Boundaries of hyperbolic groups

Boundaries of strongly accessible hyperbolic groups

We consider splittings of groups over finite and two-ended subgroups. We study the combinatorics of such splittings using generalisations of Whitehead graphs. In the case of hyperbolic groups, we relate this to the topology of the boundary. In particular, we give a proof that the boundary of a one-ended strongly accessible hyperbolic group has no global cut point.

Boundaries of coned-off hyperbolic spaces

Coning off a collection of uniformly quasiconvex subsets of a Gromov hyperbolic space leaves a new space, called the cone-off. Kapovich and Rafi generalized work of Bowditch to show this space is still Gromov hyperbolic. We show that the Gromov boundary of cone-off embeds in the boundary of the original hyperbolic space. (A stronger version of this result was previously obtained by Dowdall and Taylor; see Note in text.) Moreover, under some acylindricity assumptions we give a precise description of the image. As an application, we are able to characterize the elliptic and loxodromic elements o…

Cannon-Thurston maps, subgroup distortion, and hyperbolic hydra

There is a family of hyperbolic groups known as hyperbolic hydra which contain heavily distorted free subgroups. We prove the existence of Cannon--Thurston maps (that is, maps of the boundaries induced by subgroup inclusion) for these free subgroups. It is known that Cannon--Thurston maps between hyperbolic space boundaries can exist even in the presence of arbitrarily heavy (even non-recursive) distortion. The hyperbolic hydra examples show that Cannon--Thurston maps can exist even between hyperbolic group boundaries in the presence of arbitrarily heavy primitive recursive distortion.

Hyperbolic groups with homeomorphic Gromov boundaries

We show that the Gromov boundary of the free product of two infinite hyperbolic groups is uniquely determined up to homeomorphism by the homeomorphism types of the boundaries of its factors. We generalize this result to graphs of hyperbolic groups over finite subgroups. Finally, we give a necessary and sufficient condition for the Gromov boundaries of any two hyperbolic groups to be homeomorphic (in terms of the topology of the boundaries of factors in terminal splittings over finite subgroups).

Hyperbolic isometries and boundaries of systolic complexes

Given a group $G$ acting geometrically on a systolic complex $X$ and a hyperbolic isometry $h \in G$, we study the associated action of $h$ on the systolic boundary $\partial X$. We show that $h$ has a canonical pair of fixed points on the boundary and that it acts trivially on the boundary if and only if it is virtually central. The key tool that we use to study the action of $h$ on $\partial X$ is the notion of a $K$-displacement set of $h$, which generalises the classical minimal displacement set of $h$. We also prove that systolic complexes equipped with a geometric action of a group are a…

Boundaries of Dehn fillings

We begin an investigation into the behavior of Bowditch and Gromov boundaries under the operation of Dehn filling. In particular we show many Dehn fillings of a toral relatively hyperbolic group with 2-sphere boundary are hyperbolic with 2-sphere boundary. As an application, we show that the Cannon conjecture implies a relatively hyperbolic version of the Cannon conjecture.

双曲边界与双曲群

Hyperbolic boundaries vs. hyperbolic groups

该讲义将双曲群、相对双曲群理论与流形、克莱因群理论关联，给出相对双曲群及其边界的定义与实例，综述领域研究，内容源于2018年夏季CIRM的讲座。

Boundaries of graphs of relatively hyperbolic groups with cyclic edge groups

We prove that the fundamental group of a finite graph of convergence groups with parabolic edge groups is a convergence group. Using this result, under some mild assumptions, we prove a combination theorem for a graph of convergence groups with dynamically quasi-convex edge groups (Theorem 1.3). To prove these results, we use a modification of Dahmani's technique [Dah03]. Then we show that the fundamental group of a graph of relatively hyperbolic groups with edge groups either parabolic or infinite cyclic is relatively hyperbolic and construct Bowditch boundary. Finally, we show that the homeo…

Surface-like boundaries of hyperbolic groups

We classify the boundaries of hyperbolic groups that have enough quasiconvex codimension-1 surface subgroups with trivial or cyclic intersections.

