The pseudofinite monadic second order theory of words

02/15/2022

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by   Deacon Linkhorn, et al.

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We analyse the pseudofinite monadic second order theory of words over a fixed finite alphabet. In particular we present an axiomatisation of this theory, working in a one-sorted first order framework. The analysis hinges on the fact that concatenation of words interacts nicely with monadic second order logic. More precisely, give a signature under which for each natural number k, equivalence of (monadic second order versions of) words with respect to formulas of quantifier depth at most k is a congruence for concatenation. We use our analysis to present an alternative proof of a theorem connecting recognisable languages and finitely generated free profinite monoids via extended Stone duality, due to Gehrke, Grigorieff, and Pin.