Subgroups of cyclic groups are cyclic

Proposition (Subgroups of cyclic groups are cyclic). Let $G$ be a group that is cyclic, meaning $G=\langle g\rangle$ for some $g\in G$ (so $G$ is a cyclic group). Let $H\le G$ be a subgroup.

If $G$ is infinite cyclic (isomorphic to $\mathbb Z$ ), then either $H=\{e\}$ or there exists a unique integer $m\ge 1$ such that $H=\langle g^m\rangle$ .
If $|G|=n<\infty$ , then there exists a divisor $d\mid n$ such that $|H|=d$ , and moreover $H=\langle g^{n/d}\rangle$ .

Remarks

Context. This result gives a complete classification of subgroups of cyclic groups: they are completely controlled by divisibility (finite case) or by a single positive integer (infinite case). It is frequently paired with Lagrange's theorem in finite group computations.