相对双曲群边界中的抛物切割对

Parabolic cut pairs in boundaries of relatively hyperbolic groups

本文针对相对双曲群边界中未被探索的抛物切割对现象，提出组合定理构造相关群示例并证明其完备性，同时给出两种适用的边界拓扑描述。

Divergence spectra and Morse boundaries of relatively hyperbolic groups

We introduce a new quasi-isometry invariant, called the divergence spectrum, to study finitely generated groups. We compare the concept of divergence spectrum with the other classical notions of divergence and we examine the divergence spectra of relatively hyperbolic groups. We show the existence of an infinite collection of right-angled Coxeter groups which all have exponential divergence but they all have different divergence spectra. We also study Morse boundaries of relatively hyperbolic groups and examine their connection with Bowditch boundaries.

相对双曲群的鲍迪奇边界之间的拟共形映射

Quasiconformal Maps between Bowditch Boundaries of Relatively Hyperbolic Groups

研究相对双曲群的拟等距分类，核心方法是通过鲍迪奇边界引入拟共形映射概念，主要贡献是证明相对双曲群间粗保尖点的拟等距与粗保horoball阴影的鲍迪奇边界拟共形同胚相互诱导。

Constructing Bowditch boundaries of some relatively hyperbolic groups that are homeomorphic to the $n$-dimensional Sierpiński carpet

In this paper we prove that if some relatively hyperbolic groups have Bowditch boundary homeomorphic to the $n$-sphere, then they are also relatively hyperbolic with respect to another set of parabolic subgroups and its Bowditch boundary is homeomorphic to the $n-1$-dimensional Sierpiński carpet.

Hierarchically hyperbolic groups are determined by their Morse boundaries

We generalize a result of Paulin on the Gromov boundary of hyperbolic groups to the Morse boundary of proper, maximal hierarchically hyperbolic spaces admitting cocompact group actions by isometries. Namely we show that if the Morse boundaries of two such spaces each contain at least three points, then the spaces are quasi-isometric if and only if there exists a 2-stable, quasi-möbius homeomorphism between their Morse boundaries. Our result extends a recent result of Charney-Murray, who prove such a classification for CAT(0) groups, and is new for mapping class groups and the fundamental group…

Some groups with planar boundaries

In this expository note, we illustrate phenomena and conjectures about boundaries of hyperbolic groups by considering the special cases of certain amalgams of hyperbolic groups. While doing so, we describe fundamental results on hyperbolic groups and their boundaries by Bowditch and Haissinsky, along with special treatments for the boundaries of free groups by Otal and Cashen.

On the topological dimension of the Gromov boundaries of some hyperbolic $Out(F_N)$-graphs

We give upper bounds, linear in rank, to the topological dimensions of the Gromov boundaries of the intersection graph, the free factor graph and the cyclic splitting graph of a finitely generated free group.

Hausdorff dimension of boundaries of relatively hyperbolic groups

In this paper, we study the Hausdorff dimension of the Floyd and Bowditch boundaries of a relatively hyperbolic group, and show that for the Floyd metric and shortcut metrics respectively, they are are both equal to a constant times the growth rate of the group. In the proof, we study a special class of conical points called uniformly conical points and establish that, in both boundaries, there exists a sequence of Alhfors regular sets with dimension tending to the Hausdorff dimension and these sets consist of uniformly conical points.

