Subgroup is normal if and only if it is the kernel of a homomorphism

Written by Patrick Stevens last updated

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 .

Proof

"Normal" implies "is a kernel"

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 :

  • Certainly (because for all , since is closed as a subgroup of );
  • We have because if then in particular (where is the group identity) so .

"Is a kernel" implies "normal"

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.