Alg7.18
From Exampleproblems
Prove that a group of order 56 has a normal Sylow p-subgroup for some prime divding its order.
The number of Sylow 2-subgroups is either 1 or 7.
The number of Sylow 7-subgroups is either 1 or 8.
Suppose there are 8 Sylow 7-subgroups which account for 
elements without the identity, and two Sylow 2-subgroups, the maximum order of intersection of which is 4, including identity, which accounts for 12 elements. But 
, which is a contradiction of the group order 56.
