# Which abelian groups aren't homotopy groups of spheres?

Someone asked me if all finite abelian groups arise as homotopy groups of spheres. I strongly doubted it, and I bet ten bucks that $$\mathbb{Z}_5$$ is not $$\pi_k(S^n)$$ for any $$n,k$$. But I don't know how to prove it's not.

Which finite abelian groups are known to not arise as homotopy groups of spheres?

Is $$\mathbb{Z}_5$$ the smallest one? Or maybe $$\mathbb{Z}_4$$?

I conjecture that for no odd prime $$p \gt 3$$ is $$\mathbb{Z}_p \cong \pi_k(S^n)$$ for some $$n,k$$.

• I think every cyclic group appears as a subgroup of a homotopy group of spheres. Indeed, I think this is already true for the image of j. But I think homotopy groups of spheres are typically groups are “small” unless n-k=0 mod 4, in which case the image of j is already quite big, so it does seem likely that most finite groups will not appear. Nov 26 '20 at 23:09
• The cyclic group of order 4 appears in the table <en.wikipedia.org/wiki/…> of stable homotopy groups of spheres reproduced in Wikipedia's entry for "Homotopy groups of spheres". So it seems that this group appears as $\pi_{n+60}(S^n)$ for $n$ sufficiently large. I do not see a 5-element group tabulated anywhere on that page. Nov 26 '20 at 23:15
• Are there heuristics (a la Cohen-Lenstra) for p-part of the homotopy groups of spheres mod the image of J? Or maybe in the stable case? Nov 26 '20 at 23:57
• I can't give you any concrete answer, but my inclination is to say that it seems highly likely that you're correct. The 2-primary torsion forms a real thicket, whereas there can be no p-torsion until you are at least at $\pi_{n+2p-3}(S^n)$. This means it's not clear to me whether there are infinitely many different groups of odd order appearing. (But the unstable part is really outside my wheelhouse...) Nov 27 '20 at 5:58
• @CalicoJackRackham I think you mean "only two normal covering spaces" rather than "covering space". After all non-cyclic simple groups have a lot of subgroups. But since the question is only interesting for higher homotopy groups (which are always abelian) the relevance of your comment is not clear to me. Dec 11 '20 at 15:22