Classifying toposes for non-geometric theories

Abstract

The classifying topos of a geometric theory is a topos such that geometric morphisms into it correspond to models of that theory. We study classifying toposes for different infinitary logics: first-order, sub-first-order (i.e. geometric logic plus implication) and classical. For the first two, the corresponding notion of classifying topos is given by restricting the use of geometric morphisms to open and sub-open geometric morphisms respectively. For the last one, we restrict ourselves to Boolean toposes instead. Butz and Johnstone proved that the first-order classifying topos of an infinitary first-order theory exists precisely when that theory does not have too many infinitary first-order formulas, up to intuitionistically provable equivalence. We prove similar statements for the sub-first-order and Boolean case. Along the way we obtain completeness results of infinitary sub-first-order logic and infinitary classical logic with respect to (Boolean) toposes.