An algebraic study of S5-modal Gödel logic

Abstract: In this paper we continue the study of the variety $\mathbb{MG}$ of monadic G\"odel algebras. These algebras are the equivalent algebraic semantics of the S5-modal expansion of G\"odel logic, which is equivalent to the one-variable monadic fragment of first-order G\"odel logic. We show three families of locally finite subvarieties of $\mathbb{MG}$ and give their equational bases. We also introduce a topological duality for monadic G\"odel algebras and, as an application of this representation theorem, we characterize congruences and give characterizations of the locally finite subvarieties mentioned above by means of their dual spaces. Finally, we study some further properties of the subvariety generated by monadic G\"odel chains: we present a characteristic chain for this variety, we prove that a Glivenko-type theorem holds for these algebras and we characterize free algebras over $n$ generators.