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

**1**

vote

**2**answers

208 views

### A table for irreducible integral representation of finite cyclic groups

Is there such a table where the irreducible integral representations of finite cyclic groups
are listed?
Edited:
Thanks for Todd Leason's comment.Acutally,i want to know all inequivalent ...

**1**

vote

**1**answer

72 views

### Projectors onto the invariant subspaces of a unitary representation $U \otimes U^* \otimes U \otimes U^*$

Let $$U \mapsto U \otimes U^* \otimes U \otimes U^*$$ be a unitary representation of the unitary group $U(n)$ acting on the vector space $V$ (where $U^*$ is the complex conjugate of $U$). We can ...

**33**

votes

**2**answers

741 views

### Do unit quaternions at vertices of a regular 4-simplex, one being 1, generate a free group?

Choose unit quaternions $q_0, q_1, q_2, q_3, q_4$ that form the vertices of a regular 4-simplex in the quaternions. Assume $q_0 = 1$. Let the other four generate a group via quaternion ...

**4**

votes

**0**answers

59 views

### A relation on the set of isomorphism classes of finitely generated groups

Let $G$ be the set of finitely generated groups up to isomorphism hence its elements will be noted $[B]$ where $B$ is some finitely generated group.
On this set we put a relation $\mathcal{ND}$ ...

**1**

vote

**1**answer

85 views

### Collections in direct products and freeness

I am looking for references about the following type of questions:
Let $G$ and $H$ be two groups,
let $(g_i:i\in I)\subset G$ and $(h_i:i\in I)\subset H$ be collections of group elements,
and ...

**5**

votes

**0**answers

149 views

### What is the kernel of the map $Out(\widehat{F_2})\rightarrow GL_2(\widehat{\mathbb{Z}})$?

Let $F_2$ be the free group on two generators, and $\widehat{F_2}$ its profinite completion. Let $Out(\widehat{F_2})$ be the outer automorphism group of $\widehat{F_2}$, ie, $Out(\widehat{F_2}) = ...

**17**

votes

**0**answers

205 views

### Guess that group via product queries

Suppose someone (person B) knows a finite group $G$ of order $n$.
You (person A) know only the order $n$,
and that $1$ is the name of the identity element.
The group elements are named $1,2,\ldots,n$ ...

**9**

votes

**2**answers

208 views

### Embedding $\mathrm{PGL}(n,q^h)$ in $\mathrm{PGL}(nh,q)$

It is not very hard to see that for each prime power $q$ and natural numbers $n,h$, we have an embedding
$$\iota \colon \mathrm{GL}(n,q^h) \hookrightarrow \mathrm{GL}(nh, q),$$
obtained by choosing a ...

**11**

votes

**2**answers

462 views

### Obstructions for a group to be the multiplicative group of a field [duplicate]

It is well known that every finite multiplicative subgroup of a field is cyclic.
I somehow got interested in a possible reverse implication:
Assume we have an abelian group $G$ whose every finite ...

**1**

vote

**0**answers

104 views

### How generic are Cayley graphs of non-Abelian groups with logarithmic girth?

Given a non-Abelian group $G$ I want to choose a symmetric generating set $S \subset G$ such that $Cay(G,S)$ has girth logarithmic in the size of the set. I want to know,
For which $G$ can the ...

**2**

votes

**1**answer

186 views

### Explicit examples of Dehn presentations of hyperbolic groups

It is well known fact that a (f.g.) group is hyperbolic if and only if it admits a (finite) Dehn presentation.
My question concerns something I'm struggling with since the first time I read the proof ...

**-3**

votes

**0**answers

84 views

### assume subgroup $H$ of $G$ such that $N$ is also a subgoup of $H$, then $ P_{G/N}(H/N) = P_{G}(H)/N$ [closed]

The
permutizer of a subgroup $H$ of $G$ is defined to be the subgroup generated by all cyclic subgroups of $G$ that permute with $H$,
i.e. $\langle x \in G | \langle x \rangle H = H \langle x \rangle ...

