Adjoint Jordan Blocks

Abstract

Let G be a quasisimple algebraic group over an algebraically closed field of characteristic p>0. We suppose that p is very good for G; since p is good, there is a bijection between the nilpotent orbits in the Lie algebra and the unipotent classes in G. If the nilpotent X in Lie(G) and the unipotent u in G correspond under this bijection, and if u has order p, we show that the partitions of ad(X) and Ad(u) are the same. When G is classical or of type G_2, we prove this result with no assumption on the order of u. In the cases where u has order p, the result is achieved through an application of results of Seitz concerning good A_1 subgroups of G. For classical groups, the techniques are more elementary, and they lead also to a new proof of the following result of Fossum: the structure constants of the representation ring of a 1-dimensional formal group law F are independent of F.