Equivalence of Definitions of Complement of Subgroup
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Theorem
Let $G$ be a group with identity $e$.
Let $H$ and $K$ be subgroups.
The following definitions of the concept of Complement of Subgroup are equivalent:
Definition $1$
$K$ is a complement of $H$ if and only if:
- $G = H K$ and $H \cap K = \set e$
Definition $2$
$K$ is a complement of $H$ if and only if:
- $G = K H$ and $H \cap K = \set e$
Proof
Definition $1$ implies Definition $2$
Let $G = H K$.
Then $H K$ is a group.
By Subset Product of Subgroups:
- $H K = K H$
Thus $K H = G$.
$\Box$
Definition $2$ implies Definition $1$
Let $G = K H$.
Then $K H$ is a group.
By Subset Product of Subgroups:
- $H K = K H$
Thus $H K = G$.
$\blacksquare$