On the homotopy theory of Grothendieck \infty-groupoids
We present a slight variation on a notion of weak \infty-groupoid introduced by Grothendieck in Pursuing Stacks and we study the homotopy theory of these \infty-groupoids. We prove that the obvious definition for homotopy groups of Grothendieck \infty-groupoids does not depend on any choice. This allows us to give equivalent characterizations of weak equivalences of Grothendieck \infty-groupoids, generalizing a well-known result for strict \infty-groupoids. On the other hand, given a model category M in which every object is fibrant, we construct, following Grothendieck, a fundamental \infty-groupoid functor Π_\infty from M to the category of Grothendieck \infty-groupoids. We show that if X is an object of M, then the homotopy groups of Π_\infty(X) and of X are canonically isomorphic. We deduce that the functor Π_\infty respects weak equivalences.