proof of second isomorphism theorem for groups
First, we shall prove that is a subgroup of :Since and , clearly .Take .Clearly .Further,
Since is a normal subgroup of and ,then .Therefore ,so is closed under multiplication
.
Also, for , , since
and since is a normal subgroup of .So is closed under inverses, and is thus a subgroup of .
Since is a subgroup of ,the normality of in follows immediately fromthe normality of in .
Clearly is a subgroup of ,since it is the intersection of two subgroups of .
Finally, define by .We claim that is a surjective homomorphism from to .Let be some element of ;since , then , and .Now
and if , then we must have . So
Thus, since and ,by the First Isomorphism Theorem we see that is normal in and that there is a canonical isomorphism between and .