**-3**

votes

**0**answers

60 views

### Schur multiplier [closed]

Are there real world applications of Schur multiplier?
I am interested in applications of topics specifically coming from Schur multiplier. for example, in biology, computer sience and other branch.

**1**

vote

**0**answers

205 views

### Analogues of the Monster for central charges different from 24

One way to define the Monster group is to consider a conformal field theory (CFT) corresponding to central charge $c=24$ and look at the automorphism group of its vertex operator algebra. For one of ...

**2**

votes

**0**answers

79 views

### Does every group that satisfies the maximal permutizer condition then satisfy the permutizer condition?

The
permutizer of a subgroup $H$ of $G$ is defined to be the subgroup generated by all cyclic subgroups of $G$ that permute with $H$,
i.e. $\langle x \in G | \langle x \rangle H = H \langle x \rangle ...

**10**

votes

**1**answer

374 views

### Class field towers

It is known (Golod and Shafarevich) that the class field tower of a finite extension $K$ of $\mathbb{Q}$ may be infinite. But is it always finite for $K=\mathbb{Q}[\zeta]$ where $\zeta$ is a root of ...

**1**

vote

**0**answers

87 views

### Does Kaplansky's Zero Divisor Conjecture hold valid for (torsion-free) residually finite groups?

Kaplansky's Zero Divisor Conjecture states that the group algebra $KG$ has no zero divisor for any field $K$ and any torsion-free group $G$.
Does Kaplansky's Zero Divisor Conjecture hold valid ...

**1**

vote

**1**answer

204 views

### Limit Group decomposition

I would need a clarification about a statement in the article Limit groups and groups acting freely on $\mathbb{R}^n$-trees by Vincent Guirardel.
First recall that a limit group is a finitely ...

**1**

vote

**0**answers

77 views

### When can the rank of a submodule be bigger than the rank of the module itself? [migrated]

It is well known that the dimension of a subspace is less than or equal to the dimension of the vector space it is contained in. The same is true e.g. for modules over a principal ring.
I am looking ...

**0**

votes

**0**answers

57 views

### Action of semidirect products of cyclic groups

Is there anything known about group actions of $C_{p}\rtimes C_{p}^{*}$ on the ring of real polynomials $\mathbb{R}[X_{1},\ldots,X_{n}]$, where $C_{p}$ denotes the cyclic group of order $p$ and $p$ is ...

**5**

votes

**2**answers

84 views

### “Relative cone types” for groups relative to some collection of subgroups

It is a well known fact that an infinite hyperbolic group contains an element of infinite order (see e.g. Bridson, Haefliger, Metric spaces of non-positive curvature, Prop. 2.22 on p. 458)
I am ...

**-1**

votes

**0**answers

30 views

### Rank of finite solvable group [migrated]

I am very interested in the following question.
Is there a finite solvable group G with the property that rank G - rank G_ab > n for n > 2? Here G_ab denotes the abelianization of G. For all the ...

**4**

votes

**2**answers

136 views

### Orbits of the maximal compact subgroup on the light cone for $p$-adic groups

It is known that if $Q$ is an indefinite non-degenerate quadratic form on $ \mathbb{R}^n$ with $n \ge 3$, then any maximal compact subgroup $K$ of the orthogonal group $SO(Q)$ acts transitively on the ...

**2**

votes

**0**answers

62 views

### Pro-G_p*G_q topology, profinite topology

Let $W=G_p*G_q$, where $G_p$ and $G_q$ are pseudovarieties of
all finite $p$-groups and all finite $q$-groups respectively, with $p$ and $q$ fixed prime numbers. The pro-$W$ topology on a group $G$ is ...

