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TheoremStatement: Literature-sourcedProof: AI-adaptedprecheck passaudited 2026-08-02
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Second isomorphism theorem for groups: H/(H∩N)≅HN/N

Statement

Second isomorphism theorem for groups: H/(H∩N)≅HN/N.

If H≤G and N⊴G, then

H/(H∩N)≅HN/N.

Facts & Assumptions

Given: A subgroup H≤G and a normal subgroup N⊴G.

[L2]

A homomorphism modulo its kernel is isomorphic to its image (First isomorphism theorem for groups: G/ker⁡f≅im⁡f).

[L3]

The canonical quotient map has the given normal subgroup as kernel (The canonical projection π:G→G/N, π(g)=gN, is a surjective group homomorphism).

[L4]

Quotients by normal subgroups are groups of cosets (The quotient group G/N and coset product (gN)(hN)=ghN).

Proof

technique · direct
1.1L1L2L3L4givenconstruct

Restrict the quotient map HN→HN/N to ϕ:H→HN/N, ϕ(h)=hN; [L1] and [L4] make this a homomorphism.

2.1step 1.1L1L2L3L4givenalgebra

Its kernel is {h∈H:hN=N}=H∩N, while every hnN=hN shows that its image is HN/N.

3.1step 2.1∎

The kernel and image calculation in step 2.1 gives H/(H∩N)≅HN/N.

Depends on

Dependency tree · 2 levels

Sources