# Cohomology of central extensions of groups

Let G be a central extension of a finite group H by $Z/2$. I need an explicit description of the differentials $d_2$ and $d_3$ in the Lyndon-Hochschild-Serre spectral sequence which converges to the cohomology of G with coefficients in either $Z/2$ or $U(1)$. It appears that $d_2$ is related to the cup product with the extension class, while $d_3$ is related to the Bockstein of the extension class, but I could not find it in any standard books on cohomology of groups.

• The paper by Johannes Heubschmann mentioned in Andy Putman's answer to this question mathoverflow.net/questions/590/… might have some useful information. You could also try using Kudo's Transgression theorem to make sense of your claim about $d_3$. – Mark Grant Mar 22 '16 at 7:11
• We've done lots of computations for $H=Z/2$, see pages 14-17 in arxiv.org/abs/hep-th/0701071. There we used a so-called 7-term exact sequence in order to determine $d_2$. Maybe that can help, good luck! – Konrad Waldorf Mar 22 '16 at 15:38

Ok, maybe not in a standard book, but a classic anyway: the description of the differentials $d_2$ and $d_3$ can be found in the paper
As expected in the question, $d_2$ is the cup product with the extension class and $d_3$ is related to the Bockstein of the extension class via the product structure on the $E_3$-page. I actually found this in the introduction of the following paper
which gives some more discussion and identifies the $d_4$-differential.