Elementary Multimodal Logics

09/10/2018

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by   Jakub Michaliszyn, et al.

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We study multimodal logics over universally first-order definable classes of frames. We show that even for bimodal logics, there are universal Horn formulas that define set of frames such that the satisfiability problem is undecidable, even if one or two of the binary relations are transitive.