Bounding extensions for finite groups and Frobenius kernels

Abstract

Let G be a simple, simply connected algebraic group defined over an algebraically closed field k of positive characteristic p. Let σ:G->G be a strict endomorphism (i. e., the subgroup G(σ) of σ-fixed points is finite). Also, let G_σ be the scheme-theoretic kernel of σ, an infinitesimal subgroup of G. This paper shows that the degree m cohomology H^m(G(σ),L) of any irreducible kG(σ)-module L is bounded by a constant depending on the root system Φ of G and the integer m. A similar result holds for the degree m cohomology of G_σ. These bounds are actually established for the degree m extension groups Ext^m_{G(σ)}(L,L') between irreducible kG(σ)-modules L and L', with again a similar result holding for G_σ. In these Ext^m results, of interest in their own right, the bounds depend also on L, or, more precisely, on length of the p-adic expansion of the highest weight associated to L. All bounds are, nevertheless, independent of the characteristic p. These results extend earlier work of Parshall and Scott for rational representations of algebraic groups G. We also show that one can find bounds independent of the prime for the Cartan invariants of G(σ) and G_σ, and even for the lengths of the underlying PIMs. These bounds, which depend only on the root system of G and the "height" of σ, provide in a strong way an affirmative answer to a question of Hiss, for the special case of finite groups G(σ) of Lie type in the defining characteristic.