2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/148416This is an expository paper on the subject of the title. It assumes basic scheme theory, commutative and homological algebra.52 pages. Plain TeX file, with AMS fonts and Eplain macro package (included). Many typos have been corrected, and some material has been addedAlgebraic Geometrytext