All normal extensions of S5-squared are finitely axiomatizable
Nick Bezhanishvili, Ian Hodkinson

Abstract:

We prove that every normal extension of the bi-modal system 
${\bf S5}^2$ is finitely axiomatizable and that every proper normal
extension has NP-complete satisfiability problem.

Keywords: S5-squared, normal extensions, finite axiomatization,
better-quasi-orederings