# Condition for Element of Quotient Group of Additive Group of Reals by Integers to be of Finite Order

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## Theorem

Let $\struct {\R, +}$ be the additive group of real numbers.

Let $\struct {\Z, +}$ be the additive group of integers.

Let $\R / \Z$ denote the quotient group of $\struct {\R, +}$ by $\struct {\Z, +}$.

Let $x + \Z$ denote the coset of $\Z$ by $x \in \R$.

Then $x + \Z$ is of finite order if and only if $x$ is rational.

## Proof

From Additive Group of Integers is Normal Subgroup of Reals, we have that $\struct {\Z, +}$ is a normal subgroup of $\struct {\R, +}$.

Hence $\R / \Z$ is indeed a quotient group.

By definition of rational number, what is to be proved is:

- $x + \Z$ is of finite order

- $x = \dfrac m n$

for some $m \in \Z, n \in \Z_{> 0}$.

Let $x + \Z$ be of finite order in $\R / \Z$.

Then:

\(\, \displaystyle \exists n \in \Z_{\ge 0}: \, \) | \(\displaystyle \paren {x + \Z}^n\) | \(=\) | \(\displaystyle \Z\) | Definition of Quotient Group: Group Axiom $\text G 2$: Existence of Identity Element | |||||||||

\(\displaystyle \leadstoandfrom \ \ \) | \(\displaystyle n x\) | \(\in\) | \(\displaystyle \Z\) | Condition for Power of Element of Quotient Group to be Identity | |||||||||

\(\displaystyle \leadstoandfrom \ \ \) | \(\displaystyle n x\) | \(=\) | \(\displaystyle m\) | for some $m \in \Z$ | |||||||||

\(\displaystyle \leadstoandfrom \ \ \) | \(\displaystyle x\) | \(=\) | \(\displaystyle \dfrac m n\) |

$\blacksquare$

## Sources

- 1965: J.A. Green:
*Sets and Groups*... (previous) ... (next): Chapter $6$: Cosets: Exercise $14$