Questions tagged [gr.group-theory]
Questions about the branch of algebra that deals with groups.
8,182 questions
15
votes
2
answers
968
views
Semidirect product decomposition of the Borromean rings group
Let $X=S^3\setminus B$ be the link complement of the Borromean rings.
(source)
Then $G=\pi_1(X)$ has a presentation of the form
$$
G = \langle \; a,b,c \mid [a,[b^{-1},c]],\; ...
- 35.9k
15
votes
2
answers
951
views
Does $\mathbb{K}[G]\simeq\mathbb{K}[H]$ for some field $\mathbb{K}$ of characteristic $p$, imply $\mathbb{F}_p[G]\simeq\mathbb{F}_p[H]$?
Due to the first (and very helpful) answer I received, I've reformulated the question a little: $G$ and $H$ are now assumed to be $p$-groups.
Let $p$ be a prime, and let $\mathbb{F}_p$ be the field ...
- 151
15
votes
2
answers
626
views
Finitely presented group containing every $\mathrm{GL}_n(\mathbb{Z})$
Does there exist a concrete example of a finitely presented group that contains an isomorphic copy of $\operatorname{GL}_n(\mathbb{Z})$ for every $n\in\mathbb{N}$? I think the Higman embedding theorem ...
- 3,740
15
votes
2
answers
838
views
factorization of the regular representation of the symmetric group
Let $\mathbb{C}[S_n]$ be the regular representation of the symmetric group $S_n$, and let $\mathbb{C}^n$ be the vector representation.
Question: Does there exist a representation $V$ (of dimension $(...
- 2,537
15
votes
3
answers
1k
views
Braid group on 4 strands
Consider the braid group $B_4$ with generators
$a=\ $,
$b=\ $
and $c=\ $
Assume
$$\alpha, \alpha'
\in
\langle a,c\rangle{\smallsetminus}(\langle c\rangle{\cdot}\langle a\rangle{\cdot}\langle c\...
- 45k
15
votes
1
answer
498
views
For what LCH groups is the Haar measure $\mu(U x U)$ bounded?
Let $G$ be a locally compact Hausdorff (LCH) topological group with left Haar measure $\mu$. Given a compact unit neighborhood $U$, consider the function
$$
\Phi: \quad G \to (0,\infty), \quad x \...
- 734
15
votes
3
answers
926
views
Lower central series quotients in terms of (co)homology
Let $G$ be a group. It is well-known that $H_1(G,\mathbb{Z})=G/[G,G]$. Also (at least up to torsion) $[G,G]/[G,[G,G]]=\Lambda^2H^1(G,\mathbb{Z})/H_2(G,\mathbb{Z})$ as explained, for example, in this ...
- 7,377
15
votes
1
answer
895
views
Residually nilpotent vs residually p
A well-known theorem of Gruenberg implies that a finitely generated residually torsion-free nilpotent group is residually $p$-finite for all primes $p$. What about the converse?
Question:
Are there ...
- 3,181
15
votes
1
answer
1k
views
Multiply transitive groups, continued
This is related to this question. It is well-known that $S_n$ and $A_n$ are the only six transitive permutation groups, and it is likewise well-known that the proof of this requires the classification ...
- 96.4k
15
votes
3
answers
687
views
Do decidable properties of finitely presented groups depend only on the profinitization?
This is a just-for-fun question inspired by this one. Let $P$ be a property of finitely presentable groups. Suppose that
The truth of $P(G)$ only depends on the isomorphism class of $G$.
Given a ...
- 156k
15
votes
2
answers
613
views
Existence of a regular semisimple element over $\mathbb{F}_{q}$
This is probably old, a Chevalley level of old, but I'm not at all an expert in this field so I need help.
Let $G$ be a simply connected (almost) simple linear algebraic group defined over $K=\mathbb{...
- 455
15
votes
1
answer
639
views
What is the centralizer of a Young subgroup of $S_n$?
In their celebrated paper "A new approach to the representation theory of the symmetric group. II", Okounkov and Vershik prove that $Z(n-1,1)$, the centralizer of $\mathbb{C}[S_{n-1}]$ in $\...
