component of identity of a topological group is a closed normal subgroup
Theorem - Let be a topological group and its identity element
. The connected component
of is a closed normal subgroup
of .
Proof: Let be the connected component of . All components of a topological space are closed, so is closed.
Let . Since the multiplication and inversion functions in are continuous, the set is also connected, and since we must have . Hence, for every we have , i.e. is a subgroup of .
If is an arbitrary element of , then is a connected subset containing . Hence for every , i.e. is a normal subgroup.