Smallest Normal Subgroup containing Set
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Theorem
Let $S \subseteq G$ where $G$ is a group.
Then there exists a unique smallest normal subgroup of $G$ which contains $S$.
Proof
Let $\Bbb S$ be the set of all normal subgroups of $G$ that contain $S$.
Since $S \subseteq G \lhd G$, $\Bbb S \ne \O$.
Let $N = \bigcap \Bbb S$, that is, the intersection of all elements of $\Bbb S$.
By Intersection of Normal Subgroups is Normal, $N \lhd G$.
By the definition of intersection, $S \subseteq N$.
By the method of construction, $N$ is the smallest such subgroup.
By the definition of "smallest", $N$ is unique.
$\blacksquare$