Let be a subgroup of group . Then is normal in if and only if there is a group and a group homomorphism such that the kernel of is .
Let be normal, so it is closed under conjugation. Then we may define the quotient_group , whose elements are the left cosets of in , and where the operation is that . This group is well-defined (proof).
Now there is a homomorphism given by . This is indeed a homomorphism, by definition of the group operation .
The kernel of this homomorphism is precisely ; that is simply :
Let have kernel , so if and only if . We claim that is closed under conjugation by members of .
Indeed, since . But that is , so .
That is, if then , so is normal.