2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/113943Given a suitable functor T:C -> D between model categories, we define a long exact sequence relating the homotopy groups of any X in C with those of TX, and use this to describe an obstruction theory for lifting an object G in D to C. Examples include finding spaces with given homology or homotopy groups.28 pagesAlgebraic Topology55U35; 55P65, 55Q35, 18G55text