Toposes with enough points as categories of étale spaces
Umberto Tarantino, IRIF
As originally showed by Barr, a topology on a set X can be equivalently described as a ‘convergence relation’ between elements of X and ultrafilters on X: in other words, a spatial locale can be recovered from its set of points once it is endowed with appropriate extra structure defined in terms of ultrafilters. In this talk, I will present a similar reconstruction result for (Grothendieck) toposes with enough points, a categorification of spatial locales: every such topos can be recovered up to equivalence from its category of points, provided that the latter is endowed with appropriate extra structure involving ultrafilters. In logical terms, this reads as a (strong) conceptual completeness theorem for geometric logic. Towards this goal, I will introduce ultraconvergence spaces, a profunctorial generalization of Makkai’s ultracategories inspired by Barr’s convergence relations. This talk is based on joint work with Quentin Aristote, Sam van Gool and Jérémie Marquès.