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The symmetric and asymmetric version of Goursat's Lemma
Electronic Research Archive 2025, 33(11): 6952-6970
Published: 18 November 2025
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Goursat's lemma gives a good method to describe subgroups of the direct product of two groups G 1 , G 2 , and to determine whether subgroups of G 1 × G 2 are direct products. However, the usual symmetric version of Goursat's lemma is difficult to describe subgroups of a direct product of a finite number of groups. Fortunately, the asymmetric version of Goursat's lemma provide a new method to solve the difficulty. In this paper, we used additional conditions π i ( H ) = G i , the injectivity ρ i , and H i 22 = H i 12 H i 21 for 1 i 3 to give some related results about groups (resp. R-modules, R-algebras (rings as corollary)), and then we gave the answer on whether a R-submodule M of M 1 × × M n has the form M = N ~ × N n and M = N 1 × × N n . Further, we extended similar conclusions to R-algebras (rings as corollary).

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