# Abelian Group of Order Twice Odd has Exactly One Order 2 Element/Proof 2

## Theorem

Let $G$ be an abelian group whose identity element is $e$.

Let the order of $G$ be $2 n$ such that $n$ is odd.

Then there exists exactly one $g \in G$ with $g \ne e$ such that $g = g^{-1}$.

## Proof

By Even Order Group has Order 2 Element, $G$ has an element $x$ of order $2$.

Aiming for a contradiction, suppose $y$ is another element of order $2$.

Then $x y = y x$ is another element of order $2$.

The subset $H = \set {g \in G: g^2 = e} = \set {e, x, y, x y}$ of $G$ forms a subgroup of $G$.

Thus $\order H = 4$.

But as $n$ is odd, it follows that $\order H$ is not a divisor of $2 n$.

This contradicts Lagrange's Theorem (Group Theory).

The result follows.

$\blacksquare$

## Sources

Raoul (https://math.stackexchange.com/users/109289/raoul), Prove that an abelian group of order 2n contains precisely one element of order 2, URL (version: 2014-10-05): https://math.stackexchange.com/q/959613