Z. Liang was supported by NSFC (11071149) and SNSF (2010011001-1), J. Su was supported by NSFC111T1204, NSFC10831005, KZ201010028027, and PHR201106118.
Let G be a finite group and π(G) = { p 1 , p 2 , ··· , p k } be the set of the primes dividing the order of G. We define its prime graph Γ(G) a