# Fourth isomorphism theorem

(Redirected from Correspondence theorem)

## Contents

This article gives the statement, and possibly proof, of a basic fact in group theory.
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## Name

This result is termed the lattice isomorphism theorem, the fourth isomorphism theorem, and the correspondence theorem.

## Statement

### Statement with symbols

Let $G$ be a group and let $N$ be a Normal subgroup (?) of $G$. Then, we have a bijection:

Set of subgroups of $G$ containing $N$ $\leftrightarrow$ Set of subgroups of $G/N$

If $\varphi:G \to G/N$ is the quotient map, then this bijection is given by: $H \mapsto \varphi(H)$

in the forward direction, and: $K \mapsto \varphi^{-1}(K)$

in the reverse direction. Moreover:

1. Under the bijection, normality is preserved. In other words, a subgroup containing $N$ is normal if and only if its image under $\varphi$ is normal.
2. The bijection is an isomorphism between the lattice of subgroups of $G$ containing $N$, and the lattice of subgroups of $G/N$. In other words, the bijection preserves partial order: $A \le B$ if and only if $\varphi(A) \le \varphi(B)$. It also preserves intersections and joins.
3. The bijection preserves index. If $A,B$ are subgroups of $G$ containing $N$, with $A \le B$, then $[B:A] = [\varphi(B):\varphi(A)]$.