Subgroup of Elements whose Order Divides Integer
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Theorem
Let $A$ be an abelian group.
Let $k \in \Z$ and $B$ be a set of the form:
- $\left\{{x \in A : x^k = e}\right\}$
Then $B$ is a subgroup of $A$.
Proof
First note that the identity $e$ satisfies $e^k = e$ and so $B$ is non-empty.
Now assume that $a, b \in B$.
Then:
\(\ds \left({ab^{-1} }\right)^k\) | \(=\) | \(\ds a^k \left({b^{-1} }\right)^k\) | Power of Product in Abelian Group | |||||||||||
\(\ds \) | \(=\) | \(\ds a^k \left({b^k}\right)^{-1}\) | Powers of Group Elements | |||||||||||
\(\ds \) | \(=\) | \(\ds ee^{-1}\) | as $a^k = b^k = e \in B$ | |||||||||||
\(\ds \) | \(=\) | \(\ds e\) | Identity is Self-Inverse |
Hence, by the One-Step Subgroup Test, $B$ is a subgroup of $A$.
$\blacksquare$