Failure of Interpolation in Combined Modal Logics
Maarten Marx, Carlos Areces

Abstract:
We investigate transfer of interpolation in such combinations of modal
logic which lead to interaction of the modalities.  Combining logics
by taking products often blocks transfer of interpolation.  The same
holds for combinations by taking unions, a generalization of
Humberstone's inaccessibility logic.  Viewing first order logic as a
product of modal logics, we derive a strong counterexample for failure
of interpolation in the finite variable fragments of first order
logic.  We provide a simple condition stated only in terms of frames
and bisimulations which implies failure of interpolation.  Its use is
exemplified in a wide range of cases.

To appear in `Notre Dame Journal of Formal Logic'.