The prime graph $Gamma(G)$ of a group $G$ is
a graph with vertex set $pi(G)$, the set of primes dividing the
order of $G$, and two distinct vertices $p$ and $q$ are adjacent
by an edge written $psim q$ if there is an element in $G$ of
order $pq$. Let $pi(G)={p_{1},p_{2},...,p_{k}}$. For
$pinpi(G)$, set $deg(p):=|{q inpi(G)| psim q}|$, which is
called the degree of $p$. We also set
$D(G):...
