# Prove that the intersection of two subgroups of a group is again a subgroup

### 1 Answer | Add Yours

Prove that the intersection of two subgroups is a subgroup:

To show that a subset with an operation is a subgroup, we need to show that ` a,b in H => ab^(-1) in H` where H is the subset of a group.

---------------------------------------------------------------

Let `H,K subset G` . Let `M=H cap K` .

Suppose `a,b in M` ; then `a in H,b in H, a in K, b in K` .

Since H is a subgroup, `a,b in H ==> ab^(-1) in H`

``Since K is a subgroup, `a,b in K ==> ab^(-1) in K`

`a,b in M ==> a,b in H; a,b, in K` thus `ab^(-1) in M` .

** We know that the intersection is nonempty, since H,K are subgroups implies that they both have the identity element of the group as their identity element. **

``

**Sources:**