Large scale properties of groups; growth functions; Dehn functions; small cancellation properties; hyperbolicity and CAT(0); actions and representations; combinatorial group theory; presentations
4
votes
0answers
115 views
Uniqueness of the boundary of a hierarchically hyperbolic group
Hierarchically hyperbolic groups and spaces (HHG and HHS for short) were defined by Behrstock, Hagen and Sisto (see here and here). Examples include mapping class groups, Right angled Artin groups, ...
0
votes
0answers
50 views
Which $CAT(0)$-polygonal complexes are median spaces?
$CAT(0)$-polygonal complexes are simply connected collections of glued (on their faces) polyhedra of varying dimension such that each link is flag.
Which $CAT(0)$-polygonal complexes with appropriate ...
0
votes
0answers
54 views
Do $CAT(0)$-polygonal complexes have hyperplanes?
$CAT(0)$-polygonal complexes are simply connected collections of glued (on their faces) polyhedra of varying dimension such that each link is flag.
Do $CAT(0)$-polygonal complexes have "hyperplanes" ...
3
votes
1answer
116 views
Are $CAT(0)$-polygonal complexes median spaces?
A median space is a metric space $X$ for which for any three points $x, y , z \in X $ there exists a unique point $m$ such that $d(x,m)+ d(m, y)= d(x , y ), d(x,m)+ d(m, z)= d(x , z ), d(y,m)+ d(m, z)=...
5
votes
0answers
122 views
The preimage of bounded real intervals under homomorphisms on hyperbolic groups
Let $G$ be a hyperbolic group with a fixed (finite, symmetric) generating set and suppose that $\varphi : G \to \mathbb{R}$ is a group homomorphism. Write $W_n = \{ g \in G: |g|=n\}$, where $|g|$ ...
6
votes
4answers
480 views
What is a geodesic in Outer space?
The Culler-Vogtmann Outer space $\text{CV}_n$ is an analogue of Teichmuller space for the group $\text{Out}(F_n)$.
Is there any notion of a geodesic path in $\text{CV}_n$? Are there different ...
2
votes
1answer
236 views
About the growth rate of a group
Let $G$ be a f.g. group and $d$ be a word metric w.r.t. a symmetric generating set. For $g\in G$, define $|g|:=d(g,e)$, where $e$ is the group identity. For $k\in\mathbb N$, put
$$n_k:=\#\{g\in G: |g|...
5
votes
2answers
167 views
Codimension-1 subgroups of 3-manifold groups
Let $G$ be a finitely generated group and let $H$ be a subgroup of $G$. $H$ is a codimension-1 subgroup of $G$ if $C_{G}/H$ has more than one end, where $C_{G}$ is the Cayley graph of $G$.
Do all ...
9
votes
0answers
235 views
Sharpness of the $1/6$-constant in the Cancellation Theorem
I originally posted this question over at Stackexchange, before realising it is much better suited for Overflow:
Let $\langle \; S \; | \; R \; \rangle$ be a presentation of a group $G$ with a set $...
12
votes
2answers
500 views
Modern references on hyperbolic groups
Several good references dedicated to hyperbolic groups have been written during the 90s, including:
Hyperbolic groups, written by M. Gromov.
Géométrie et théorie des groupes : les groupes ...
1
vote
0answers
111 views
Codimension-1 subgroups and submanifolds
Let $G$ be a finitely generated group and let $H$ be a subgroup of
$G$. $H$ is a codimension-1 subgroup of $G$ if $C_{G}/H$ has more than one end, where $C_{G}$ is the Cayley graph of $G$.
Let $...
3
votes
1answer
101 views
Group acting on a CAT(0) cube complex then acting also on a tree
If a group $G$ acts on a CAT(0) cube complex, then does $G$ act on a simplicial tree?
1
vote
1answer
163 views
Group action on quasi-isometric geodesic metric space [closed]
If a group $G$ acts on a geodesic metric space $X$, then does $G$ act on a geodesic metric space $Y$ which is quasi-isometric to $X$?
8
votes
1answer
172 views
If a group $G$ has decidable word problem, must it have a decidable square problem?
My question is a refinement of this one about 'efficient' construction of square elements: If the word problem for a (finitely generated, finitely presented) group is decidable, must the 'square ...
1
vote
0answers
114 views
Representing curves using words
I am trying to understand how in this paper https://arxiv.org/abs/1412.0101 he represents curves with words. This is on page 10 of the paper.
Assume that two piecewise smooth closed curves $\gamma_1$ ...
30
votes
1answer
924 views
Is this conjecture strictly weaker than P=NP?
My three computability questions are related to the following group theory question (first asked by Bridson in 1996):
For which real $\alpha\ge 2$ the function $n^\alpha$ is equivalent to the Dehn ...
