Ga-actions on affine cones

Abstract

We give a criterion of existence of a unipotent group action on the affine cone over a projective variety or, more generally, on the affine quasicone over a variety which is projective over another affine variety.

16p. In a formula in the proof of Lemma 2.3, the direct sum was replaced by the usual sum . The authors are grateful to Kevin Langlois for mentioning this inaccuracy