Existence of Unique Subgroup Generated by Subset
Theorem
Let $\struct {G, \circ}$ be a group.
Let $\O \subset S \subseteq G$.
Let $\struct {H, \circ}$ be the subgroup generated by $S$.
Then $H = \gen S$ exists and is unique.
Also, $\struct {H, \circ}$ is the intersection of all of the subgroups of $G$ which contain the set $S$:
- $\ds \gen S = \bigcap_i {H_i}: S \subseteq H_i \le G$
Singleton Generator
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
Existence
First, we prove that such a subgroup exists.
Let $\mathbb S$ be the set of all subgroups of $G$ which contain $S$.
$\mathbb S \ne \O$ because $G$ is itself a subgroup of $G$, and thus $G \in \mathbb S$.
Let $H$ be the intersection of all the elements of $\mathbb S$.
By Intersection of Subgroups is Subgroup, $H$ is the largest element of $\mathbb S$ contained in each element of $\mathbb S$.
Thus $H$ is a subgroup of $G$.
Since $\forall x \in \mathbb S: S \subseteq x$, we see that $S \subseteq H$, so $H \in \mathbb S$.
Smallest
Now to show that $H$ is the smallest such subgroup.
If any $K \le G: S \subseteq K$, then $K \in \mathbb S$ and therefore $H \subseteq K$.
So $H$ is the smallest subgroup of $G$ containing $S$.
Uniqueness
Now we show that $H$ is unique.
Suppose $\exists H_1, H_2 \in \mathbb S$ such that $H_1$ and $H_2$ were two such smallest subgroups containing $S$.
Then, by the definition of "smallest", each would be equal in size.
If one is not a subset of the other, then their intersection (by definition containing $S$) would be a smaller subgroup and hence neither $H_1$ nor $H_2$ would be the smallest.
Hence one must be a subset of the other.
By definition of set equality, that means they must be the same set.
So the smallest subgroup, whose existence we have proved above, is unique.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {III}$: The Natural Numbers: $\S 14$: Orderings: Theorem $14.5$
- 1966: Richard A. Dean: Elements of Abstract Algebra ... (previous) ... (next): $\S 1.9$
- 1971: Allan Clark: Elements of Abstract Algebra ... (previous) ... (next): Chapter $2$: Subgroups and Cosets: $\S 35 \epsilon$
- 1996: John F. Humphreys: A Course in Group Theory ... (previous) ... (next): Chapter $4$: Subgroups: Definition $4.7$