Hyperbolic projections and topological invariance of sublinearly Morse boundaries

We show that the sublinearly Morse boundary of a CAT(0) cubical group with a factor system is well-defined up to homeomorphism with respect to the visual topology. The key tool used in the proof is a new topology on sublinearly Morse boundaries that is induced by group actions on hyperbolic spaces that are sufficiently nice, for example, largest acylindrical actions. Using the same techniques, we obtain a explicit description of this new topology on the sublinearly Morse boundary of any hierarchically hyperbolic group in terms of medians. Finally, we explicitly describe the sublinear Morse bou…

双曲空间边界上的可视化度量

Visual metrics on boundaries of hyperbolic spaces

本文是基于2024年里弗赛德几何群论研讨会迷你课程的综述，介绍双曲度量空间边界可视化度量的构造、拟对称映射及不变量、相关维数、Gromov圆形树的构造与应用，并辅以示例。

关于相对双曲TDLC群的组合定理与Bowditch边界

On combination theorems and Bowditch boundaries of relatively hyperbolic TDLC groups

本文为全不连通局部紧群引入相对双曲性概念，证明其与Arora-Pedroza定义等价，并建立融合积与HNN扩张的组合定理，构造Bowditch边界，进而刻画粗糙端无限时边界的唯一确定性和单端时的连通性，最后给出Gromov边界全不连通时群分裂为紧群有限图的结论。

Local simple connectedness of boundaries of hyperbolic groups

In this paper we prove a theorem describing the local topology of the boundary of a hyperbolic group in terms of its global topology: the boundary is locally simply connected if and only if the complement of any point in the boundary is simply connected. This generalises a theorem of Bestvina and Mess.

Semiconjugacies Between Relatively Hyperbolic Boundaries

We prove the existence of Cannon-Thurston maps for Kleinian groups corresponding to pared manifolds whose boundary is incompressible away from cusps. We also describe the structure of these maps in terms of ending laminations.

相对双曲群作用在其边界上的拓扑稳定性

Topological stability of relatively hyperbolic groups acting on their boundaries

本文证明了双曲群在其Bowditch边界上作用的拓扑稳定性，指出在抛物子群上受半共轭控制的小扰动必全局半共轭于标准作用，从而确立了arXiv:2206.14914主要结果的相对版本。

Boundaries of Hyperbolic Metric Spaces

We investigate the relationship between the metric boundary and the Gromov boundary of a hyperbolic metric space. We show that the Gromov boundary is a quotient topological space of the metric boundary, and that therefore a word-hyperbolic group has an amenable action on the metric boundary of its Cayley graph. This result has significance for the study of Lip-norms on group C*-algebras.

Hyperbolic groups with planar boundaries

We prove that the class of convex-cocompact Kleinian groups is quasi-isometrically rigid. We also establish that a word hyperbolic group with a planar boundary different from the sphere is virtually a convex-cocompact Kleinian group provided that its boundary has Ahlfors regular conformal dimension strictly less than $2$ or if it acts geometrically on a CAT(0) cube complex.

On the Lattice of Boundaries and the Entropy Spectrum of Hyperbolic Groups

Let $Γ$ be a non-elementary hyperbolic group and $μ$ be a probability on $Γ$. We study the $μ$-proximal, stationary actions, also known as boundary actions, of $Γ$. In particular, we are interested in the spectrum of Furstenberg entropies of $(Γ,μ)$-boundaries, and the lattice-theoretic and topological structure of the set $\mathcal{BL}(Γ,μ)$ of boundaries. We prove that all hyperbolic groups have infinitely many distinct boundaries, which attain an infinite set of distinct entropies. Additionally, for simple random walks on non-abelian free groups $F_d$, we establish that there are infinitely…

相对双曲空间之间及其边界之间的映射

Maps between relatively hyperbolic spaces and between their boundaries

本文建立相对双曲群/空间间尊重外围结构的拟等距嵌入与边界拟对称嵌入的对应，并推广至多多项式失真情形，据此刻画相对虚拟幂零子群双曲的群。

Floyd maps to the boundaries of relatively hyperbolic groups

A continuous equivariant map from the Floyd boundary of a relatively hyperbolic group (RHG for short) to its Bowditch boundary is constructed. Such a map is unique unless the group is two-ended. In order to optimize the proof and the usage of the map theorem we propose two new definitions of relative hyperbolicity equivalent to the other known definitions. In our approach some "visibility" conditions in graphs are essential. We introduce a class of "visibility actions" that contains the class of relatively hyperbolic actions. The convergence property still holds for the visibility actions. We …

