# Homotopy Type Theory homotopy groups of spheres (Rev #2)

Here’s a quick reference for the state of the art on homotopy groups of spheres in HoTT. Everything listed here is also discussed on the page on Formalized Homotopy Theory.

## In progress

### $\pi_4(S^3)$

• Guillaume has proved that there exists an n such that π4(S3) is Z_n, and given a computational interpretation, we could run this proof and check that n is 2.

### $\pi_3(S^2)$

• Peter L. has constructed the Hopf fibration as a dependent type. Lots of people around know the construction, but I don’t know anywhere it’s written up. Here’s some Agda code with it in it.
• Guillaume’s proof that the total space of the Hopf fibration is $S^3$, together with $\pi_n(S^n)$, imply this by a long-exact-sequence argument, but this hasn’t been formalized.

## At least one proof has been formalized

### Freudenthal Suspension Theorem

Implies $\pi_k(S^n) = \pi_{k+1}(S^{n+1})$ whenever $k \le 2n - 2$

• Peter’s encode/decode-style proof, formalized by Dan, using a clever lemma about maps out of two n-connected types.

### π2(S2)

• Guillaume’s proof using the total space the Hopf fibration.
• Dan’s encode/decode-style proof.

### π1(S1)

• Mike’s proof by contractibility of total space of universal cover (HoTT blog).
• Dan’s encode/decode-style proof (HoTT blog). A paper mostly about the encode/decode-style proof, but also describing the relationship between the two.
• Guillaume’s proof using the flattening lemma.

Revision on March 3, 2014 at 13:34:13 by Mike Shulman. See the history of this page for a list of all contributions to it.