Decidability of Order-Based Modal Logics

Abstract Decidability of the validity problem is established for a family of many-valued modal logics, notably Godel modal logics, where propositional connectives are evaluated according to the order of values in a complete sublattice of the real unit interval [ 0 , 1 ] , and box and diamond modalities are evaluated as infima and suprema over (many-valued) Kripke frames. If the sublattice is infinite and the language is sufficiently expressive, then the standard semantics for such a logic lacks the finite model property. It is shown here, however, that, given certain regularity conditions, the finite model property holds for a new semantics for the logic, providing a basis for establishing decidability and PSPACE-completeness. Similar results are also established for S5 logics that coincide with one-variable fragments of first-order many-valued logics. In particular, a first proof is given of the decidability and co-NP-completeness of validity in the one-variable fragment of first-order Godel logic.

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