双曲群在其边界上作用的跟踪性

Shadowing of actions of hyperbolic groups on their boundaries

本文证明任意双曲群在其 Gromov 边界上的典范作用具有跟踪（伪轨跟踪）性质，并由此重新得到 Mann 等人所述此类作用拓扑稳定的结论。

Relations between various boundaries of relatively hyperbolic groups

Suppose a group $G$ is relatively hyperbolic with respect to a collection $\PP$ of its subgroups and also acts properly, cocompactly on a $\CAT(0)$ (or $δ$--hyperbolic) space $X$. The relatively hyperbolic structure provides a relative boundary $\partial(G,\PP)$. The $\CAT(0)$ structure provides a different boundary at infinity $\partial X$. In this article, we examine the connection between these two spaces at infinity. In particular, we show that $\partial (G,\PP)$ is $G$--equivariantly homeomorphic to the space obtained from $\partial X$ by identifying the peripheral limit points of the sam…

Quasisymmetric maps of boundaries of amenable hyperbolic groups

In this paper we show that if $Y=N \times \mathbb{Q}_m$ is a metric space where $N$ is a Carnot group endowed with the Carnot-Caratheodory metric then any quasisymmetric map of $Y$ is actually bilipschitz. The key observation is that $Y$ is the parabolic visual boundary of a mixed type locally compact amenable hyperbolic group. The same results also hold for a larger class of nilpotent Lie groups $N$. As part of the proof we also obtain partial quasi-isometric rigidity results for mixed type locally compact amenable hyperbolic groups. Finally we prove a rigidity result for uniform subgroups of…

Some applications of l_p-cohomology to boundaries of Gromov hyperbolic spaces

We study quasi-isometry invariants of Gromov hyperbolic spaces, focussing on the l_p-cohomology and closely related invariants such as the conformal dimension, combinatorial modulus, and the Combinatorial Loewner Property. We give new constructions of continuous l_p-cohomology, thereby obtaining information about the l_p-equivalence relation, as well as critical exponents associated with l_p-cohomology. As an application, we provide a flexible construction of hyperbolic groups which do not have the Combinatorial Loewner Property, extending and complementing earlier examples. Another consequenc…

相对双曲群边界的局部连通性

Local connectedness of boundaries for relatively hyperbolic groups

本文证明了相对单端的相对双曲群对的 Bowditch 边界具有局部连通性，去除了 Bowditch 先前结果中对外围子群的有限展示、端数及挠子群等全部额外限制。

作用于其边界的双曲群的稳定性

Stability of hyperbolic groups acting on their boundaries

本文利用边界点的动力学编码，证明双曲群在其 Gromov 边界上的作用具有拓扑稳定性，并为圆边界情形的全局稳定性提供新证明。

建筑格子的泊松边界及具有类双曲目标的刚性

Poisson boundaries of building lattices and rigidity with hyperbolic-like targets

本文证明正则厚欧氏建筑及其格的泊松边界为无穷远房室空间并带调和测度类，进而推广了从建筑格到负曲率群的态射与上闭链的刚性结果。

非正曲率群的拟重定向边界

Quasi-redirecting boundaries of non-positively curved groups

本文证明多类群的拟重定向边界存在，刻画Croke-Kleiner容许群的边界并证明其满射到Bowditch边界，拓宽了该边界的存在性。

Trees of manifolds as boundaries of spaces and groups

We show that trees of manifolds, the topological spaces introduced by Jakobsche, appear as boundaries at infinity of various spaces and groups. In particular, they appear as Gromov boundaries of some hyperbolic groups, of arbitrary dimension, obtained by the procedure of strict hyperbolization. We also recognize these spaces as boundaries of arbitrary Coxeter groups with manifold nerves, and as Gromov boundaries of the fundamental groups of singular spaces obtained from some finite volume hyperbolic manifolds by cutting off their cusps and collapsing the resulting boundary tori to points.

