Rule extensions of the implication--negation fragment of intuitionistic logic

Abstract

We present an algebraic study of extensions of the implication--negation fragment $\mathrm{IPC}_{\to,\neg}$ of intuitionistic propositional logic. Such extensions by inference rules correspond to subquasivarieties of the variety $\var{Ho}$ of Hilbert algebras with zero. We describe the lattice of such extensions of $\mathrm{IPC}_{\to,\neg}$ by means of its generators: it is isomorphic to the lattice of upsets of a countable poset, and in particular it is completely distributive. Consequently, $\var{Ho}$ is not Q-universal, in contrast to several other varieties related to $\mathrm{IPC}$. Nevertheless, this lattice is very complex: at each suitably defined height it has intervals containing continuum many quasivarieties which are not varieties. To the best of our knowledge $\mathrm{IPC}_{\to,\neg}$ is the only natural fragment of intuitionistic logic known to combine a completely distributive lattice of extensions with continuum many non-axiomatic extensions.