2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/163792We examine the proof of a classical localization theorem of Bousfield and Friedlander and we remove the assumption that the underlying model category be right proper. The key to the argument is a lemma about factoring in morphisms in the arrow category of a model category.A small mistake in the published version is fixedAlgebraic TopologyNote on a theorem of Bousfield and Friedlandertext