Uniform Lyndon Interpolation for Basic Non-normal Modal Logics

Abstract

In this paper, a proof-theoretic method to prove uniform Lyndon interpolation for non-normal modal logics is introduced and applied to show that the logics E, M, MC, EN, MN have that property. In particular, these logics have uniform interpolation. Although for some of them the latter is known, the fact that they have uniform Lyndon interpolation is new. Also, the proof-theoretic proofs of these facts are new, as well as the constructive way to explicitly compute the interpolants that they provide. It is also shown that the non-normal modal logics EC and ECN do not have Craig interpolation, and whence no uniform (Lyndon) interpolation.