Questions about the branch of abstract algebra that deals with groups.

**17**

votes

**1**answer

520 views

### Which philosophy for reductive groups?

I am just beginning to look further into trace formulas and automorphic forms in a quite general setting. For long I have noticed that the natural assumption on the groupe $G$ we work on is to be ...

**5**

votes

**0**answers

88 views

### Second homology of mapping class group via universal coefficient theorem

Let $S_g$ be the closed, orientable surface of genus $g\ge 2$ and $Mod(S_{g,1})$ the mapping class group of homeomorphisms fixing a base point $x_0\in S_g$ modulo isotopies fixing $x_0$.
It is known ...

**0**

votes

**0**answers

78 views

### When can the restriction map be zero or onto?

Let $G$ be a finite $p$ group of order $p^n$ and $N$ be a subgroup of $G$ of index $p$. Let $res:H^2(G,A) \rightarrow H^2(N,A)$ be the restriction map on cohomology, where $A$ is a trivial $G$ module. ...

**3**

votes

**1**answer

87 views

### Finding a semigroup that maximizes the trace of a sum of matrices

Let $H$ be a finite semigroup containing $n$ elements from a compact group $G$. I am trying to solve
$$\max_{h_i,\ h_j\ \in\ H} \operatorname{tr} \sum_{i,\ j\ \leq\ n} \rho(h_i^{-1} h_j)(A_j ...

**6**

votes

**0**answers

154 views

### What's the name of the cohomology class associated to a projective representation?

Suppose $\rho : G \to PGL_n(k)$ is a projective representation of a group $G$ over a field $k$. It's classical that the obstruction to lifting this to a linear representation $G \to GL_n(k)$ is a ...

**3**

votes

**1**answer

170 views

### Homology of solvable Lie groups made discrete

In what follows "homology" will mean group homology, i.e. $H_*(BG^\delta;{\mathbf R})$ for the group $G$ with the discrete topology.
It is well-known how to compute the homology of abelian groups, ...

**6**

votes

**1**answer

139 views

### Are the integer matrices in SO(3,2) “boundedly generated”?

Let $G$ be the subgroup of integer matrices in $\mathrm{SO}(3,2)$.
(The invertible linear maps from a $5$ dimensional real vector space to itself which leave invariant a nondegenerate symmetric ...

**1**

vote

**0**answers

57 views

### Salvaging Howson's theorem for free profinite groups

This is an attempt to find a correct version of Do free profinite groups satisfy Howson's theorem?
Let $F$ be a free profinite group, and let $A,B \leq F$ be closed finitely generated subgroups ...

**3**

votes

**2**answers

218 views

### Do limit groups satisfy Howson's theorem?

Let $G$ be a limit group, and let $A,B \leq G$ be finitely generated
subgroups generating $G$ (i.e. $\langle A \cup B \rangle = G$). Must
$A \cap B$ be finitely generated?
Recall that a limit ...

**1**

vote

**2**answers

193 views

### Is $K\cap \langle H\cup N\rangle\subseteq \langle H\cup (K\cap N)\rangle$? [closed]

If $G$ is a arbitrary group, $H,K,N\leq G$ such that $H\subseteq K$, then $K\cap \langle H\cup N\rangle\subseteq \langle H\cup (K\cap N)\rangle$?. If it is not true, how can I find an ...

**9**

votes

**3**answers

311 views

### Integer matrix that does not belong to a free group of rank 2

I'm given two matrices in $SL_2(\mathbb{Z})$
$$
A = \left(\begin{array}{cc}
2 & 3\\
3 & 5
\end{array}\right), \ \
B = \left(\begin{array}{cc}
5 & 3\\
3 ...

**1**

vote

**0**answers

133 views

### Invariant free factors for automorphisms of free products

I am thinking the following question: for a group that splits as a free product $G = G_1 \ast F_n $, where $G_1$ is any infinite group (preferebly freely indecomposable) and we denote by $F_n$ the ...

**0**

votes

**1**answer

112 views

### Averaging measurable functions over amenable group actions

Let $G$ be an amenable group acting on a space $X$.
Amenability means there is a $G$-invariant mean on $L^\infty(G,{\mathbf R})$.
Given a bounded function $f\colon X\to {\mathbf R}$ one can use the ...

**1**

vote

**0**answers

123 views

### Subgroup generated by elements of finite order

Let G be any group and let H be the subgroup of G generated by all elements of finite order. Is there a name for H?

**10**

votes

**2**answers

278 views

### Conjugacy of matrices of order three in $PGL(2,k)$, where $k$ is any field

We know that every matrix of order three in $PGL(2,\mathbb Z)$ is conjugate to the following matrix
$$
\left(
\begin{array}[cc]
&1 & -1 \\
1&0
\end{array}
\right)
$$
I want to know if ...

**0**

votes

**1**answer

193 views

### Computational Algebra and Symbolic Computation - Where? [closed]

Following the line of this question, I'm in my last year of M.Sc., and I'm looking for a place where I can start my PHD. Since that question has been asked 4 years ago, I thought it may be wise to ask ...

