Completeness for monadic fuzzy logics via functional algebras

Abstract

We study S5-modal (monadic) expansions of extensions of Hájek's basic logic . Hájek proposed Hilbert-style systems axiomatizing these logics and we prove that completeness theorems for these logics follow from algebraic representation results, namely, functional representations of finitely subdirectly irreducible algebras. We prove a general theorem linking these concepts and give two major applications, namely, for the S5-modal expansions of Łukasiewicz and Gödel logics.