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1answer
16 views

Another description for the map $\text{Ext}^1_\mathbb{Z}(A,G)\to H^2(G,A)$

Group extensions of $G$ by $A$ $0\to A\to E\to G\to 0$ up to equivalence (where $G$ and $E$ may be nonabelian) are in bijection with the second group cohomology ...
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19 views

Extension to rational and real chains

In the paper on stable commutator length, D. Calegari says that generalized $\operatorname{scl}$ function can be extended by linearity to rational group $1$-chains and by continuity to real chains ...
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1answer
21 views

Supplement for reading Group cohomology from Serre Local Fields

I am doing a reading course on Group cohomology... I am supposed to start reading Group cohomology part in Serre's Local fields Book ...
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2answers
168 views

How can I find $\mathbb Z_4$ as an extension of $\mathbb Z_2$ by $\mathbb Z_2$?

Let $H=\{1,h\}$ and $A=\{0,a\}$ be groups, and $\pi:H\rightarrow \text{Aut}(A)$ be the trivial homomorphism. I have found $FS(H,A,\pi)=\{f_0,f_1\}$ and $IFS(H,A,\pi)={f_0}$ where ...
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1answer
37 views

First cohomology group of direct products

Let $p$ be a prime number and H be a finite group with $|H|=p-1$ and consider $\varphi: H \times Z_{p^k} \rightarrow Aut(Z_{p^k})$ as a non-trivial action of $H \times Z_{p^k}$ on $Z_{p^k}$ such that ...
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1answer
75 views

Understanding a proof about splitting of short exact sequences.

I am reading a paper by Keith Conrad about the splitting of exact sequences. I have a few questions about one particular section. This is Theorem 3.3 in this paper ...
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1answer
92 views

Cohomology group $H^n(G, \mathbb{R}/\mathbb{Z})$ as a continuous group or a Lie group?

Is there any example of Borel cohomology group $H^n(G, \mathbb{R}/\mathbb{Z})$ for any $G$ such that $H^n(G, \mathbb{R}/\mathbb{Z})$ is a continuous group? Such as a Lie group? Most of the examples ...
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1answer
27 views

Schur Multiplier of general linear group

Ideally I would like to know the Schur multiplier of $Gl(n, F_3)$, but perhaps this is not reasonable to ask. But for a small fixed $n$, this should be known, but i could not find any result when ...
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0answers
27 views

Kunneth formula for group homology

I'm trying to prove Kunneth formula for group homology. $$ 0 \to \bigoplus_p H_p(G,M)\otimes H_{n-p}(G',M') \to H_n(G\times G',M \times M') \to \bigoplus_p Tor_1^{\mathbb Z}(H_p(G,M),H_{n-p-1}(G',M')) ...
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52 views

Homology groups of $SL(2,\mathbb Z)$

I am reading Brown's book "Cohomology of Groups" and I can't solve exercise II.7.1.3.: "It's a classical fact that $SL_2(\mathbb Z) \cong \mathbb Z_6 *_{\mathbb Z_2}\mathbb Z_4.$ Use Mayer-Vietoris ...
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19 views

Two nonequivalent extensions with the same factors

Let $0 \rightarrow M \overset{\alpha}{\rightarrow} E \overset{\beta}{\rightarrow} P \rightarrow 0$ and $0 \rightarrow M \overset{\alpha'}{\rightarrow} E' \overset{\beta'}{\rightarrow} P \rightarrow 0$ ...
1
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1answer
131 views

What can we say about groups $G$ with $H_3(G)=0$?

Let $G$ be a group. What can we say about groups such that $H_3(G)=0$? If a characterization is not possible, then knowing examples of such groups would be good? Any help is appreciated. Thanks
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0answers
24 views

Is group cohomology functorial in the first argument?

Ie, suppose you have a group $G$ acting on a group $A$ (allowing both to be nonabelian), then suppose you have another group $G'$ acting on $A$, and a group homomorphism $G\rightarrow G'$ (maybe we ...
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0answers
37 views

How does the first group cohomology classify torsors?

I know this is true, but I'm having some trouble finding any references on this. I'm in particular interested in the nonabelian case. Specifically, let $G$ be a group acting on a group $A$ (both ...
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1answer
53 views

Recovering a character from a cohomology class

Let $G$ be a finite group, and consider $\mathbf Z, \mathbf Q$ and $\mathbf Q/\mathbf Z$ as trivial $G$-modules. Then $H^1(G, \mathbf Q/\mathbf Z) = {\rm Hom}(G, \mathbf Q/\mathbf Z) = \widehat{G}$. ...
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1answer
56 views

Why do we have that Hom is an exact functor in the situation described below?

