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If $H \trianglelefteq G$ is a normal subgroup, a complement of $H$ is a subgroup $K \subseteq G$ with $\lvert H \rvert \lvert K \rvert = \lvert G \rvert$ and $\lvert H \cap K \rvert = 1$.

In order for a complement to exist, the sequence $$1 \to H \to G \to G/H \to 1$$ must split, in which case the complements are precisely the images of the possible splittings $G/H \to G$. All complements are isomorphic to $G/H$, and if $K$ is any complement then $G$ can be described as the internal semidirect product $H \rtimes K$.

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• Review status: beta
• Last edited by David Roe on 2021-06-21 06:09:56
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