__Definition__

If is a homomorphism of a group into a , then the set of all those elements of which is mapped by onto the identity of is called the kernel of the homomorphism .

__Theorem__**:**

Let and be any two groups and let and be their respective identities. If is a homomorphism of into , then

**(i)**

**(ii)** for all

**(iii)** is a normal subgroup of .

** **

__Proof__**:**

(i) We know that for , .

and therefore by using left cancellation law, we have or

(ii) Since for any , , we get

Similarly , gives

Hence by definition of in we obtain the result

(iii) Since , . This shows that , now let , , ,

This establish that is a subgroup of .

Now, to show that it is also normal we prove the following.

Therefore, Hence the result.