We are given a finite $p$-group $G$ and a finite $G$-module $M$ such that $pM=0$ (therefore $M$ is in particular a $\mathbb{F}_p$-vector space). In addition we have an arbitrary $G$-module $N$ which ...
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1answer
25 views

Group cohomology of mapping-class-group

Let $MCG_g$ be the mapping class group of closed genus $g$ Riemannian surface. What is the group cohomology $H^n(MSG_g,Z)$ for $n=2$ (and other values).
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37 views

$\operatorname{Br}(\Bbb Q_{47})$

I'm looking for division algebras over $\Bbb Q_{47}$. I guess my best bet is to calculate the Brauer group $\operatorname{Br}(\Bbb Q_{47})$. What's the best way of performing this calculation? Should ...
2
votes
1answer
53 views

Computing $H^\bullet(\Bbb Z/n\Bbb Z)$

This is related to this other question of mine Showing that $\operatorname {Br}(\Bbb F_q)=0$ in which I also got stuck at writing a free resolution. I want to compute the group cohomology ...
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2answers
87 views

Showing that $\operatorname {Br}(\Bbb F_q)=0$

I want to prove that $\operatorname {Br}(\Bbb F_q)=0$ using the cohomological description of the Brauer group. We have: $\operatorname {Br}(\Bbb F_q)=H^2(\operatorname {Gal}(\overline {\Bbb ...
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0answers
24 views

free resolutions of $\mathbb Z$ in Mod(G)

Lang Algebra, XX.2,3 I'm asked to show that $E_\bullet \cong F_\bullet$ are isomorphic free resolutions of $\mathbb Z$, in Mod(G), where $E_\bullet$ is the standard complex: $E_i$ is the free ...
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21 views

Sum rules for group-cohomology cocycles?

Consider a cocycle ($\omega$) in $H^n(G,U(1))$, where $U(1)$ is the group of unitary matrices over ${\rm C}$ of dimension 1. Thinking of $U(1)$ as a circle in the complex plane, one can multiply and ...
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1answer
86 views

Second cohomology group of a perfect group

Consider a finite perfect group $G$ and a $G$-module, $U\left(1\right)$, on which $G$ acts trivially. Here $U\left(1\right)$ is the set of $1 \times 1$ unitary matrices over $\mathbb{C}$. Are there ...
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1answer
84 views

Understanding the Bockstein homomorphism in group cohomology

Let $k$ be an algebraically closed field of characteristic $p>0$. Let $G$ be a finite group. In group cohomology, the Bockstein homomorphism is the connecting homomorphism ...
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1answer
94 views

How do you compute group cohomology in practice?

If you have a finite group $G$ and a finite $G$-module $K$, and you need to know $H^1(G,K)$ or $H^2(G,K)$, how do you do it? Do you use a computer algebra system? (If so, which one?) Do you use a ...
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0answers
53 views

Central extensions of elementary abelian p-groups

Given an elementary abelian $p$-group $E$, we can consider $E$ as a trivial $E$-module; my first question is : How can one compute the rank of of the cohomology group $\operatorname{H}^n(E,E)$, $n ...
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1answer
37 views

Property of G-modules involving the invariant elements under the G-action

I am stuck at some basic fact I would like to prove. I tried proving it using $G-$orbits and cardinalities, but without success. Let $p$ be some prime number, $G$ be a finite $p-$group and $A$ a ...
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0answers
75 views

Can group cohomology be used to study fiber bundles?

Is (non-abelian) cohomology used to study vector and principal bundles? Can you give me a text or an article? For example: Consider a vector bundle $E$ with fiber $V$ and base manifold $M$. Consider ...
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0answers
31 views

Group cohomology of $\mathbb Z^n$.

I know very little about group cohomology but the following came up in something else I was looking at: Let $c_1,c_2,\ldots,c_n \in \mathbb C$ and let $\mathbb Z^n$ act on $\mathbb C$ via the ...
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1answer
64 views

Reference request: Introduction to Finite Group Cohomology

I don't know anything about group cohomology and I'd like to. What is the best text to learn this subject? I'd prefer as soft an introduction as possible - that is, lots of motivation, lots of ...
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37 views

Uniqueness of the cohomological functor

This question is from the chapter 'Cohomology of Groups' by Atiyah and Wall in Cassels' and Frohlich's book 'Algebraic Number Theory'. Let $G$ be a group. Theorem 1 on page 95 says that there is a ...
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1answer
32 views

A G-isomorphism of certain Hom groups

This question is from 'Cohomology of Groups' by Atiyah and Wall, p.95 of Cassels' and Frohlich's book 'Algebraic Number Theory'. Let $G$ be a group and $A={\rm Hom}_{\mathbb Z}(\mathbb Z[G],X)$ where ...
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0answers
61 views

inflation restriction exact sequence

Let $G$ be a profinite group and $H$ be a normal closed subgroup of $G$. For every discrete $G$-module $A$ we have the inflation-restriction exact sequence $$0 \to H^1(G/H,A^H) \to H^1(G,A) \to ...
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0answers
93 views

Cohomology of covering space

Let $B$ be a base space and $E$ be a covering space of $B$ what is the relation between $H^2(B,\mathbb{Z})$ and $H^2(E,\mathbb{Z})$.?
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2answers
57 views

Inductive definition of group cohomology?