**4**

votes

**1**answer

159 views

### Existence of some character degree in a solvable group

Let $p=2^n-1$ be a Mersenne prime. We know that $H=PSL(2,p^4)$ has an irreducible character of degree $ p^4$.
Is there any solvable group of order $H$ with an irreducible character of degree $ p^4$. ...

**5**

votes

**1**answer

81 views

### Embedding abelian cancellative Hausdorff topological semigroups into abelian Hausdorff topological groups

An abelian cancellative semigroup embeds (via a semigroup monomorphism) into an abelian group. What about an abelian cancellative Hausdorff topological semigroup that does not embed (via a ...

**0**

votes

**0**answers

115 views

### Rational cohomological dimension of the lattices in $SL_n(\mathbb{Q}_p)$

What is the rational cohomological dimension of the lattices in $SL_n(\mathbb{Q}_p)$, where $n\geq 3$ ? A reference would be appreciated.
For the definition of "cohomological dimension of a group ...

**4**

votes

**1**answer

183 views

### reference on classfication of multiply transitive permutation groups

It seems to be true that multiply transitive permutation groups have been classified completely (using CFSG), but I am having trouble finding a reference where this classification is actually stated. ...

**3**

votes

**1**answer

84 views

### Unipotent radical of minimal parabolic subgroup of a unitary group over an arbitrary field

I am looking for an explicit description of the unipotent radical of a minimal parabolic subgroup of a unitary group, i.e. the group of isometries of a hermitian form, over an arbitrary field.
In his ...

**0**

votes

**0**answers

59 views

### Homology of product of two groups [duplicate]

There is well known formula for the homology of product of two groups with coefficient in integers, that is
$0 \rightarrow \oplus_{p+q=n}H_p(G,\mathbb{Z}) \otimes H_q(H,\mathbb{Z}) \rightarrow H_n(G ...

**2**

votes

**0**answers

72 views

### On subdirect products [closed]

I'm sorry if this question doesn't fit with MO rules, but I've asked on Math SE yet without answers, so I post here with the hope someone will answer me.
I want to show, knowing the Goursat's ...

**5**

votes

**0**answers

242 views

### Quotients of an extension of the Higman group

(Note: this started as a different question that soon changed form, thanks to the answers.)
Let $G = \mathbb{Z}/4\mathbb{Z} \ltimes H_4$, where $H_4$ is the Higman group and $\mathbb{Z}/4\mathbb{Z}$ ...

**2**

votes

**0**answers

70 views

### Positive definiteness via Folner sequence

Let $\Gamma$ be a countable discrete group. A function $\phi:\Gamma\to\mathbb{C}$ is called positively definite if for every finite subset $F$ of $\Gamma$, the |F| by |F| matrix ...

**4**

votes

**2**answers

256 views

### Structure of the group generated by two specific symplectic matrices

Consider the following two symplectic matrices
$$
A \ = \
\left(\begin{array}{rrrr}%
1&0&0&0\\%
0&1&0&0\\%
0&0&-1&1\\%
0&0&-1&0\\%
\end{array}\right), \ ...

**4**

votes

**1**answer

240 views

### Recognize this countably generated abelian group?

I recently came across a construction that, in abstraction, leads to the following family of abelian groups: Fix $1<q<p$ with $q$ and $p$ relatively prime. The group $G_{(p,q)}$ is given by the ...

**10**

votes

**1**answer

340 views

### Extending an infinite simple group

Maybe the question does not fit here.
Yesterday in my logic course, I presented a nice example about an application of model theory to group theory. The example is due to Hodges and as following: For ...

**6**

votes

**2**answers

261 views

### Faithful projective representations of symmetric groups

This is a reference request.
Do you know where I can find the dimensions of the faithful projective representations of $S_n$ and $A_n$ for $n\ge 5$?
Thank you in advance.

**7**

votes

**3**answers

509 views

### Beyond Brauer's theorem

Brauer's classic theorem states that any character of a finite group can be expressed as a linear combination with integer coefficients of characters induced by linear characters of p-elementary ...

**17**

votes

**1**answer

626 views

### “Squeezing” a finite group between symmetric groups

For a finite group $G$, take $m$ as the largest integer such that $G$ has a subgroup $H\cong S_m$ and $n$ as the smallest integer such that $G$ is itself isomorphic to a subgroup of $S_n$. We then ...

**1**

vote

**0**answers

125 views

### Is there a brute force method for determining irreducible representations?

Suppose I have some groups $G_1$, $G_2$, $G_3$, etc... Then the direct product is given by $G = G_1 \times G_2 \times G_3 \ldots$
I know that the sub-representations of a reducible representation ...

**-1**

votes

**1**answer

108 views

### Example of a group in which centralizers of every element are non-abelian [closed]

I am studying $AC$-groups, i.e. groups in which the centralizer of every
non-identity element is abelian. Now I need an example of a group in which
the centralizer of every non-identity element is ...

