Abstract
The alternation hierarchy in two-variable first-order logic FO2[ < ] over words was recently shown to be decidable by Kufleitner and Weil, and independently by Krebs and Straubing. In this paper we consider a similar hierarchy, reminiscent of the half levels of the dot-depth hierarchy or the Straubing-Thérien hierarchy. The fragment \(\Sigma^2_m\) of FO2 is defined by disallowing universal quantifiers and having at most m − 1 nested negations. One can view \(\Sigma^2_m\) as the formulas in FO2 which have at most m blocks of quantifiers on every path of their parse tree, and the first block is existential. Thus, the m th level of the FO2-alternation hierarchy is the Boolean closure of \(\Sigma^2_m\). We give an effective characterization of \(\Sigma^2_m\), i.e., for every integer m one can decide whether a given regular language is definable by a two-variable first-order formula with negation nesting depth at most m. More precisely, for every m we give ω-terms U m and V m such that an FO2-definable language is in \(\Sigma^2_m\) if and only if its ordered syntactic monoid satisfies the identity U m ≤ V m . Among other techniques, the proof relies on an extension of block products to ordered monoids.
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Fleischer, L., Kufleitner, M., Lauser, A. (2014). Block Products and Nesting Negations in FO2 . In: Hirsch, E.A., Kuznetsov, S.O., Pin, JÉ., Vereshchagin, N.K. (eds) Computer Science - Theory and Applications. CSR 2014. Lecture Notes in Computer Science, vol 8476. Springer, Cham. https://doi.org/10.1007/978-3-319-06686-8_14
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DOI: https://doi.org/10.1007/978-3-319-06686-8_14
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