B}{\Sigma }_{2}\). Our first main result is an effective characterization of the Boolean closure of the alphabetic topology, that is, given an ω-regular language L, it is decidable whether L is a Boolean combination of open sets in the alphabetic topology. This is then used for transferring Place and Zeitoun’s recent decidability result for \(\mathbb {B}{\Sigma }_{2}\) from finite to infinite words.

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Notes

  1. During the preparation of this submission, we learned that Pierron, Place and Zeitoun [10] independently found another proof for the decidability of \(\mathbb {B}{\Sigma }_{2}\) over infinite words. For documenting the independency of the two proofs, we also include the technical report [7] of this paper in the list of references.

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Correspondence to Manfred Kufleitner.

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This work was supported by the German Research Foundation (DFG) under grants DI 435/5-2 and DI 435/6-1.

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Kufleitner, M., Walter, T. Level Two of the Quantifier Alternation Hierarchy Over Infinite Words. Theory Comput Syst 62, 467–480 (2018). https://doi.org/10.1007/s00224-017-9801-x

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