**2**

votes

**0**answers

31 views

### Local zetafunction of T-group and Lie-ring coincide

Grunewald, Segal and Smith define in "Subgroups of finite index in nilpotent groups" a zetafunction associated to a finitely generated, torsion-free nilpotent group G by counting normal subgroups ...

**10**

votes

**1**answer

206 views

### A generalization of residual finiteness to topological groups

Consider the following generalization of residual finiteness to
topological groups.
A locally compact Hausdorff group $G$ is called residually compact if
for every compact $K \subseteq G$ there is a ...

**2**

votes

**0**answers

109 views

### Divisible fundamental group [duplicate]

I apologize if this question seems trivial or elementary. Is there any concrete topological space with divisible fundamental group? For example, is there any such a space the fundamental group in ...

**4**

votes

**1**answer

251 views

### Number of different positions of rooks on chessboard

I know that this topic as been mentioned before, but no accurate answer has been provided.
Suppose we have to place $n$ rooks on $n \times n$ chessboard so that no one attacks another. How to count ...

**1**

vote

**0**answers

87 views

### A finite distributive lattice which may be represented as the normal subgroup lattice of a supersolvable group

Is there a supersolvable group $G$ with the lattice of all its normal subgroups, order-isommorphic to the 18-element lattice of down-sets of this poset:
?
It has been proved that not every finite ...

**5**

votes

**0**answers

106 views

### Connectedness of cones in the boundary of a 1-ended hyperbolic group

Let $G$ be a one-ended hyperbolic group. We can think of the boundary of $G$ as consisting of geodesic rays originating at the identity in some Cayley graph, modulo the relationship of being ...

**2**

votes

**1**answer

85 views

### Looking for a modern source about Ulm Invariants

I'm looking for a modern, approachable text (preferably a website, textbook, or expository article, and preferably one easily available online or at a library) which can explain the concept of Ulm ...

**4**

votes

**1**answer

145 views

### Obstruction for two subgroups to be conjugated by an automorphism

Altough this sounds as a very basic question, I didn't receive any answer on stack exchange and by people more knowledgeable than me
Take $p$ a prime number and $P$ an abelian finite $p$-group. Let ...

**12**

votes

**4**answers

524 views

### Normal subgroups of an extension of the Higman group

Let $G = \mathbb{Z}/4\mathbb{Z} \ltimes H_4$, where $H_4$ is the Higman group and $\mathbb{Z}/4\mathbb{Z}$ acts on $H_4$ in the obvious way (permuting the four standard generators cyclicly). The group ...

**5**

votes

**0**answers

102 views

### A group acting acylindrically on a fine hyperbolic graph with infinite edge stabilizers

I am looking for an example of a group $G$ that acts (cocompactly and) acylindrically on a hyperbolic graph $\Gamma$, such that
a) the graph $\Gamma$ is fine,
b) $\Gamma$ is not a tree,
c) not all ...

**12**

votes

**1**answer

412 views

### Realizing symmetric groups by diffeomorphisms

Let $M$ be a (closed, smooth) manifold of dimension $d$. For $n$ a positive integer, fix $n$ points $x_1, \dots, x_n \in M$. The group of diffeomorphisms of $M$ that permutes the points $x_i$ surjects ...

**4**

votes

**0**answers

93 views

### Example of “exotic” verbal subgroups of free groups

This will be an ambiguous question.
I am interested in various examples that appear in the literature of verbal subgroups of free groups, but which are not part of the "classical examples" like ...

**4**

votes

**1**answer

326 views

### Counting cyclic subgroups of order $p^{2}$: $p$ an odd prime vs. $p=2$

While playing with Frobenius' problem (about finite groups $G$ in which, for some positive integer $n \mid |G|$, there are exactly $n$ elements of order dividing $n$), I came up with the following ...

**3**

votes

**1**answer

152 views

### Intersection of maximal subgroups of PSL(2,q)

Let $G := PSL(2,2^r)$, and let $M$ be a maximal subgroup of $G$ isomorphic to $PSL(2,2^s)$. I need to compute $H := M \cap M^g$ for $g \in G-M$. It seems to me that $|H|$ must be $2^r, 2^r\pm 1$ or ...

**6**

votes

**2**answers

335 views

### Counting matrices over finite fields of a given order

How can I count/enumerate matrices in ${\rm GL}(2,{\rm GF}(2^5))$
of order $3$? In general, how can I obtain the number of matrices in
${\rm GL}(2,{\rm GF}(q))$, where $q$ is a power of a prime, of ...

**5**

votes

**0**answers

40 views

### “Quasi-orthogonal” subgroup of a group with length?

For my project in bivariant K-theory for locally convex algebras, I'm looking how to call a particular notion of groups, too simple to be never considered elsewhere.
Let $G$ b a group with length ...

**8**

votes

**2**answers

255 views

### Groups with trivial rational homology and their finite index subgroups

For a short exact sequence $0 \to G \to H \to K \to 0$ of (discrete) groups with $K$ finite we have, as a consequence of the Hochschild-Serre spectral sequence, that $H^{\ast}(H;\mathbb Q) = ...