Finite groups admitting a dihedral group of automorphisms

Let D = alpha, beta be a dihedral group generated by the involutions alpha and beta and let F = alpha beta). Suppose that D acts on a finite group G by automorphisms in such a way that C-G(F)= 1. In the present paper we prove that the nilpotent, length of the group Cr' is equal to the maximum of the nilpotent lengths of the subgroups C-G (alpha) and C-G(beta).