Normalizers and centralizers
Let
- Prove that
is a subgroup of . - Prove that
is a normal subgroup of and that is isomorphic to a subgroup of .
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Let $G$ be a group and suppose $H\leq G$. The {\bfseries normalizer} of $H$ in $G$ is defined to be $N(H)=\{g\in G\,|\, gH=Hg\}$ and the {\bfseries centralizer} of $H$ in $G$ is defined to be $C(H)=\{g\in G\,|\, gh=hg\text{ for all }h\in H\}$.
\begin{enumerate}[label=(\alph*)]
\item Prove that $N(H)$ is a subgroup of $G$.
\item Prove that $C(H)$ is a normal subgroup of $N(H)$ and that $N(H)/C(H)$ is isomorphic to a subgroup of $\operatorname{Aut}(H)$.
\end{enumerate}