Birational automorphisms of a three-dimensional double quadric with an elementary singularity
arXiv:math/9808073 · doi:10.1070/SM1998v189n01ABEH000294
Abstract
It is proved that the group of birational automorphisms of a three-dimensional double quadric with a singular point arising from a double point on the branch divisor is a semidirect product of the free group generated by birational involutions of a special form and the group of regular automorphisms. The proof is based on the method of `untwisting' maximal singularities of linear systems.
18 pages