Degree of reductivity of a modular representation

Abstract

For a finite-dimensional representation V of a group G over a field F, the degree of reductivity δ(G,V) is the smallest degree d such that every nonzero fixed point υ ∈ VG/{0} can be separated from zero by a homogeneous invariant of degree at most d. We compute δ(G,V) explicitly for several classes of modular groups and representations. We also demonstrate that the maximal size of a cyclic subgroup is a sharp lower bound for this number in the case of modular abelian p-groups. © 2017 World Scientific Publishing Company.