Published online by Cambridge University Press: 08 October 2014
Let $G$ be a finite group, let
${\it\pi}(G)$ be the set of prime divisors of
$|G|$ and let
${\rm\Gamma}(G)$ be the prime graph of
$G$. This graph has vertex set
${\it\pi}(G)$, and two vertices
$r$ and
$s$ are adjacent if and only if
$G$ contains an element of order
$rs$. Many properties of these graphs have been studied in recent years, with a particular focus on the prime graphs of finite simple groups. In this note, we determine the pairs
$(G,H)$, where
$G$ is simple and
$H$ is a proper subgroup of
$G$ such that
${\rm\Gamma}(G)={\rm\Gamma}(H)$.
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