Jordan blocks of Richardson classes in the classical groups and the Bala--Carter Theorem

Abstract

This paper provides new, relatively simple proofs of some important results about unipotent classes in simple linear algebraic groups. We derive the formula for the Jordan blocks of the Richardson class of a parabolic subgroup of a classical group. This result was originally due to Spaltenstein. Secondly, we derive, for good characteristic, the description of the natural partial order of unipotent classes of a classical group in terms of their Jordan blocks. This result was originally due to Gerstenhaber and Hesselink. As a consequence we obtain a proof of the Bala--Carter Theorem which holds even in certain bad characteristics (this proof requires the prior classification of unipotent classes, unlike the original proofs due to Bala, Carter and Pommerening).