# Generalized Postnikov square

Following Wikipedia (https://en.wikipedia.org/wiki/Postnikov_square), a Postnikov square is a certain cohomology operation from a first cohomology group $$H^1$$ to a third cohomology group $$H^3$$, introduced by Postnikov (1949). Eilenberg (1952) described a generalization taking classes in $$H^t$$ to $$H^{2t+1}$$.

I checked Eilenberg's paper, I find that the generalized Postnikov square requires that $$t$$ is odd.

In Browder and Thomas's paper: Axioms for the generalized Pontryagin cohomology operations (https://www.maths.ed.ac.uk/~v1ranick/papers/browthom.pdf), they also defined a generalized Postnikov square $$\mathfrak{P}:H^{2n}(-,\mathbb{Z}/2^r)\to H^{4n+1}(-,\mathbb{Z}/{2^{r+1}}).$$

My question: Is there a generalized Postnikov square $$\mathfrak{P}:H^t(-,\mathbb{Z}/p^r)\to H^{2t+1}(-,\mathbb{Z}/{p^{r+1}})$$ for even $$t$$ and odd prime $$p$$?

Thank you!

• can you give Refs for Postnikov (1949). Eilenberg (1952)? – annie heart Oct 23 '18 at 4:04
• I'm not clear on what properties you want. Browder and Thomas give an explicit formula for their Pontrjagin square: it sends $u$ to $\phi(u \beta u)$ where $\beta$ is the Bockstein and $\phi$ is the inclusion of $\Bbb Z/2^r$ into $\Bbb Z/2^{r+1}$. This formula is still valid at odd primes. In fact, for $t < 2p-3$ the only cohomology operations $H^t \to H^{2t+1}$ are of the form $u \mapsto k \beta'(u^2)$ for some scalar $k$, where $\beta'$ is the Bockstein associated to $0 \to \Bbb Z/p^{r+1} \to \Bbb Z/p^{2r+1} \to \Bbb Z/p^r \to 0$. What do you need from the Postnikov square? – Tyler Lawson Oct 26 '18 at 10:37
• @TylerLawson Thank you very much! I want to know whether Postnikov square can be defined for even $t$ and odd prime $p$. Can you elaborate on the fact you mentioned above? – Borromean Oct 26 '18 at 11:15
• @Tyler Lawson, do you have any Refs? – annie heart Oct 26 '18 at 15:10