Goursat's lemma

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Not to be confused with Goursat's integral lemma from Complex analysis

Goursat's lemma is an algebraic theorem about subgroups of the direct product of two groups.

It can be stated as follows.

Let , be groups, and let be a subgroup of such that the two projections and are surjective (i.e., is a subdirect product of and ). Let be the kernel of and the kernel of . One can identify as a normal subgroup of , and as a normal subgroup of . Then the image of in is the graph of an isomorphism .

An immediate consequence of this is that the subdirect product of two groups can be described as a fiber product and vice versa.

Proof of Goursat's lemma

Before proceeding with the proof, and are shown to be normal in and , respectively. It is in this sense that and can be identified as normal in G and G', respectively.

Since is a homomorphism, its kernel N is normal in H. Moreover, given , there exists , since is surjective. Therefore, is normal in G, viz:


It follows that is normal in since


The proof that is normal in proceeds in a similar manner.

Given the identification of with , we can write and instead of and , . Similarly, we can write and , .

On to the proof. Consider the map defined by . The image of under this map is . This relation is the graph of a well-defined function provided , essentially an application of the vertical line test.

Since (more properly, ), we have . Thus , whence , that is, . Note that by symmetry, it is immediately clear that , i.e., this function also passes the horizontal line test, and is therefore one-to-one. The fact that this function is a surjective group homomorphism follows directly.