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Semantic characterization of Kracht formulas

Stanislav Kikot



Abstract: Kracht formulas are first-order correspondents of modal Sahlqvist formulas. In this paper we present a model-theoretic characterization of Kracht formulas similar to Van Benthem's theorem saying that a first-order formula is equivalent to a modal formula iff it is invariant under bisimulation. Our characterization yields a method to prove that a given first-order formula is not equivalent to any Kracht formula. In particular, we prove that the first-order formula, expressing the ‘cubic property’ of a 3-dimensional modal frame does not have a Kracht equivalent.


© Steklov Math. Inst. of RAS, 2024