- 492
15
votes
1
answer
784
views
The completion of the space of finite groups
Edit: I revise the question based on the comment conversations
Let $\mathcal{F}$ be the set of all equivalence classes of finite groups under the "Isomorphism" equivalence relation.
We define ...
- 356
15
votes
1
answer
2k
views
Which finite groups have no irreducible representations other than characters?
A classical result states that all the irreducible representations of a finite group over $\mathbb{C}$ are characters if and only if $G$ is abelian. I would like to know what happens if we consider a ...
- 11.3k
15
votes
3
answers
913
views
Is there a characterization of groups in which at least one subgroup is not an endomorphism kernel?
This is a crosspost from MSE:
Is there a characterization of groups with the property $\exists N \unlhd G : \not \exists H \leq G \;\; \text{s.t.} \;\; H\cong G/N$?
A common mistake among beginning ...
- 1,173
15
votes
2
answers
5k
views
Isomorphism of semidirect products
Let $N,H$ be groups, $\phi \colon H\rightarrow Aut(N)$ be a homomorphism, $f\in Aut(N)$ and $\hat{f}$ be an inner automorphism of $Aut(N)$ induced by $f$. Then $$N\rtimes_{\phi} H \cong N\rtimes_{\hat{...
- 161
15
votes
1
answer
687
views
Probability that a random element of a group is trivial
Let $G$ be an infinite group with a finite generating set $S$. For $n \geq 1$, let $p_n$ be the probability that a random word in $S \cup S^{-1}$ of length at most $n$ represents the identity. Is it ...
- 153
15
votes
1
answer
821
views
Conjugated subgroups in $\mathsf{GL}(m+n,\mathbb{Z})$ implies conjugated subgroups in $\mathsf{GL}(n,\mathbb{Z})$?
In my research I came up with the following question:
Question: Let $H_1$ and $H_2$ be finite abelian subgroups of $\mathsf{GL}(n,\mathbb{Z})$. Define $$ H_1'=\left\{\begin{pmatrix} I_m &0\\0&...
15
votes
1
answer
679
views
Submodules of $({\mathbb Z}/6{\mathbb Z})^n$ intersecting $\{0,1\}^n$ trivially
$\newcommand{\F}{{\mathbb F}}$
$\newcommand{\Z}{{\mathbb Z}}$
Suppose that $\F$ is a finite field of prime order $p:=|\F|$, and let $n$ be a positive integer. I consider the regime where $\F$ is ...
- 23k
15
votes
1
answer
640
views
Torsion-free group that is not of type F but is virtually of type F
Recall that a group $G$ is of type F if there exists a compact $K(G,1)$.
There are many examples of groups which are not of type F but which are virtually of type F, that is, they have finite-index ...
- 153
15
votes
2
answers
886
views
Is there a finitely generated residually finite group with solvable word problem that does not embed in a finitely presented residually finite group?
The famous Higman embedding theorem says that every recursively presented group embeds in a finitely presented group. This is a convenient tool to construct finitely presented groups with bizarre ...
- 38.6k
15
votes
2
answers
1k
views
A Non-Commutative Nullstellensatz
In studying presentations of pro-$p$-groups via generators and relations, one is led (via the so-called Magnus embedding) to questions involving power series in non-commuting variables. Results from ...
- 8,467
15
votes
1
answer
352
views
$p$-groups with trivial $H^3$
Let $Q_8$ be the group of quaternions of order $8$. It is a non-abelian $2$-group such that $H^3(Q_8,\mathbb{Z})=0$, where $\mathbb{Z}$ has the trivial action. For a proof, see the book "Homological ...
- 153
15
votes
2
answers
1k
views
infinite dimensional CAT(0) groups
Usually a CAT(0) group is defined to be a group acting properly isometrically and cocompactly on a CAT(0) space, but I would like to consider only those groups that act properly, isometrically and ...
