Order of Homomorphic Image of Group Element
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Theorem
Let $G$ and $H$ be groups whose identities are $e_G$ and $e_H$ respectively.
Let $\phi: G \to H$ be a homomorphism.
Let $g \in G$ be of finite order.
Then:
- $\forall g \in G: \order {\map \phi g} \divides \order g$
where $\divides$ denotes divisibility.
Proof
Let $\phi: G \to H$ be a homomorphism.
Let $\order g = n, \order {\map \phi g} = m$.
\(\ds \paren {\map \phi g}^n\) | \(=\) | \(\ds \map \phi {g^n}\) | Homomorphism of Power of Group Element | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \phi {e_G}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds e_H\) | Homomorphism to Group Preserves Identity |
It follows from Element to Power of Multiple of Order is Identity that $m \divides n$.
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $8$: Homomorphisms, Normal Subgroups and Quotient Groups: Exercise $4 \ \text{(ii)}$
- 1996: John F. Humphreys: A Course in Group Theory ... (previous) ... (next): Chapter $8$: The Homomorphism Theorem: Exercise $4$