Projective unification in weakly transitive and weakly symmetric modal logics

Abstract

Abstract We prove that $\textbf {K4}^{\textbf {n+}}\textbf {B}^{\textbf {k+}}$ has projective unification, for any $k,n\geq 1$. It means, in particular, that any weakly transitive and weakly symmetric modal logic is unitary. Some consequences of projective unification concerning structural completeness and passive rules are provided.