On special quadratic birational transformations of a projective space

Abstract

A birational map from a projective space onto a not too much singular projective variety with a single irreducible non-singular base locus scheme (special birational transformation) is a rare enough phenomenon to allow meaningful and concise classification results. We shall concentrate on transformations defined by quadratic equations onto some varieties (especially projective hypersurfaces of small degree), where quite surprisingly the base loci are interesting projective manifolds appearing in other contexts; for example, exceptions for adjunction theory, small degree or small codimensional manifolds, Severi or more generally homogeneous varieties. In particular, we shall classify: quadro-quadric transformations into a quadric hypersurface; quadro-cubic transformations into a del Pezzo variety; transformations whose base locus (scheme) has dimension at most three.