An Arad and Fisman's theorem on products of conjugacy classes revisited

Abstract

A theorem of Z. Arad and E. Fisman establishes that if $A$ and $B$ are two conjugacy classes of a finite group $G$ such that either $AB=A\cup B$ or $AB=A^{-1}\cup B$, then $G$ cannot be non-abelian simple. We demonstrate that, in fact, $⟨A⟩=⟨B⟩$ is solvable, the elements of $A$ and $B$ are $p$-elements for some prime $p$, and $⟨A⟩$ is $p$-nilpotent. Moreover, under the second assumption, it turns out that $A=B$ and this is the only possible case. This research is done by appealing to recently developed techniques and results that are based on the Classification of Finite Simple Groups.