Existence of Unique Subgroup Generated by Subset/Singleton Generator

Theorem
Let $\struct {G, \circ}$ be a group. Let $a \in G$.

Then $H = \gen a = \set {a^n: n \in \Z}$ is the unique smallest subgroup of $G$ such that $a \in H$.

That is:
 * $K \le G: a \in K \implies H \subseteq K$

Proof
From Powers of Element form Subgroup, $H = \set {a^n: n \in \Z}$ is a subgroup of $G$.

Let $K \le G: a \in K$.

Then $\forall n \in \Z: a^n \in K$.

Thus, $H \subseteq K$.