**2**

votes

**1**answer

171 views

### Uncountable cardinals and Prufer $p$-groups

Let $A$ be an elementary Abelian uncountable $p$-group. Is it known if there is an action of a Prufer $q$-group (here $q$ is a prime not necessarily distinct from $p$) $C_{q^{\infty}}$ onto $A$ such ...

**3**

votes

**1**answer

117 views

### amalgamation of locally finite groups

It is well known that in category of groups there are Push-outs so it is possible to realize amalgamation in some kind of most free way. My question is what about category of locally free groups? I ...

**1**

vote

**1**answer

127 views

### Generating finite groups using subgroups

For finite groups $L \leq K$ we define $d(L,K)$ to be the least $n \in \mathbb{N}$ for which there exist $a_1, \dots, a_n \in K$ such that $\langle L, a_1 \dots, a_n \rangle = K$.
Is there some $m ...

**1**

vote

**0**answers

59 views

### Why is the polynomial relating the invariants of a binary polyhedral group fixed by an overgroup?

Let $G$ be a finite subgroup of $\mathrm{SL}(2,\mathbb{C})$ and $N \triangleleft G$ a normal subgroup. Let $x, y, z$ be the fundamental invariants for the standard action of $N$ on $\mathbb{C}^2$, ...

**3**

votes

**3**answers

432 views

### The free group of a group and the kernel of a canonical morphism

Let $G$ be a group and $F_G$ the free group on the set $G$. Then there exists a canonical surjective morphism ${\rm can}: F_G \to G \to 1$ constructed as follows: let $(e_x)_{x \in G}$ be a copy as a ...

**14**

votes

**2**answers

348 views

### Explicit permutation representation of the Thompson sporadic simple group?

The Thompson group Th of order $90745943887872000$ is one of the sporadic simple groups occurring in the classification of finite simple groups.
Its maximal subgroups are known (see ...

**2**

votes

**1**answer

78 views

### Generators of the colored braid group (two colors), reference

I consider the group $B_{n,n}$, the braids, colored in two colors, say all odd strings are black and all even strings are white.
It is easy to find a set of generators for $B_{n,n}$:
$$
\begin{cases}
...

**0**

votes

**0**answers

43 views

### projective representation of supergroup

In fact, I am not very clear about what I am asking, but I am looking for a concrete example of supergroup which has non-trivial projective representation(some supergroup similar to usual Lie group ...

**6**

votes

**1**answer

206 views

### For discrete groups, does the Haagerup property imply the AP of Haagerup-Kraus?

I don't expect to find an explicit counterexample to my question, because any example which was known to have the Haagerup property yet not have AP would have given an exact group without AP, and the ...

**4**

votes

**0**answers

63 views

### Topological systems of imprimitivity

Let $G$ be a group acting by homeomorphisms on a topological space $X$. $G$ is topologically transitive if every open $G$-invariant subset of $X$ is empty or dense.
Here is an attempt to define ...

**0**

votes

**0**answers

16 views

### Is trace of regular representation in Lie group a delta function? [migrated]

My major is physics. I need to use some tools in group theory, but I am really
confused by the trace in compact infinite groups. The following is my question:
In discrete group theory, the ...

**4**

votes

**3**answers

309 views

### Must the powers of some element always grow linearly with respect to a word metric?

Suppose we have a group $G$ which is finitely generated , and let $|\cdot |$ denote some word metric on it. Must there be an element $a\in G$ such that $|a^n|\ge c\cdot n$ for some $c>0$?
My ...

**-1**

votes

**1**answer

156 views

### The name of a group of order 24 [closed]

I encountered a group $G =\langle(1,3,2,4),(3,5,4,6)\rangle\subseteq S_6$ in my study, but I do not know its name.
Let $f=(1,3,2,4)$ and $g=(3,5,4,6)$. We have $g^2=fg^2f$, and thus $\langle ...

**1**

vote

**0**answers

90 views

### Homology and Burnside ring

If $G$ is a finite groupe, we denote $\mathcal{S}(G)$ the category of finite $G$-sets and $\mbox{I}(G)$ the set of isomorphism classes of it's objects. The Burnside ring of $G$, denoted by ...

