2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/224432The first part is expository: it explains how finite fields may be used to prove theorems on infinite fields by a reduction mod p process. The second part gives a variant of P.Smith's fixed point theorem which applies in any characteristic.12 pages; to appear in Proc.Conference Marseille-Luminy 2007, Contemp. Math. series, AMS, 2009Algebraic Geometry14-XXtext