Index two subgroup of group is normal

https://arbital.com/p/index_two_subgroup_of_group_is_normal

by Patrick Stevens Jun 17 2016

An easy (though not very widely applicable) criterion for a subgroup to be normal.


Let be a Subgroup of the Group , of [index_of_subgroup index] . Then is a Normal subgroup of .

Proof

We must show that is closed under conjugation by elements of .

Since has index in , there are two left cosets: and for some specific . There are also two right cosets: and .

Now, since , it must be the case that ; so without loss of generality, .

Hence and so .

It remains to show that is closed under conjugation by every element of . But every element of is either in , or in ; so it is either or , for some .

This completes the proof.