# Cohomology of Groups at Gregory Berhuy's Book

Regarding Gregory Berhuy's book "An Introduction to Galois Cohomology and its Applications":

The book defined a cohomology sets for non-abelian $G$-groups. Let $A$ be a $G$-group, we define a 1-cocycle as follows: it is a map $$\alpha : G \to A \quad , \quad \sigma \mapsto \alpha_\sigma$$ such that for all $\alpha \in A$: $\alpha_1 = 1$ (1 is the unit element of each group) and it satisfies the following relation for all $\sigma , \tau \in G$: $$\alpha_{\sigma \tau} = \alpha_\sigma \sigma \cdot \alpha_\tau \ .$$

In page 52, Berhuy's uses that fact to deduce that for an exact sequence of groups $$1 \to A \stackrel{f}{\to} B \stackrel{g}{\to} C \to 1$$ where $f(A)$ is a central subgroup of $b$, that $$g( \beta_\sigma (\sigma \cdot \beta_\tau) \beta_{\sigma \tau}^{-1}) = \gamma_\sigma \sigma \cdot \gamma_\tau \gamma_{\sigma \tau}^{-1} = 1$$ however, in page 53, he writes $$f(\alpha_{\sigma, \tau}) = \beta_\sigma (\sigma \cdot \beta_\tau) \beta_{\sigma \tau}^{-1}$$ (notice the brackets on $\sigma \cdot \beta_\tau$) but he doesn't say it is equal to 1, why? If we use the 1-cocycle definition (the equation with the $\alpha$'s) then $$\beta_\sigma (\sigma \cdot \beta_\tau) = \beta_{\sigma \tau}$$

Where is the difference and why?

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It seems to me like he's using the fact that $g(\beta_\sigma(\sigma \cdot \beta_\tau)\beta_{\sigma\tau}^{-1}) =1$ to say that \beta_\sigma(\sigma \cdot \beta_\tau)\beta_{\sigma\tau}^{-1} is in the image of $f$, which is of course true by the assumption that we have a short exact sequence of $G$-groups. – stankewicz Jun 24 '12 at 11:36
I'm not sure. He first states that $$g( \beta_\sigma (\sigma \cdot \beta_\tau ) \beta_{\sigma \tau}^{-1}) = \gamma_\sigma \sigma \cdot \gamma_\tau \gamma_{\sigma \tau}^{-1} = 1$$ and then deduces from that, that $$\beta_\sigma (\sigma \cdot \beta_\tau ) \beta_{\sigma \tau}^{-1} = f( \alpha_{\sigma , \tau})$$ (I'm quoting: " = 1, so $\beta ... = f( \alpha_{\sigma , \tau})$ for some unique $\alpha_{\sigma , \tau} \in A$). However, what about the general case? Why it is not =1 regardless the $f,g$ homomorphisms? – Zachi Evenor Jun 24 '12 at 12:05
@stankewicz See my last comment, it may help. – Zachi Evenor Jun 24 '12 at 17:15
In this book on page 52, $\beta_\sigma$ is SOME preimage of $\gamma_\sigma$. Although $\gamma$ is a cocycle, $\beta$ in general is NOT a cocycle, and this is the point! It is a cochain, whose coboundary lives in $f(A)$ (because $\gamma$ is a cocycle), which gives us a 2-cocycle in $A$. The cohomology class of this cocycle is, by definition, $\delta^1(\gamma)$. – Mikhail Borovoi Jun 24 '12 at 20:23
Thanks. Now that this understood, I got the rest (I recall the since the sequence is exact $g \circ f = 1$ the trivial map. – Zachi Evenor Jun 25 '12 at 10:00

R. Brown, Fibrations of groupoids'', J. Algebra 15 (1970) 103-132.