Peripheral structures of relatively hyperbolic groups

In this paper, we introduce and characterize a class of parabolically extended structures for relatively hyperbolic groups. A characterization of relative quasiconvexity with respect to parabolically extended structures is obtained using dynamical methods. Some applications are discussed. The class of groups acting geometrically finitely on Floyd boundaries turns out to be easily understood. However, we also show that Dunwoody's inaccessible group does not act geometrically finitely on its Floyd boundary.

具有平面边界的相对双曲群

Relatively hyperbolic groups with planar boundaries

本文在相对双曲群边界为平面且无 Sierpinski carpet 或割点、群单端且实质上无 2-挠的条件下，证明了 Martin-Skora 猜想的一个版本，即该群实质 Kleinian，并应用于 Cannon 猜想及 S^2 上的收敛群。

Harmonic measures on Bowditch boundaries of groups hyperbolic relative to virtually nilpotent subgroups

For a group hyperbolic relative to virtually nilpotent subgroups, on a cusped graph associated to the group, we construct a random walk whose Martin boundary is the Bowditch boundary of the group. Moreover, the harmonic measure is a conformal density corresponding to a hyperbolic Green metric and is exact dimensional on the Bowditch boundary. The latter equipped with a visual distance induced by the Green metric is an Ahlfors-regular metric measure space. The dimension is given in terms of the drift, a Green drift and the asymptotic entropy. The Patterson-Sullivan density for the action on the…

Stability phenomena for Martin boundaries of relatively hyperbolic groups

Let $Γ$ be a relatively hyperbolic group and let $μ$ be an admissible symmetric finitely supported probability measure on $Γ$. We extend Floyd-Ancona type inequalities up to the spectral radius of $μ$. We then show that when the parabolic subgroups are virtually abelian, the Martin boundary of the induced random walk on $Γ$ is stable in the sense of Picardello and Woess. We also define a notion of spectral degenerescence along parabolic subgroups and give a criterion for strong stability of the Martin boundary in terms of spectral degenerescence. We prove that this criterion is always satisfie…

Cubulated groups: thickness, relative hyperbolicity, and simplicial boundaries

Let G be a group acting geometrically on a CAT(0) cube complex X. We prove first that G is hyperbolic relative to the collection P of subgroups if and only if the simplicial boundary of X is the disjoint union of a nonempty discrete set, together with a pairwise-disjoint collection of subcomplexes corresponding, in the appropriate sense, to elements of P. As a special case of this result is a new proof, in the cubical case, of a Theorem of Hruska--Kleiner regarding Tits boundaries of relatively hyperbolic CAT(0) spaces. Second, we relate the existence of cut-points in asymptotic cones of a cub…

在群复形上粘合相对双曲群

Combining relatively hyperbolic groups over a complex of groups

本研究针对满足特定条件的相对双曲群构成的群复形，通过构造其基本群的Bowditch边界模型并结合Yaman定理，证明该基本群是相对双曲的，还给出Bowditch边界的一般分解方法，为相关拓扑问题研究提供工具。

Geodesics in trees of hyperbolic and relatively hyperbolic groups

We present a careful approximation of the geodesics in trees of hyperbolic or relatively hyperbolic groups. As an application we prove a combination theorem for finite graphs of relatively hyperbolic groups, with both Farb's and Gromov's definitions.

Boundaries and automorphisms of hierarchically hyperbolic spaces

Hierarchically hyperbolic spaces provide a common framework for studying mapping class groups of finite type surfaces, Teichmüller space, right-angled Artin groups, and many other cubical groups. Given such a space $\mathcal X$, we build a bordificationcompatible with the hierarchically hyperbolic structure. If $\mathcal X$ is proper, we get a compactification of $\mathcal X$; we also prove that our construction generalizes the Gromov boundary of a hyperbolic space. In our first main set of applications, we introduce a notion of geometrical finiteness for hierarchically hyperbolic subgroups of…

Parabolic groups acting on one-dimensional compact spaces

Given a class of compact spaces, we ask which groups can be maximal parabolic subgroups of a relatively hyperbolic group whose boundary is in the class. We investigate the class of 1-dimensional connected boundaries. We get that any non-torsion infinite f.g. group is a maximal parabolic subgroup of some relatively hyperbolic group with connected one-dimensional boundary without global cut point. For boundaries homeomorphic to a Sierpinski carpet or a 2-sphere, the only maximal parabolic subgroups allowed are virtual surface groups (hyperbolic, or virtually $\mathbb{Z} + \mathbb{Z}$).

