Correspondence Theorem (Groups)

Correspondence Theorem (Groups). Let $G$ be a group and let $N \trianglelefteq G$ be a normal subgroup. Let $\pi: G \to G/N$ be the canonical projection. Then the assignments

• $A \mapsto A/N := \{aN : a \in A\}$, for subgroups $A$ with $N \subseteq A \subseteq G$, and
• $B \mapsto \pi^{-1}(B)$ (the preimage) for subgroups $B \le G/N$,

are inverse, inclusion-preserving bijections between:

1. subgroups $A$ of $G$ with $N \subseteq A$, and
2. subgroups of $G/N$.

Moreover:

• $A \trianglelefteq G$ if and only if $A/N \trianglelefteq G/N$, and
• if $[G:A]$ is finite, then $[G:A] = [G/N : A/N]$.