Projectivity meets Uniform Post-Interpolant: Classical and Intuitionistic Logic

We examine the interplay between projectivity (in the sense that was introduced by S. Ghilardi) and uniform post-interpolant for the classical and intuitionistic propositional logic. More precisely, we explore whether a projective substitution of a formula is equivalent to its uniform post-interpolant, assuming the substitution leaves the variables of the interpolant unchanged. We show that in classical logic, this holds for all formulas. Although such a nice property is missing in intuitionistic logic, we provide Kripke semantical characterisation for propositions with this property. As a main application of this, we show that the unification type of some extensions of intuitionistic logic are finitary. In the end, we study admissibility for intuitionistic logic, relative to some sets of formulae. The first author of this paper recently considered a particular case of this relativised admissibility and found it useful in characterising the provability logic of Heyting Arithmetic.