2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/74102These notes reproduce the content of a short, 50-minutes, survey talk given at the Nice University in September, 2004. We added a few topics that have not been touched on in the lecture by lack of time.14 pages, few style changes and additional remarksAlgebraic Geometry14P25; 14J10text