Coverages and Grothendieck Toposes

Abstract

These notes detail the basics of the theory of Grothendieck toposes from the viewpoint of coverages. Typically one defines a site as a (small) category equipped with a Grothendieck topology. However, it is often desirable to generate a Grothendieck topology from a smaller structure, such as a Grothendieck pretopology, but these require some pullbacks to exist in your underlying category. There is an even more light-weight structure one can generate a Grothendieck topology from called a coverage. Coverages don't require any limits or colimits to exist in the underlying category. We prove in detail several results about coverages, such as closing coverages under refinement and composition, to be what we call a saturated coverage, which doesn't change its category of sheaves. We show that Grothendieck topologies are in bijection with saturated coverages. We give an explicit description of the saturated coverage and the Grothendieck topology generated from a coverage. We furthermore give a readable account of some of the most important parts of Grothendieck topos theory, with an emphasis placed on coverages. These include constructing sheafification using the plus construction and also in ``one go,'' the equivalence between left exact localizations of presheaf toposes and saturated coverages, morphisms of sites using the fully general notion of covering flatness, points of a Grothendieck topos and Giraud's theorem. We show that Giraud's theorem is equivalent to Rezk's notion of weak descent. Also included is a section devoted to many examples of sites and Grothendieck toposes appearing in the literature, and appendices covering set theory and category theory background, localization and locally presentable categories.