# Geometric characteristics or combinatorics analog for homotopy groups of spheres other than $\pi_n(S^n)$

The fact that $$\pi_1(S^1) = \mathbb Z$$ is geometrically intuitive. It is linked to a wealth of concepts and results, notably the winding number, residue theorem on $$\mathbb C$$, etc. There is a combinatorics analog with Sperner lemma, when extended to labellings of the first triangle that wind more than $$1$$ time.

The fact that $$\pi_n(S^n) = \mathbb Z$$ still appeals to intuition, although more difficult to "visualize" for $$n \ge 3$$, and this kind of intuition is potentially misleading of course. The continuous map degree, fixed-point index, engulfing number, extend the idea of winding number. Sperner lemma has extensions to higher dimensions (although I am not sure about the extension of the multiplicity result).

Then, what happens with other non-trivial homotopy groups of spheres?

Take $$\pi_4(S^2) = \mathbb Z_2$$ for example. We could hope to have a boolean associated to each mapping (some kind of "orientation"?). I have not seen anything like that, although that may just mean I am too new to these topics. Similarly, there does not seem to be any combinatorics result associated.

Question: I kindly ask for references that present some cases of $$\pi_k(S^n), k > n$$, with either a geometric intuition, or a named concept (e.g. an "orientation" when $$\pi_k(S^n) = \mathbb Z_2$$) having interesting consequences, or a correspondence with combinatorics results.

• You can convert these to framed cobordisms and those reduce the "intuiting" dimensionality, so are easier to wrap you head around Commented Sep 15, 2023 at 11:59
• @Trebor Thanks. What would be a good (= simple, and preferably online) resource about framed cobordisms? Commented Sep 15, 2023 at 12:59
• Trebor is referring to what is called the Pontryagin-Thom isomorphism. Usually you will see this stated in terms of the stable homotopy groups of spheres but there is also an unstable version. Unfortunately I don't know a reference off the top of my head and googling is strangely unhelpful. Commented Sep 15, 2023 at 20:00