2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/132725This paper has been withdrawn by the author; its content is properly cantained in the paper arXiv:0706.4447, entitled "Pure motives, mixed motives and extensions of motives associated to singular surfaces", and submitted on June 29, 2007.This paper has been withdrawnAlgebraic GeometryK-Theory and Homologytext