Finite groups whose prime graph on class sizes is a block square

Abstract

Let G be a finite group, and let be the prime graph built on its set of conjugacy class sizes: this is the (simple undirected) graph whose vertices are the prime numbers dividing some conjugacy class size of G, and two distinct vertices p, q are adjacent if and only if pq divides some class size of G. In this paper, we characterize the structure of those groups G whose prime graph is a block square.