## Second homotopy group of the mod 2 Moore spectrum

Let $S/2$ be the mod 2 Moore spectrum (i.e. the cofiber of $2: S \to S$). Then multiplication by 2 acts nontrivially on this spectrum: the homotopy groups of $S/2$ are all $\mathbb{Z}/4$-modules by a formal argument, but not $\mathbb{Z}/2$-modules. For instance, $\pi_2(S/2) = \mathbb{Z}/4$. The long exact sequence in homotopy groups enables one to determine the homotopy groups of $S/2$ (in the range that the stable homotopy groups of spheres are known) up to extension, but the extension problems are generally nontrivial: $\pi_2(S/2)$ is a case in point (as the aforementioned exact sequence $0 \to \mathbb{Z}/2 \to \pi_2(S/2) \to \mathbb{Z}/2 \to 0$ shows).

Is there a general technique for resolving these kinds of extension problems? I can get it in this case using some computation in the cobar complex to get the $t-s = 1$ line of the Adams spectral sequence for $S/2$ and see that the Bockstein acts nontrivially. I'm curious about a more efficient method of doing this. (Another example I had in mind was $bo \wedge \mathbb{CP}^2$: this is $bu$ by the "theorem of Reg Wood" and this is visible in the ASS, but directly computing $KO$-groups of complex projective spaces yields nontrivial extension problems, I think.)

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## 1 Answer

This is a comment, not an answer, I suppose. Just a reference to Adams and Walker "On complex Stiefel manifolds''. This follow up to Adams' "Vector fields on spheres'' directly computes the $KO$-groups of complex projective spaces (see Theorem 2.2) by the methods of VFS, which computed the complex $K$-theory of complex and real projective spaces and the real $K$-theory of real projective spaces. There are no extension problems in sight in the calculation of $KO(\mathbf{C}P^n)$.

Aside from the obvious, multiplication by $h_0$ in the ASS, method of detection of multiplication by $2$, there aren't a whole lot of systematic methods for detecting multiplication by $2$, let alone less simple extensions. Massey products/Toda brackets can help.

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 Thanks. I'll take a look at Adams's paper -- I had only seen VFS. – Akhil Mathew Jun 22 at 20:57 Unless I'm mistaken, there is an extension problem in calculating $KO_∗(\mathbb{CP}^2)$ (specifically, there's an extension of $\mathbb{Z}$ and $\mathbb{Z}/2$ for $KO_6(\mathbb{CP}^2)$ in the AHSS). – Akhil Mathew Jun 22 at 21:03 (It seems the paper does $KO^*$ rather than $KO_*$.) – Akhil Mathew Jun 22 at 21:06 I should have realized you meant KO homology, but you just wrote KO, and so did I. – Peter May Jun 23 at 1:05 I should have realized you meant KO homology, but you just wrote KO, and so did I. – Peter May Jun 23 at 1:06