- 11.1k
15
votes
2
answers
970
views
Groups with a rational generating function for the word problem
This question comes more from curiosity than a specific research problem. Let G be a group and S a finite symmetric generating set. By the WP(G,S) I mean the set of all words in the free monoid on S ...
- 38.6k
15
votes
3
answers
3k
views
Entropy of a measure
Let $\mu$ be a probability measure on a set of $n$ elements and let $p_i$ be the measure of the $i$-th element. Its Shannon entropy is defined by
$$
E(\mu)=-\sum_{i=1}^np_i\log(p_i)
$$
with the ...
- 6,034
15
votes
1
answer
1k
views
Is there a group whose cardinality counts non-intersecting paths?
Introduction
Graphs are not only important combinatorial objects, but also related to many topological/algebraic structures. In this question I am going to talk about various group structures with ...
- 85.6k
15
votes
1
answer
1k
views
Symmetric groups which are not quotients of Z/2Z*Z/3Z
Somehow this question made me think of instances of small exceptions in general, and I remembered the statement I heard once that $S_5,A_6,S_6,A_7,A_8,S_8$ are the only instances of symmetric/...
- 16.9k
15
votes
1
answer
413
views
Equivalence of surjections from a surface group to a free group
Let $g \geq 2$. Let $S = \langle a_1,b_2,...,a_g,b_g | [a_1,b_1] \cdots [a_g,b_g] \rangle$ be the fundamental group of a genus $g$ surface and let $F_g$ be a free group with $g$ generators. Given ...
- 5,349
15
votes
1
answer
1k
views
Tarski monster groups: for which primes they don't exist?
What can be said about the set of primes $p$ for which it is proven that an infinite group with all non-trivial proper subgroups cyclic of order $p$ doesn't exist? Specifically, what is the largest ...
- 3,630
15
votes
1
answer
629
views
Characteristic classes of symmetric group $S_4$
For the symmetric group $S_3$, it is classically known that \begin{equation} H^*(S_3;\mathbb{Z})\cong \mathbb{Z}[x,y]/(2x,6y,x^2-3y), \end{equation} where $|x|=2$ and $|y|=4$. Moreover, $x$ can be ...
- 439
15
votes
1
answer
761
views
Permutation Groups Containing non-commuting $p$-cycles
I noticed that the following is true, and that there is a reasonably elementary proof of it (in particular, the classification of finite simple groups is not needed). Let $G$ be a finite permutation ...
- 44.4k
15
votes
1
answer
1k
views
Amenable groups with finite classifying space
A group $G$ is said to be elementary amenable if it can be obtained from finite and abelian groups by subgroups, quotients, extensions and increasing unions. It is well-known that all such groups are ...
- 25.5k
15
votes
1
answer
2k
views
Number of conjugacy classes in GL(n,Z)
Like every other group also $\mbox{GL}(n,\mathbb{Z})$ acts on the set of all its subgroups, by conjugation: if $\phi \in \mbox{GL}(n,\mathbb{Z})$, then $\phi$ acts by $H \mapsto \phi H \phi^{-1}$, ...
- 907
15
votes
2
answers
527
views
Is there a natural notion of completion of a Coxeter system?
Let $(W,S)$ be a Coxeter system. Then any element of $W$ can be written as a finite products of elements of $S$. I want some notion of a "completion" of $W$, call it $\hat{W}$, whose elements are (...
- 24.2k
15
votes
1
answer
742
views
Computing van Kampen diagrams
If G is a finitely presented group (with generating set X) and w is a word over X such that
w=1 in G, then the latter can be witnessed by a so called van Kampen diagram for w, which is
a planar ...
- 151
15
votes
1
answer
512
views
fundamental groups of complements to countable subsets of the plane
This question is a follow-up of this MSE post and a comment by Henno Brandsma:
Question 1. Let $S$ be the set of isomorphism classes of fundamental groups $\pi_1(E^2 - C)$, where $C$ ranges over all ...
- 12.3k
15
votes
1
answer
474
views
Dirichlet's unit theorem for reductive schemes
Let $O_{K,S}$ be the ring of $S$-integers in a number field $K$. Dirichlet's unit theorem implies that the group of units in $O_{K,S}$ is a finitely generated group. In other words, the group $\mathbb ...
