Center is Characteristic Subgroup
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Theorem
Let $G$ be a group.
Then its center $\map Z G$ is characteristic in $G$.
Proof
By Identity Mapping is Group Automorphism, there exists at least one automorphism of $G$.
Let $\phi$ be an automorphism of $G$.
Let $x \in \map Z G, y \in G$.
Then:
\(\ds \map \phi x y\) | \(=\) | \(\ds \map \phi x \map \phi {\map {\phi^{-1} } y}\) | automorphisms are bijections | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \phi {x \map {\phi^{-1} } y}\) | Definition of Group Homomorphism | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \phi {\map {\phi^{-1} } y x}\) | Definition of Center of Group | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \phi {\map {\phi^{-1} } y} \map \phi x\) | Definition of Group Homomorphism | |||||||||||
\(\ds \) | \(=\) | \(\ds y \map \phi x\) |
Hence $\map \phi x \in \map Z G$.
So we have $\phi \sqbrk {\map Z G} \subseteq \map Z G$.
Since $\phi^{-1}$ is also an automorphism:
- $\phi^{-1} \sqbrk {\map Z G} \subseteq \map Z G$
Since $\phi$ is a bijection:
- $\map Z G = \phi \sqbrk {\phi^{-1} \sqbrk {\map Z G}} \subseteq \phi \sqbrk {\map Z G}$
Therefore we conclude that $\phi \sqbrk {\map Z G} = \map Z G$.
Hence $\map Z G$ is characteristic in $G$.
$\blacksquare$