Elementary topology of champs

TL;DR

This paper develops a homotopy theory for topological stacks (champ), introducing a representability theorem, a homotopy 2-group, and applications to algebraic geometry, including generalizations of Lefschetz theorems.

Contribution

It provides a homotopy complement to Giraud's work, establishing a framework for homotopy groups, covering spaces, and the homotopy 2-group for champs, with applications to algebraic geometry.

Findings

Representability theorem for maps to topological champs.

Construction of the homotopy 2-group $\

,

Abstract

Broadly speaking the present is a homotopy complement to the book of Giraud, albeit in a couple of different ways. In the first place there is a representability theorem for maps to a topological champ (a.k.a. stack) and whence an extremely convenient global atlas, i.e. the path space, which permits an immediate importation of the familiar definitions of homotopy groups and covering spaces as encountered in elementary text books. In the second place, it provides the adjoint to Giraud's co-homology, i.e. the homotopy 2-group $\Pi_2$, by way of the 2-Galois theory of covering champs. In the sufficiently path connected case this is achieved by much the same construction employed in constructing 1-covers, i.e. quotients of the path space by a groupoid. In the general case,so inter alia the pro-finite theory appropriate for algebraic geometry, the development parallels the axiomatic Galois theory of SGA1. The resulting explicit description of the homotopy 2-type can be applied to prove theorems in algebraic geometry: optimal generalisations to $\Pi_2$ (by a very different method, which even gives improvements to the original case) of the Lefschetz theorems (over a locally Noetherian base) of SGA2, and a counterexample to the extension from co-homology to homotopy of the smooth base change theorem. These limited goals are achieved, albeit arguably at the price of obscuring the higher categorical structure, without leaving the 2-category of groupoids.