Homotopy Type Theory
homotopy groups of spheres (Rev #6, changes)

Showing changes from revision #5 to #6: Added | Removed | Changed

Idea

The homotopy groups of spheres are a fundemental concept in algebraic topology. They tell you about homotopy classes of maps from spheres to other spheres which can be rephrased as the collection of different ways to attach a sphere to another sphere. The homotopy type of a CW complex is completely determined by the homotopy types of the attaching maps.

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

π 4(S 3)\pi_4(S^3)

  • Guillaume has proved that there exists an nn such that π 4(S 3)\pi_4(S^3) is n\mathbb{Z}_n. Given a computational interpretation, we could run this proof and check that nn is 2. Added June 2016: Brunerie now has a proof that n=2n=2, using cohomology calculations and a Gysin sequence argument.

At least one proof has been formalized

π 3(S 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 3S^3, together with π n(S n)\pi_n(S^n), imply this by a long-exact-sequence argument.
  • This was formalized in Lean in 2016.

π n(S 2)=π n(S 3)\pi_n(S^2)=\pi_n(S^3) for n3n\geq 3

  • This follows from the Hopf fibration and long exact sequence of homotopy groups.
  • It was formalized in Lean in 2016.

Freudenthal Suspension Theorem

Implies π k(S n)=π k+1(S n+1)\pi_k(S^n) = \pi_{k+1}(S^{n+1}) whenever k2n2k \le 2n - 2

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

π n(S n)\pi_n(S^n)

π k(S n)\pi_k(S^n) for k<nk \lt n

π 2(S 2)\pi_2(S^2)

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

π 1(S 1)\pi_1(S^1)

  • 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 September 14, 2018 at 05:31:55 by Ali Caglayan. See the history of this page for a list of all contributions to it.