A fibrational study of realizability toposes

Abstract

This is the author's PhD thesis. It is a contribution to categorical logic, in particular to the theory of realizability toposes. While the tools of categorical logic have proven very successful in analyzing and organizing proof theoretic realizability interpretations, it was remarked by experts (notably Peter Johnstone) that the field of realizability toposes itself was not clearly delineated, and lacked a powerful theory analogous to that of Grothendieck toposes. The present work sets out to remedy this situation to a certain extent. We argue that realizability toposes are best understood using Grothendieck fibrations, and develop a framework of fibrational cocompletions, which allows to view certain constructions from realizability in precise analogy to constructions of presheaf and sheaf toposes. Using these techniques, and a class of posetal fibrations that we call uniform preorders, we are able to give an extensional characterization of partial combinatory algebras and of the realizability toposes that are constructed from these algebras. Striving to develop the analogy to Grothendieck toposes further, we outline how to apply our techniques on arbitrary base toposes, and give a decomposition theorem for constant objects functors induced by triposes, analogous to the known decompositions of geometric morphisms. Finally, we sketch an approach of how to find a unified framework for Grothendieck toposes and realizability toposes, based on the observation that uniform preorders can be identified with preorders internal to a category of sheaves.