# Math Help - Left and Right Cosets

1. ## Left and Right Cosets

Let G be a group and H a subgroup of G. Let a, b in G. Prove the following:

If aH = Ha and bH = Hb, then (ab)H = H(ab).

2. ## Re: Left and Right Cosets

Originally Posted by jzellt
Let G be a group and H a subgroup of G. Let a, b in G. Prove the following:

If aH = Ha and bH = Hb, then (ab)H = H(ab).

So we want to show that $(ab)H \subset H(ab)$

let $k \in (ab)H$

Then $k=(ab)h$ for some $h \in H$

By the relation $bH=Hb$ we know that there exists an $h_1 \in H$ such that $bh=h_1b$

and we know that by $aH=Ha$ there exists an $h_2 \in H$ such that $ah_1=h_2a$

Putting all of these together we get

$k=(ab)h=a(bh)=a(h_1b)=(ah_1)b=h_2(ab) \in H(ab)$

Now you just need to show or justify the other direction (it is exactly the same.)