# What is the 31th homotopy group of the 2 - sphere ?

What is $\pi_{31}(S^2)$, the 31th homotopy group of the 2 - sphere ?

This question has a physics motivation:

1) There are relations between (2nd and 3rd) Hopf fibrations and (2- and 3-) qbits (quantum bits) entanglement, see this reference

2) Maybe there are relations between classification of qbits entanglements and sphere homotopy groups, and we are interested in the classification of 4-qbits entanglements.

I tried fo find the solution on the net, with help of math fans, but without success.

Wikipedia gives only to the 22th group homotopy of the 2-sphere

This article of John Baez gives interesting references, like Allen Hatcher, Stable homotopy groups of spheres or a link with braids. One speak of a book of Kochman Stanley O. : Stable Homotopy Groups of Spheres A Computer-Assisted Approach

But I am totally unable to find the answer.

A subsidiary question would be : Until what rank do we know these high homotopy group of the 2-sphere ?

• But there is no fourth Hopf fibration! – Mariano Suárez-Álvarez May 17 '13 at 7:45
• @David: $\pi_{31}(S^2)$ is well out of the stable range. There is a homomorphism to the 29th stable group, but it is unlikely to be injective or surjective. – Neil Strickland May 17 '13 at 7:54
• You could deduce some information on $\pi_{32}(S^3)$ from Bob Bruners unstable chart on the bottom of math.wayne.edu/~rrb/cohom/index.html (if that's of any help) – Christian Nassau May 17 '13 at 8:08
• Actually, I remember seeing more unstable charts in the "Oaxtapec proceedings" (CONM146) (Appendix 2 by Paul Shick), but I don't know if this relates to $S^2$. Be warned that it's usually nontrivial to deduce the homotopy from these charts, so this info might not be of much use to you. – Christian Nassau May 17 '13 at 8:27
• «For separable states, the original Hilbert space $S^7$ simplifies to $S^2\times S^2$» Reading physics papers requires a lot of restraint :-) – Mariano Suárez-Álvarez May 17 '13 at 8:31

One simple observation is that $\pi_{31}(S^2)\cong\pi_{31}(S^3)$, by the long exact sequence of the Hopf fibration.

The homotopy groups $\pi_i(S^3)$ for $i\le 64$ are apparently computed in:

Curtis, Edward B.,Mahowald, Mark, The unstable Adams spectral sequence for $S^3$, Algebraic topology (Evanston, IL, 1988), 125–162, Contemp. Math., 96, Amer. Math. Soc., Providence, RI, 1989.

Unfortunately I wasn't able to get a hold of that reference to check for an explicit answer. Maybe someone has it on their shelf and can check.

• The beginning of the article can apparently be read on Google Books – Trimok May 17 '13 at 9:34
• I don't know if it is the correct result, but it seems that the answer should be $Z_2 + Z_2 + Z_2 + Z_2$ (Found page 204, in 1st paper reference below) Nobuyuki Oda, On the 2.Components of the Unstable Homotopy Groups of Spheres I. Source: Proc. Japan Acad. Ser. A Math. Sci. Volume 53, Number 7 (1977), 215-218. 1st paper – Trimok May 17 '13 at 10:29
• @Trimok: Good find! Note that this is only supposed to be the $2$_primary part; there may be other summands of the form $\mathbb{Z}_{p^n}$ with $p$ an odd prime. – Mark Grant May 17 '13 at 11:01
• @Mark : Yes, I understand this is not the final answer... – Trimok May 17 '13 at 11:04
• I checked, and unfortunately the reference you've listed only seems to compute the 2-primary part as well. – Mike Miller Eismeier Sep 4 '14 at 1:09
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