2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/136284We give a homotopy theoretic characterization of sheaves on a stack and, more generally, a presheaf of groupoids on an arbitary small site C. We use this to prove homotopy invariance and generalized descent statements for categories of sheaves and quasi-coherent sheaves. As a corollary we obtain an alternate proof of a generalized change of rings theorem of Hovey.20 pagesAlgebraic TopologyAlgebraic Geometry14A20, 18G55, 55U10text