This question is related to [Lyndon-Hochschild-Serre spectral sequence and cup products](https://mathoverflow.net/questions/130008/lydon-hochschild-serre-spectral-sequence-and-cup-products).


I have the followin result by J.S Milne in his book Arithmetic duality theorems pg 105.


Let $$0 \rightarrow C \rightarrow W \rightarrow G \rightarrow 0$$ is an exact sequence of groups corresponding to the canonical class $u \in H^{2}(G,C)$, and $C$ is of finite index in $W$


Let $M$ be a $W$ module on which $C$ acts trivially. Then by Hochschild-serre gives a spectral sequence $$0 \rightarrow H^{1}(G,M) \rightarrow H^{1}(W,M) \rightarrow H^{1}(C,M)^{G} \overset{\tau}\rightarrow H^{2}(G,M) $$ where $\tau$ is the transgression map. Now he goes on to prove that in this case $\tau(\alpha) = - \alpha \cup u$ (cup-product) for some $\alpha \in H^{0}(G,Hom(C,M))$.


Now consider the LHS for group homology, with $C,W,G,M$ as above ($C$ acting trivially on $M$) 


$$ H\_{2}(G,M) \overset{\tau'}\longrightarrow H\_{1}(C,M)\_{G} \overset{cor}\longrightarrow H\_1(W,M) \overset{coinf}\longrightarrow H\_{1}(G,M) \longrightarrow 0$$


Does it follow that $\tau'$ is induced by a cup product?


Thank you



Cup-products and Transgression maps.
The homology analogue of the result stated by Milne is that the transgression in the Lyndon-Hochschild-Serre spectral sequence for group homology 
$$
H\_2(G;M)\to H\_0(G;H\_1(C;M)) = H\_1(C;M)\_G
$$
is given (up to a sign) by *cap* product with the extension class $H^2(G;C)$. This makes sense since the cap product is a map
$$
H^2(G;C)\otimes H\_2(G;M)\to H\_0(G;C\otimes M)=H\_0(G;H\_1(C; M)),
$$
the final equality since $C$ is abelian.


This is supposedly well-known, see page 3 of [this paper](http://arxiv.org/pdf/0812.2092v1.pdf). I don't know a reference to an explicit proof, though.