相对双曲群上随机游走的比率极限边界

Ratio-limit boundaries for random walks on relatively hyperbolic groups

本文研究相对双曲群上随机游走的比率极限边界，扩展了Woess策略并揭示其局限，证明谱非退化情形下极小点闭包为唯一最小不变子空间，并推广了Toeplitz C*-代数共泛商的存在性，且构造了反例。

具有有限边群的群图的莫尔斯边界

Morse boundaries of graphs of groups with finite edge groups

本文证明自由积的莫尔斯边界只由因子的莫尔斯边界决定，并将该结论推广到具有有限边群和无穷多个端的群图，从而推广了Martin-Swiatkowski在非双曲群情形下的结果。

Statistical hyperbolicity of relatively hyperbolic groups

We prove that a non-elementary relatively hyperbolic group is statistically hyperbolic with respect to every finite generating set. We also establish statistical hyperbolicity for certain direct products of two groups, one of which is relatively hyperbolic.

Small cancellation in acylindrically hyperbolic groups

We generalize a version of small cancellation theory to the class of acylindrically hyperbolic groups. This class contains many groups which admit some natural action on a hyperbolic space, including non-elementary hyperbolic and relatively hyperbolic groups, mapping class groups, and groups of outer automorphisms of free groups. Several applications of this small cancellation theory are given, including to Frattini subgroups and Kazhdan constants, the construction of various \exotic"' quotients, and to approximating acylindrically hyperbolic groups in the topology of marked group presentation…

Dyer群：中心、双曲性与柱形双曲性

Dyer groups: Centres, hyperbolicity, and acylindrical hyperbolicity

该研究刻画了所有Dyer群的中心，并给出了可通过Dyer图直接判定的Dyer群双曲性与柱形双曲性的完整分类结果。

Hierarchical hyperbolicity of hyperbolic-2-decomposable groups

Let G be a graph of hyperbolic groups with 2-ended edge groups. We show that G is hierarchically hyperbolic if and only if G has no distorted infinite cyclic subgroup. More precisely, we show that G is hierarchically hyperbolic if and only if G does not contain certain quotients of Baumslag-Solitar groups. As a consequence, we obtain several new results about this class, such as quadratic isoperimetric inequality and finite asymptotic dimension.

Relative hyperbolicity and Artin groups

This paper considers the question of relative hyperbolicity of an Artin group with regard to the geometry of its associated Deligne complex. We prove that an Artin group is weakly hyperbolic relative to its finite (or spherical) type parabolic subgroups if and only if its Deligne complex is a Gromov hyperbolic space. For a 2-dimensional Artin group the Deligne complex is Gromov hyperbolic precisely when the corresponding Davis complex is Gromov hyperbolic, that is, precisely when the underlying Coxeter group is a hyperbolic group. For Artin groups of FC type we give a sufficient condition for …

Piecewise Visual, Linearly Connected Metrics on Boundaries of Relatively Hyperbolic Groups

Suppose a finitely generated group $G$ is hyperbolic relative to $\mathcal P$ a set of proper finitely generated subgroups of $G$. Established results in the literature imply that a "visual" metric on $\partial (G,\mathcal P)$ is "linearly connected" if and only if the boundary $\partial (G,\mathcal P)$ has no cut point. Our goal is to produce linearly connected metrics on $\partial (G,\mathcal P)$ that are "piecewise" visual when $\partial (G,\mathcal P)$ contains cut points. %Visual metrics for $\partial (G,\mathcal P)$ are tightly linked to inner products of geodesic rays in "cusped" spaces…