At the start of Atiyah and Wall's section on group cohomology (in the Cassels-Frhlich collection of Algebraic Number Theory notes) they, of course, define group cohomology (actually, a 'cohomological ...
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1answer
63 views

Cohomology of finite p-groups

Given a finite abelian $p$-group $A$ acted on by a finite $p$-group $G$. Under the assumption $\operatorname{H}^1(G,A_1)=0$, where $A_1$ is the set of elements of $A$ having order at most $p$, what ...
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1answer
61 views

Show that image of $res$ lies in $H^n(H,A)^{G/H}$

Let $G$ and $G^{\prime}$ be groups, $A$ and $A^{\prime}$ be $G$-module and $G^{\prime}$-module respectively, $C^n(G,A)$ be set of all maps from $G \times \cdots \times G$ ($n$ times) to $A$, $d_n ...
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0answers
66 views

Corestriction map in lie algebra cohomology

Given a lie algebra $\mathfrak{g}$ over a field $k$, we can define the cohomology groups of $\mathfrak{g}$ as follows: $$H^n(\mathfrak{g},k):=\mathrm{Ext}_{U(\mathfrak{g})}^n(k,k)$$ where ...
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1answer
79 views

Cohomological ($p$-)dimension of a pro-$p$ group

I have a question concerning the cohomological dimension and $p$-dimension of a pro-$p$-group. Let's first recall the definitions of that The cohomological dimension $cd \ G$ of a pro-finite group ...
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1answer
107 views

Why are projective modules cohomologically trivial?

Let $G$ be a finite group, $H\subset G$ a subgroup, $k$ a commutative ring, $M$ a $kG$-module, $n\in\mathbb{Z}$, and $\hat{H}\,^n(H,M)$ the $n$th Tate cohomology group as defined in this question, ...
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1answer
86 views

Finite generation of Tate cohomology groups

Let $G$ be a finite group, and let $F$ be a complete resolution for $G$. In other words, $F$ is an acyclic chain complex of projective $\mathbb{Z}G$-modules together with a map ...
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1answer
57 views

1-Cocycle of an Algebra

Is there a good definition of a 1-Cocycle of an algebra A that is relatively easy to understand? I am rather new to cohomology and representation theory, but it seems like this is a fundamental ...
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0answers
90 views

A group theory exercise in Brown's group cohomology book

My question is taken from exercise IV.3.4(b) in Brown's group cohomology book. Let $E$ be a finitely generated group, and suppose that $C$, the center of $E$, has finite index in $E$, say $[E:C]=n$. ...
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1answer
79 views

Extensions of $\mathbb{Z}_n$ by $\mathbb{Z}$

Given that $H^2(\mathbb{Z}_n,\mathbb{Z})=\mathbb{Z}_n$, it follows that up to equivalence there should be $n$ extensions of $\mathbb{Z}_n$ by $\mathbb{Z}$, one for each cohomology class. I'd like to ...
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1answer
132 views

Computing the action of $S_3$ on $H^n(\mathbb{Z}_3,\mathbb{Z})$

Let $G=S_3$ and let $H$ be the Sylow $3$-subgroup in $G$. If $\mathbb{Z}$ is the trivial module, then it can be shown that $$H^n(H,\mathbb{Z})=\begin{cases}\mathbb{Z}&n=0\\0&n\text{ ...
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0answers
58 views

Exact Sequences and Cohomology.

I am currently working through exercises in "Cohomology of Groups" by Kenneth S. Brown. In particular I am struggeling with exercise 4 (d) on page 90. My question is concerning the last step in that ...
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2answers
104 views

The norm map in group cohomology is an isomorphism if $M$ is a projective $G$-module

This is exercise III$.1.1(c)$ in Brown's "Cohomology of Groups." Let $G$ be a finite group, $M$ a $G$-module, and $\overline{N}:M_G\to M^G$ the norm map defined by $[m]\to Nm$, where $N=\sum_{g\in ...
3
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1answer
66 views

Short exact sequences of non abelian groups

It's a long time I am not working with groups. Maybe you know the answer to these questions. Suppose that $1\rightarrow A\xrightarrow{i_1} B_1\xrightarrow{\pi_1}C\rightarrow 1$ and $1\rightarrow ...
4
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2answers
109 views

Cohomology of finite groups with finite coefficients

I'm wondering if the group cohomology of a finite group $G$ can be made nontrivial with a nice choice of a finite $G$-module M. In other words, given a finite group $G$ and a number $n$, does there ...
5
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1answer
246 views

The Zig Zag Lemma in Cohomology

I´m reading the Zig Zag lemma in Cohomology and i want to prove the exactness of cohomology sequence at $ H^k(A)$ and $H^k(B)$ : A short exact sequence of cochain complexes $ 0 \to A \ ...