7
votes
4answers
259 views
Lattices of PU(n,1) with large abelianization
I am interested in properly discontinuous cocompact subgroups of the group $PU(n,1)$ of automorphisms of the complex hyperbolic space $H^n_{\mathbb{C}}$, says for $n=2,3$. Is there such a lattice $G$ ...
5
votes
1answer
221 views
Cayley graph properties
Consider an infinite graph that satisfies the following property: if any finite set of vertices is removed (and all the adjacent edges), then the resulting graph has only one infinite connected ...
26
votes
1answer
425 views
What is the minimal dimension of a complex realising a group representation?
This question is inspired by this one, which was about representations that can be realised homologically by an action on a graph (i.e., a 1-dimensional complex).
Many interesting integral ...
10
votes
0answers
262 views
Amalgamated product of automatic groups
In Gersten's "Problems on Automatic Groups", Problem 14, he asks the following question: Let $G=A\ast_{C}B$ where $A$ and $B$ are automatic and $C$ is infinite cyclic. Is $G$ automatic?
Is this ...
11
votes
1answer
246 views
Genus of Cayley graph of $A_5$ with two generators of order 5
The Cayley graph of $A_5$ with two generators of order 5 seems rather complicated. I am wondering what is its graph genus (orientable or non-orientable). The best I could get by trial and error is an ...
5
votes
0answers
45 views
Computing centralizers of finite sets in right angled Artin groups (RAAGs) / partially commutative groups / graph groups
This question concerns right angled Artin groups (RAAGs), also called partially commutative groups or graph groups.
A student of mine, Adi Ben-Zvi, needs for an algorithm in RAAGs, a subalgorithm ...
9
votes
1answer
322 views
Divergence of Groups and Metric Spaces
Several papers, including this and this claim that divergence of finitely generated groups and metric spaces have been introduced by Misha Gromov in his paper "Asymptotic invariants of infinite groups"...
2
votes
1answer
171 views
Quotient groups of the lower central series of a surface group
In the answer to MO question 132247, it is possible to find a nice computation of the quotient groups of the lower central series of a finitely generated free group.
Q. What are the quotient ...
8
votes
0answers
118 views
Geodesics between boundary points of a hyperbolic space
Let $X$ be a (not necessarily proper) hyperbolic space. Following Gromov, we define the boundary of $X$ as the set of equivalence classes of sequences convergent at infinity. In general, it is not ...
19
votes
1answer
413 views
Can a hyperbolic, one ended, one relator group, have a shorter trivial word?
Let $G= \langle S \mid r \rangle$ be a one-relator presentation for a one-ended hyperbolic group, with $r$ cyclically reduced.
Question: Can there be a nontrivial word $w(S)$ which is trivial in the ...
5
votes
1answer
198 views
Conformal boundary and cusp of figure-8 complement
As we know the figure-8 ($4_1$) complement can be obtained by quotienting $\mathbb{H}^3$ with an arithmetic Kleinian group, which has index 12 inside $PSL(2,\mathcal{O}_3)$. The resulting complete ...
5
votes
0answers
122 views
Filling points to a simplex in models for EG
I have a question which is related to higher Dehn functions of groups.
I also have a group $G$ with a finite $K(G,1)$. Let us denote by $EG$ the universal cover of this complex. We choose a path-...
4
votes
2answers
307 views
Are there any known examples of groups that virtually split that don't have a codimension-1 subgroup?
Are there any known examples of groups that virtually split that don't have a codimension-1 subgroup?
5
votes
1answer
284 views
Can infiniteness of finitely generated groups be read by a “paradoxical” decomposition?
(Edit) Let $G$ be a group. Two subsets $A,B$ of $G$ are said to be equidecomposable if there exists a finite partition $A=\bigsqcup_{i=1}^nA_i$ and $a_i\in G$ such that $B=\bigsqcup_{i=1}^na_iA_i$.
...
3
votes
1answer
176 views
Classifying/characterising groups acting on CAT(0)-cube complexes
What are some potential approaches to classifying/characterising (finitely presented?) groups that act (without fixed points and potentially more conditions) on CAT(0)-cube complexes? As so many ...
10
votes
1answer
137 views
Minimal length presentations of cyclic groups
By the length of a finite presentation I mean the sum of the lengths of the relators. I am interested in knowing what the minimal length of a presentation of $\mathbb{Z}/n\mathbb{Z}$. I'm even more ...
3
votes
1answer
140 views
Normal generating set for the intersection of two normal subgroups of a surface group
Let $G = \left< a_1,b_1, ... , a_g, b_g | [a_1,b_1] \cdots [a_g,b_g] \right>$ be the fundamental group of a surface of genus $g$. Let $N_1$ and $N_2$ be two normal subgroups of $G$ that are ...
5
votes
2answers
202 views
generators for the handlebody group of genus two
Is the handlebody group of genus two surface generated by Dehn twists along properly embedded disks and annuli?
Are there alternative ways to describe a set of generators that are conceptually simple ...
