Co-intersection graphs of nonabelian finite simple groups have diameter two

Abstract

Let $G$ be a finite group. The co-intersection graph $\Delta_G ^c$ of $G$ has as its vertices the nontrivial proper subgroups of $G$, with edges joining those pairs of subgroups which intersect trivially. It is clear that every connected component of $\Delta_G ^c$ has diameter at most three. In this Note, we show that when $T$ is a nonabelian finite simple group, $\Delta_T$ is connected of diameter exactly two. We also make several observations about the extent to which a finite group is determined by its co-intersection graph, and about the class of finite graphs arising as co-intersection graphs.