Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Let $G$ be a profinite group, and $A$ a G-module. If $G$ is the projective limit of {$G_{\alpha}$}, and $A$ the direct limit of {$A_{\alpha}$}, then
$H^*(G,A)$ is isomorphic to $dir lim_{\alpha} H^*(G_{\alpha},A_{\alpha})$.
Here the cohomology groups are defined via the group of continuous functions from $G$ to $A$. At page 26 of this book it is asserted that this theorem is false on discontinuous cochains. And my question is: is there an example illustrating this statement?
Thanks in advance.

share|cite|improve this question
So you're asking for an example where $H^*_{\text{cont.}}(G,A) \not\cong H^*_{\text{discont.}}(G,A)$? – Bruno Joyal Dec 4 '12 at 18:52
Actually I would like to see an example showing that the equality holds not, on discontinuous cochains. Thanks for your attention in any case. – awllower Dec 5 '12 at 1:17
Dear @awllower, I don't understand the question... What equality? Please be more precise. – Bruno Joyal Dec 5 '12 at 13:20
I mean this isomorphism, which I referred to earlier as the equality: $H^*_{\text{cont.}}(G,A) \cong dir limH^*_{\text{cont.}}(G_{\alpha},A_{\alpha})$ That is to say, the "theorem" alludes to that isomorphism exactly. – awllower Dec 5 '12 at 14:43

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.