**2**

votes

**0**answers

127 views

### Groups acting on complexes

Let $G$ be a finite group. We define a $G$-simplicial complex $\mathcal{A}(G)$ with set of vertices $G^*:=G-\{e\}$ and the simplices are the abelian subsets of $G^*$. The groupe $G$ act simplicially ...

**2**

votes

**3**answers

182 views

### Rank of a special linear group over a finite field

What is the rank (minimal number of group generators) of $SL(n,\mathbb{F})$ in the situation when $SL(n,\mathbb{F})$ is not perfect (i.e. when $SL(n,\mathbb{F})$ is different from $SL(2,\mathbb{F}_2)$ ...

**-1**

votes

**1**answer

94 views

### direct product of a finite group with an infinite symmetric group [closed]

Cross-posted from MSE: http://math.stackexchange.com/q/1226622/15624.
Let $G$ be any finite group, and $S_{\aleph_0}$ the group of all bijections $\mathbb{Z}\rightarrow \mathbb{Z}$.
Is $G\times ...

**3**

votes

**2**answers

381 views

### Upper bound of |Aut(G)| for a p-group

If G is a p-group which is finitely generated with order p^n then what is the upper bound of |Aut(G)|.

**4**

votes

**1**answer

200 views

### Coherent subgroups of $F_2 \times F_2$

A group is coherent if its finitely generated subgroups are finitely presented. For instance, $F_2 \times F_2$ is a well-known example of incoherent group. My question is:
Is a subgroup of $F_2 ...

**7**

votes

**1**answer

274 views

### Products of subgroups of a free group

Let $F$ be a free group, and let $A,B \leq F$ be two subgroups such that $AB$ contains a nontrivial normal subgroup of $F$. Must either $A$ or $B$ contain a nontrivial normal subgroup of $F$?
What if ...

**16**

votes

**0**answers

232 views

### Nonabelian topological fundamental group of a conjugate variety

Let $X$ be a pointed algebraic variety over the field of complex numbers $\mathbb{C}$.
Let $\pi_1^{\rm top}(X)$ and $\pi_1^{\mathrm{\acute{e}t}}(X)$ denote the topological and the étale fundamental ...

**4**

votes

**1**answer

141 views

### Subgroups of $Sp_{2g}$ giving rise to Shimura data

Consider the Shimura datum $(GSp_{2g},\mathcal{H}_g)$. Let $G$ be a reductive $\mathbb{Q}$-subgroup of $Sp_{2g}$. I want to know under what condition there exists a point $x\in\mathcal{H}_g$ such that ...

**19**

votes

**1**answer

331 views

### products of conjugates in free groups

While trying to carry out some technical arguments in free groups, I have encountered the following problem, to which I don't know the answer.
Let $F$ be a free group and let $g,a_1,\ldots,a_n \in ...

**0**

votes

**0**answers

102 views

### Behaviour of first $l^2$-Betti number under quotienting

Let $G$ be a finitely generated group, and let $H = G / N$ be a quotient of it. We have two observations:
1) In general, it is $\textbf{not}$ true that $\beta_1^{(2)}(G) = 0 \Rightarrow ...

**6**

votes

**0**answers

186 views

### On a permutation module for GL(n,q)

Let $G=GL(n,q)$ be the general linear group of degree $n$ over the $q$ element field. Let $X$ be the set of full rank $n\times r$ matrices where $1\leq r\leq n$. Then $G$ acts transitively on the ...

**1**

vote

**1**answer

88 views

### The automorphism groups of smallest grammars of a language string are isomorphic

Let $s \in \Sigma^*$ be a formal language string. Consider the automorphism group of $s$, defined to be the set of all permutations of positions of $s$ that leave $s$ fixed. For instance $G(abab) = ...