Bisimulations and bisimulation quotients of generalized Veltman models

We consider interpretability logic, a modal description of the interpretability predicate, and try to determine the most suitable notion of bisimulation for generalized Veltman semantics. In the first part of this paper we consider several notions of bisimulation and determine connections between them. In the second part we develop some further model theoretic properties for generalized Veltman semantics and consider difficulties that arise when studying quotient structures.