2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/71890We describe smooth rational projective algebraic surfaces X, over an algebraically closed field of characteristic different from 2, having an even set of four disjoint (-2)-curves N_1,...,N_4, i.e. such that N_1+...+N_4 is divisible by 2 in Pic(X).9 pagesAlgebraic Geometry14J26text