- 151
15
votes
2
answers
512
views
Does a classification of simultaneous conjugacy classes in a product of symmetric groups exist?
Let the symmetric group $S_n$ on $n$ letters act on $S_n^d=S_n\times\cdots\times S_n$ by simultaneous conjugation, i.e. $\pi\in S_n$ acts on $(\sigma_1,\ldots,\sigma_d)\in S_n^d$ by $\pi.(\sigma_1,\...
- 4,902
15
votes
1
answer
776
views
An infinite, amenable, finitely presentable group with no non-trivial finite quotients
My question is a simple one: is there a group with the properties in the title?
In the absence of the 'finitely presentable' hypothesis, an example is provided by Juschenko--Monod's construction of a ...
- 25k
15
votes
1
answer
2k
views
Interpretation of universal coefficients theorem for group cohomology
Suppose $G$ and $A$ are abelian groups (I'm setting $G$ abelian to keep the discussion simple, though there are analogues for non-abelian $G$) with $G$ acting trivially on $A$. By the universal ...
- 7,320
15
votes
1
answer
656
views
Linear embeddings of nilpotent pro-$p$ groups
Is it true that every finitely generated (topologically) torsion-free nilpotent pro-$p$ group is isomorphic to a subgroup of $U_d(\mathbb{Z}_p)$, the group of $d\times d$-upper triangular matrices ...
- 657
15
votes
2
answers
685
views
Coordinates of the Weyl vector of $E_8$ (and the 135 classes of $W(E_8)/W(D_8)$)
Consider the root system of $E_8$, written in its standard "even" coordinate system: i.e., it is the set of all $240$ vectors in $\mathbb{R}^8$ which whose coordinates are either all integers or all ...
- 32.5k
15
votes
1
answer
686
views
Amenability of groups in terms of a perturbation condition
Let $G$ be a countable group and $\lambda \colon G \to U(\ell^2 G)$ its left-regular representation. Suppose that there exists a constant $C>0$ such that for all $T \in B(\ell^2 G)$
$$\inf \lbrace\...
- 25.5k
15
votes
2
answers
2k
views
What is the subgroup generated by involutions?
I was recently taking some notes on the Cartan-Dieudonné theorem: if $(V,q)$ is a nondegenerate quadratic space of finite dimension $n$ over a field of characteristic not $2$, then every element of ...
- 65.4k
15
votes
1
answer
466
views
Can a torsion-free group be quasi-isometric to a torsion group?
I have looked around in the literature on group theory and geometric group theory and this looks to be an open question as far as I can tell (by torsion group, I mean as usual a group in which every ...
- 5,146
15
votes
1
answer
1k
views
Folner sequences of amenable groups of exponential growth
Let $G$ be an amenable group of exponential growth and let $S$ be a finite symmetric generating set. For each $k$, let $B_{k}$ be the closed ball of radius $k$ about the identity element in the ...
- 8,298
15
votes
2
answers
1k
views
Translation length functions of non-simplicial trees
Let $G$ be a finitely generated group. By a theorem of Culler and Morgan, the set of non-abelian (not necessarily simplicial) minimal $\mathbb{R}$-trees with isometric $G$-action injects into the ...
- 937
15
votes
0
answers
347
views
Poset defined on pairs of subgroups
Let $G$ be a group. Consider the set $P(G)$ of all pairs $(H,N)$ of subgroups of $G$ such that $N$ is a normal subgroup of $H$. Consider the relation $\leq_G$ on $P(G)$ defined as follows: $(H,N)\...
- 1,462
15
votes
0
answers
510
views
On uniform Kazhdan's property (T)
For a finitely generated group $\Gamma$ and its finite generating subset $S$, the Kazhdan constant $\kappa(\Gamma,S)$ is defined to be
$$\kappa(\Gamma,S)=\inf_{\pi,v} \max_{g\in S} \| v - \pi_g v \|,$...
- 10.1k