Authors: V Chernousov P Gille Z Reichstein
Publish Date: 2008/03/26
Volume: 126, Issue: 4, Pages: 465-480
Abstract
Let G be a reductive affine group scheme defined over a semilocal ring k Assume that either G is semisimple or k is normal and noetherian We show that G has a finite ksubgroup S such that the natural map H 1R S → H 1R G is surjective for every semilocal ring R containing k In other words Gtorsors over SpecR admit reduction of structure to S We also show that the natural map H 1X S → H 1X G is surjective in several other contexts under suitable assumptions on the base ring k the scheme X/k and the group scheme G/k These results have already been used to study loop algebras and essential dimension of connected algebraic groups in prime characteristic Additional applications are presented at the end of this paper
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