Normalizers and centralizers

Let G be a group and suppose HG. The normalizer of H in G is defined to be N(H)={gG|gH=Hg} and the centralizer of H in G is defined to be C(H)={gG|gh=hg for all hH}.

  1. Prove that N(H) is a subgroup of G.
  2. Prove that C(H) is a normal subgroup of N(H) and that N(H)/C(H) is isomorphic to a subgroup of Aut(H).