11
votes
0answers
193 views
Does Thompson's group $V$ have property AP?
Property AP: A discrete group $\Gamma$ has property AP (Approximation Property) if there exists a net $(\phi_i)_{i \in I}$ of finitely supported functions on $\Gamma$ such that $\phi_i \to 1 $ weak$^*$...
6
votes
0answers
195 views
Generators for commutator subgroup of surface group
Let $\pi_1(\Sigma_g) = \langle\text{$x_1,\ldots,x_{2g}$ $|$ $[x_1,x_2]\cdots[x_{2g-1},x_{2g}]$}\rangle$ be a surface group. Can anyone tell me an explicit free basis for the commutator subgroup of $\...
4
votes
2answers
136 views
Nielsen-Thurston decomposition from the product of Dehn twists
Given a closed surface of genus $g\geq 2$, we know that the mapping class group $Mod(S)$ is generated by the Dehn twists. My question is
Given an element as a product of Dehn twist, is it possible ...
16
votes
0answers
244 views
Finitely generated groups with Hölder-exotic space of ends?
The space of ends of a finitely generated group is always homeomorphic to 0, 1, 2 points, or a Cantor set, and in which of these 4 cases it falls is governed by Stallings' characterization (wikipedia ...
2
votes
1answer
109 views
Teichmuller uniqueness theorem with marked points
Let $S$ be a genus $g$, $g > 1$ Riemann surface, and let $h \colon S \to S$ be a homeomorphism of $S$. We denote by $[h] \in \text{Map}(S)$ the corresponding element of the mapping class group of $...
9
votes
1answer
318 views
Parabolic subgroups of relatively hyperbolic and CAT(0) groups
Let $G$ be a finitely generated group. We say that $G$ is CAT(0) if it acts properly and co-compactly by isometries on a CAT(0) space.
We say it is hyperbolic relative to a collection $\Omega$ of ...
5
votes
1answer
231 views
Relative/acylindrical hyperbolicity of free-by-cyclic groups
Is this statement true?
Let $\mathbb{F}$ denotes a finitely generated free group, $\Phi$ an automorphism of $\mathbb{F}$ and $\varphi$ its image in $\mathrm{Out}(\mathbb{F})$.
If $\varphi$ ...
2
votes
1answer
180 views
Do commutator functor and intersection commute?
For two subgroups $A, B$ in $G$,
$[A,A] \cap [B,B] = [A\cap B, A \cap B]$?
At least, if $G$ is free, is the left contained in the right?
7
votes
0answers
169 views
Elements of a group than can achieve all orders in its quotients
Let $G$ be a group. For different purposes I am now interested in a quite natural property that the elements in $G$ may or may not have, and I would like to ask if there is a standard terminology for ...
3
votes
3answers
237 views
Graphs of groups with homomorphisms not necessarily injective
I'm wondering if there is any literature on graphs of groups where the maps $G\to H$ from an edge group $G$ to its endpoint group $H$ are not necessarily $\pi_1$-injective. Or is this just too general ...
10
votes
0answers
111 views
2-generator subgroups of an Artin group of small type
Suppose I have an Artin group $G$ of small-type, meaning that the generators either commute or braid. E.g a braid group. Take two generators $g, h$ and arbitrary conjugates of these generators $xgx^{-...
4
votes
2answers
332 views
Random walk uniformly hitting a compact set
Suppose $G$ is a locally compact compactly generated group. Let $\mu$ be a probability measure that is:
Adapted to $G$, i.e. there is no proper subgroup $H$ such that $\mu(H)=1$.
Symmetric, i.e. $\...
13
votes
2answers
484 views
Subgroup of hyperbolic group generated by non-torsion elements
Let $G$ be a hyperbolic group. I know that it is an open problem whether $G$ has a torsion-free subgroup of finite index. But if we let $N$ be the subgroup of $G$ generated by its non-torsion elements,...
4
votes
2answers
150 views
Divergence of Green function of random walks at spectral radius
Let $P=(p(x,y))_{x, y\in N}$ be the transition matrix over countable states $N$.
Consider the generating Green function $G(x, y|t)=\sum_{0}^{\infty} p^n(x, y) t^n$, where $p^n(x,y)$ is the $(x,y)$-...
0
votes
1answer
77 views
Harmonicity of the Martin kernels
Let $\Gamma$ be a finitely generated group and let $\mu$ be a probability measure on $\Gamma$. Consider the Green function $G(x,y)=\sum_{n\geq 0}\mu^{*n}(x^{-1}y)$, where $\mu^{*n}$ is the $n$th ...
6
votes
0answers
175 views
Rational stable translation length
Let $G$ be a finitely generated group and $S$ a finite generating set and consider the word metric associated to $S$.
If $g\in G$, define its stable translation length as $l(g)=\lim_n \frac{d(e,g^n)}{...
