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Let $G$ be a profinite group, then there is a general notion of continuous cohomology groups $H^n_{\text{cont}}(G, M)$ for any topological $G$-module $M$ (I require topological $G$-modules to be abelian). Although this definition makes perfect sense, it has a drawback that it is not a "delta-functor" (the category of topological $G$-modules is not abelian, so I will explain below what I really mean by this). It is stated in various places that this is not true that an arbitrary short exact sequence of topological $G$-modules $$ 0 \to M' \to M \to M''\to 0 $$ induces a long exact sequence of continuous cohomology groups $$ 0 \to H^0_{\text{cont}}(G, M')\to H^0_{\text{cont}}(G, M) \to H^0_{\text{cont}}(G, M'') \to H^1_{\text{cont}}(G, M') \to H^1_{\text{cont}}(G,M) \to \dots $$ However, I've never seen an example of such a short exact sequence for a profinite group $G$. Before asking a precise question, let me explicitly say what I mean by an exact sequence of topological $G$-modules.

Definition: A short sequence of topological $G$-modules $0 \to M' \to M\to M'' \to 0$ is called exact, if it is exact as a sequence of abstract groups, topology on $M'$ is equal to the topology induced from $M$ and the map $M \to M''$ is a topological quotient map.

Question 1: What is an explicit example of a profinite group $G$ and a short exact sequence of topological $G$-modules $0 \to M' \to M \to M'' \to 0$ s.t. there is no associated long exact sequence of continuous cohomology groups?

Question 2: Is there an example of such a sequence with $M'$ being (quasi-)compact?

Apparently, the answer to the second question should positive. And I am more interested in this question, but I don't even know an example for the first question.

Remark: It is easy to show that if $M \to M''$ has a continuous section (as topological spaces), then the associated long exact sequence is exact. So, whatever these examples are the map $M \to M''$ shouldn't have a continuous section. In particular, they can't be finite-dimensional spaces over a field and $M''$ can't be a discrete module. Also, if $M'$ and $M$ itself are profinite groups, then Lemma 5.6.5 from the book "Profinite groups" by Ribes and Zalesskii guarantees that $M \to M''$ has a continuous section.

Remark 2: If you drop the assumption that $G$ is a profinite group, then it is easy to construct an example. Namely, consider $G=S^1$ and a short exact sequence $$ 0 \to \mathbf Z \to \mathbf R \to S^1 \to 0 $$ with trivial $S^1$ action. It is not hard to show that $H^1_{\text{cont}}(S^1, \mathbf R)=H^2_{\text{cont}}(S^1, \mathbf Z)=0$ but $H^1_{\text{cont}}(S^1, S^1)=\operatorname{Hom}_{\text{cont}}(S^1,S^1)\neq 0$.

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  • $\begingroup$ The cohomology of topological groups defined by Segal in [1] is a delta functor, and agrees with continuous cohomology when the group is totally disconnected. Reference: [1] Segal, G. Cohomology of topological groups. In Symposia Mathematica, Vol. IV (INDAM, Rome, 1968/69), pp. 377–387 (Academic Press, London, 1970) $\endgroup$ Oct 6, 2018 at 21:46
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    $\begingroup$ @AndréHenrique Thanks for the reference! I can't find this paper in the web as well as in university's library. Do you have a copy of it? I am a bit suspicious about this result since it seems that Bhatt and Scholze in their paper (arxiv.org/pdf/1309.1198.pdf) suggest that continuous cohomology of profinite groups shouldn't form a "delta functor" (a sentence after Remark $4.3.10$). Is it true that Segal has the same definition of exact sequence and delta functor? Of course, it is possible that they just weren't aware of this Segal's paper, but I've seen this claim many times in NTbooks $\endgroup$
    – gdb
    Oct 6, 2018 at 22:22

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