Weak del Pezzo surfaces with irregularity
Abstract
Description
I construct normal del Pezzo surfaces, and regular weak del Pezzo surfaces as well, with positive irregularity q>0. Such things can happen only over nonperfect fields. The surfaces in question are twisted forms of nonnormal del Pezzo surfaces, which were classified by Reid. The twisting is with respect to the flat topology and infinitesimal group scheme actions. The twisted surfaces appear as generic fibers for Fano--Mori contractions on certain threefolds with only canonical singularities.
32 pages, minor changes, to appear in Tohoku Math. J
32 pages, minor changes, to appear in Tohoku Math. J