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For any compact abelian group $K$.$$K\cong H_0\times \mathrm{U}(1)^k,$$where $H_0$ is a finite group.

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closed as off topic by Henry Cohn, Emil Jeřábek, Stefan Geschke, Felipe Voloch, Todd Trimble Oct 9 '12 at 15:53

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I'm guessing that this question won't last long in its current form, but let me point out that you at very least need to add some hypotheses. If the scope of $K$ is "compact topological abelian group" then your statement is false. – Ramsey Oct 9 '12 at 14:34
This is clearly not true, consider for example $\mathbb Z_2^{\mathbb N}$. I am trying to think of some context where this question makes sense. Is $k$ finite? – Stefan Geschke Oct 9 '12 at 15:05
Presumably he means compact Hausdorff abelian Lie groups, where $k$ is finite. – Todd Trimble Oct 9 '12 at 15:49
Try looking here for instance: – Todd Trimble Oct 9 '12 at 15:51
Oh sorry, two comments ago, I should have said "he or she". – Todd Trimble Oct 9 '12 at 15:52
up vote 3 down vote accepted

Any topological group of the form $K\cong H_0\times U(1)^k$ (with $H_0$ finite and $k$ a positive integer) is a closed subgroup of $U(1)^h$ (for some positive integer $h\geq k$). Furthermore, all the closed subgroups of $U(1)^h$ are of this form.

The proof is an easy application of the Pontryagin-Van Kampen duality. In fact, such groups are the duals of the finitely generated Abelian groups (which are quotients of $\mathbb Z^h$). It is well known that such groups are of the form $F\times \mathbb Z^k$ (with $F$ a finite group).

To find a general form for a compact abelian group is as difficult as giving a structure theorem for discrete abelian group (which is known to be quite a difficult, and fairly open, problem in general, even if there are nice results for countable torsion groups).

EDIT: just to answer also to the comment of Stefan Geschke. Finitely generated groups in $\mathrm{Mod}(\mathbb Z)$ can be characterized as the Noetherian objects of the category. So I guess that (by duality) the objects of the form $K\cong H_0\times U(1)^k$ should be the Artinian objects in the category of compact abelian topological ($T_2$) groups. (N.B.= here by Artinian I mean the category-theoretical notion)

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