# How to find all subgroups of a direct product?

I am wondering how do we find all subgroups of a direct product? Is there a method to find it?

For example, how can we find all the subgroups of $\mathbb{Z}_2\times\mathbb{Z}_2$? There is the answer: $\{(0, 0), (1, 0)\}, \{(0, 0), (0, 1)\}, \{(0, 0), (1, 1)\}$, and the improper and trivial subgroup. But I do not know the method how to find it.

All I can think about is the following: since the order of $\mathbb{Z}_2\times\mathbb{Z}_2$ is $4$, the order of a subgroup of it can be $1,2$ or $4$.

If order is $1$, then it is trivial, and if order is $4$, it is proper. Then, the order of a subgroup is $2$. We know all the elements of $\mathbb{Z}_2\times\mathbb{Z}_2$, so we try all the pairs.

Is that the method?

But what if we are given a bigger order product, such as $\mathbb{Z}_2\times\mathbb{Z}_8\times\mathbb{Z}_3\times\mathbb{Z}_9$? It looks very long to try all possibilities.

Can anyone help?

Thanks

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@julien, actually i did not copy and paste, i tried to use mathematical notations as much as i could –  copy_constructor Apr 14 '13 at 20:25
Ah, sorry then! But here is something you might find interesting. –  julien Apr 14 '13 at 20:26
Thank you, i will have a look at it –  copy_constructor Apr 14 '13 at 20:26
For just the normal subgroups you can take a look at mathoverflow.net/questions/23692/… (all your examples are abelian, so all subgroups are normal then). –  j.p. Apr 15 '13 at 12:26
Addendum: Thus to find all subgroups of $G\times H$, you may find all subgroups $A\in G$ and $B\in H$, and then all subdirect sums of $A\times B$.