(Non-)well-founded derivations in the provability logic $\mathsf{GLP}$

Abstract

We examine cyclic, non-well-founded and well-founded derivations in the provability logic $\mathsf{GLP}$. While allowing cyclic derivations does not change the system, the non-well-founded and well-founded derivations we consider define the same proper infinitary extension of $\mathsf{GLP}$. We establish that this extension is strongly algebraic and neighbourhood complete with respect to both local and global semantic consequence relations. In fact, these completeness results are proved for generalizations of global and local consequence relations, which we call global-local. In addition, we prove strong local neighbourhood completeness for the original system $\mathsf{GLP}$ (with ordinary derivations only).