Discrete Subgroup of Real Numbers is Closed
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Theorem
Let $G$ be a subgroup of the additive group of real numbers.
Let $G$ be discrete.
Then $G$ is closed.
Proof
By Subgroup of Real Numbers is Discrete or Dense, there exists $a \in \R$ such that $G = a \Z$.
If $a = 0$, then $G$ is closed.
Let $a > 0$.
Then:
- $\ds \R \setminus G = \bigcup_{z \mathop \in \Z} \openint {a z} {a z + a}$
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By Union of Open Sets of Metric Space is Open, $\R\setminus G$ is open.
Thus $G$ is closed.
$\blacksquare$