Subgroups of products versus products of subgroups.

 February 4, 2007 mathematics

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This is a question I wandered into accidentally years ago now, which I think other people might be amused to think about (or more likely, put on an abstract algebra exam).

Let G be a group, and H a subgroup of G \times G . Is H always isomorphic to G_1 \times G_2 , for some subgroups G_1, G_2 < G ? But beware!–I am not requiring (or expecting) any canonicity or naturality for the isomorphism: for instance G sits diagonally in G \times G , and it just so happens that G = \{ 1 \} \times G = G \times \{ 1 \} , so this is not a counterexample, in spite of the fact that the “horizontal” or “vertical” subgroup is not a canonical choice for the diagonal subgroup.

What is a good name for groups with this property? It’s not completely trivial: cyclic groups, for instance, have this property–not that I think this property is important, but names can be amusing…

I have examples of groups G and H < G \times G with H not (abstractly!) a product of subgroups of G . My challenge to you is to find some explicit examples of H < G \times G and prove that H doesn’t decompose.

In the end, I think this is a fun problem for a group theory final exam; I think it nicely highlights the difference between “being isomorphic” and “being equal,” though if one completes the challenge as stated, one probably already understands that distinction… So maybe the best reason for blogging about this is that chiastic title.