Elementary canonical formulae: extending Sahlqvist’s theorem

Annals of Pure and Applied Logic 141 (1):180-217 (2006)
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We generalize and extend the class of Sahlqvist formulae in arbitrary polyadic modal languages, to the class of so called inductive formulae. To introduce them we use a representation of modal polyadic languages in a combinatorial style and thus, in particular, develop what we believe to be a better syntactic approach to elementary canonical formulae altogether. By generalizing the method of minimal valuations à la Sahlqvist–van Benthem and the topological approach of Sambin and Vaccaro we prove that all inductive formulae are elementary canonical and thus extend Sahlqvist’s theorem over them. In particular, we give a simple example of an inductive formula which is not frame-equivalent to any Sahlqvist formula. Then, after a deeper analysis of the inductive formulae as set-theoretic operators in descriptive and Kripke frames, we establish a somewhat stronger model-theoretic characterization of these formulae in terms of a suitable equivalence to syntactically simpler formulae in the extension of the language with reversive modalities. Lastly, we study and characterize the elementary canonical formulae in reversive languages with nominals, where the relevant notion of persistence is with respect to discrete frames
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References found in this work BETA
A Sahlqvist Theorem for Distributive Modal Logic.Gehrke, Mai; Nagahashi, Hideo & Venema, Yde
Modal Logic with Names.Gargov, George & Goranko, Valentin
Topology and Duality in Modal Logic.Sambin, Giovanni & Vaccaro, Virginia

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Sahlqvist Correspondence for Modal Mu-Calculus.van Benthem, Johan; Bezhanishvili, Nick & Hodkinson, Ian

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