On quasi-reductive group schemes

Abstract

The paper was motivated by a question of Vilonen, and the main results have been used by Mirkovic and Vilonen to give a geometric interpretation of the dual group (as a Chevalley group over Z) of a reductive group. We define a quasi-reducitve group over a discrete valuation ring R to be an affine flat group scheme over R such that (i) the fibers are of finite type and of the same dimension; (ii) the generic fiber is smooth and connected, and (iii) the netural component of the reduced special fiber is a reductive group. We show that such a group scheme is of finite type over R, the generic fiber is a reductive group, the special fiber is connected, and the group scheme is smooth over R in most cases, for example when the residue characteristic is not 2, or when the generic fiber and reduced special fiber are of the same type as reductive groups. We also obtain results about group schemes over a Dedekind scheme or a noetherian scheme. We show that in residue characteristic 2 there are indeed non-smooth quasi-reductive groups and they can be classified when R is strictly henselian.