Tree-graded spaces are generalizations of R-trees. They appear as asymptotic cones of groups (when the cones have cut points). Since many questions about endomorphisms and automorphisms of groups, solving equations over groups, studying embeddings of a group into another group, etc. lead to actions of groups on the asymptotic cones, it is natural to consider actions of groups on tree-graded spaces. We develop a theory of such actions which generalizes the well known theory of groups acting on R-trees. As applications of our theory, we describe, in particular, relatively hyperbolic groups with infinite groups of outer automorphisms, and co-Hopfian relatively hyperbolic groups.
This is a (very subjective) survey paper for nonspecialists covering group actions on Gromov hyperbolic spaces. The first section is about hyperbolic groups themselves, while the rest of the paper focuses on mapping class groups and $Out(F_n)$, and the way to understand their large scale geometry using their actions on various hyperbolic spaces constructed using projection complexes. This understanding for $Out(F_n)$ significantly lags behind that of mapping class groups and the paper ends with a few open questions.
We construct an uncountable sequence of groups acting uniformly properly on hyperbolic spaces. We show that only countably many of these groups can be virtually torsion-free. This gives new examples of groups acting uniformly properly on hyperbolic spaces that are not virtually torsion-free and cannot be subgroups of hyperbolic groups.
We consider the class of countable groups possessing an action on a finite product of hyperbolic graphs where every infinite order element acts loxodromically. When the graphs are locally finite, we obtain strong structure theorems for the groups in this subclass, so that mapping class groups of genus at least 3 (and $Aut(F_n)$ and $Out(F_n)$ for $n\geq 4$) are not in this subclass. This contrasts with the general case, where Bestvina, Bromberg and Fujiwara showed the existence of proper actions of mapping class groups on a finite product of quasitrees. In particular these quasitrees cannot be…
In their first article, the authors initiated a systematic study of hyperbolic $Λ$-metric spaces, where $Λ$ is an ordered abelian group, and groups acting on such spaces. The present paper concentrates on the case $Λ= \mathbf{Z}^n$ taken with the right lexicographic order and studies the structure of finitely generated groups acting on hyperbolic $\mathbf{Z}^n$-metric spaces. Under certain constraints, the structure of such groups is described in terms of a {\em hierarchy} similar to the one established for $\mathbf{Z}^n$-free groups by Kharlampovich, Myasnikov, Remeslennikov and Serbin.
A group with a geometric action on some hyperbolic space is necessarily word hyperbolic, but on the other hand every countable group acts (metrically) properly by isometries on a locally finite hyperbolic graph. In this paper we consider what happens when a group acts isometrically on a restricted class of hyperbolic spaces, for instance quasitrees. We obtain strong conclusions on the group structure if the action has a locally finite orbit, especially if the group is finitely generated. We also look at group actions on finite products of quasitrees, where our actions may be by automorphisms o…
Suppose that a group $G$ acts non-elementarily on a hyperbolic space $S$ and does not fix any point of $\partial S$. A subgroup $H\le G$ is said to be geometrically dense in $G$ if the limit sets of $H$ and $G$ coincide and $H$ does not fix any point of $\partial S$. We prove that every invariant random subgroup of $G$ is either geometrically dense or contained in the elliptic radical (i.e., the maximal normal elliptic subgroup of $G$). In particular, every ergodic measure preserving action of an acylindrically hyperbolic group on a Borel probability space $(X,μ)$ either has finite stabilizers…
We prove that for a one-ended hyperbolic graph $X$, the size of the quotient $X/G$ by a group $G$ acting freely and cocompactly bounds from below the number of simplices in an Eilenberg-MacLane space for $G$. We apply this theorem to show that one-ended hyperbolic cubulated groups (or more generally, one-ended hyperbolic groups with globally stable cylinders à la Rips-Sela) cannot contain isomorphic finite-index subgroups of different indices.
We abstract the notion of an A/QI triple from a number of examples in geometric group theory. Such a triple (G,X,H) consists of a group G acting on a Gromov hyperbolic space X, acylindrically along a finitely generated subgroup H which is quasi-isometrically embedded by the action. Examples include strongly quasi-convex subgroups of relatively hyperbolic groups, convex cocompact subgroups of mapping class groups, many known convex cocompact subgroups of Out(Fn), and groups generated by powers of independent loxodromic WPD elements of a group acting on a Gromov hyperbolic space. We initiate the…
Convex cocompact groups with three-dimensional limit sets
Mathematisches Institut der Universität Bonn（波恩大学数学研究所）；Department of Mathematical Sciences and Research Institute of Mathematics, Seoul National University（首尔大学数学科学系及数学研究所）；Univ Rennes, CNRS, IRMAR - UMR 6625（雷恩大学、法国国家科学研究中心、IRMAR-6625联合实验室）；Department of Mathematics and Statistics - Binghamton University（宾汉姆顿大学数学与统计系）
In this paper we study group actions on quasi-median graphs, or 'CAT(0) prism complexes', generalising the notion of CAT(0) cube complexes. We consider hyperplanes in a quasi-median graph $X$ and define the contact graph $\mathcal{C}X$ for these hyperplanes. We show that $\mathcal{C}X$ is always quasi-isometric to a tree, generalising a result of Hagen, and that under certain conditions a group action $G \curvearrowright X$ induces an acylindrical action $G \curvearrowright \mathcal{C}X$, giving a quasi-median analogue of a result of Behrstock, Hagen and Sisto. As an application, we exhibit an…
Hyperbolic buildings, affine buildings and automatic groups
我们看到，一个考克斯特群是双曲的建筑物本身也是双曲的。因此，任何有限生成群共同作用于这样的建筑物是双曲线的，因此是自动的。我们把我们的注意力，仿射建筑物，并考虑一组$Γ$的行为简单地传递和“类型的”的方式上的一个局部有限厚建筑物的类型$\tilde A_n$的顶点。我们证明了$Γ$是双自动的，使用$Γ$的表示和$\tilde A_n$的每个元素的唯一范式，如D. I. Cartwright，出现，1993年科莫会议的论文集“李群及其几何”。
Splittings and automorphisms of relatively hyperbolic groups
研究了相对双曲群G的自同构。当G是单端的，我们描述出（G）使用首选的JSJ树的子群，几乎是循环或抛物线。特别地，当G是相对双曲的时，Out（G）实际上是由GL_n（Z）的映射类群和子群固定某些基元素构成的。当允许更一般的抛物群时，GL_n（Z）的这些子群必须用McCool群代替：抛物群的自同构平凡地（即通过共轭）作用于某些子群。给定一个双曲群G的一个反常拟凸子群P，我们认为G是相对于P的双曲群，我们应用前面的分析来描述P的自同构群Out（P to G）延伸到G：它实际上是一个McCool群。如果Out（P to G）是无限的，则P是G的一个分裂中的一个顶点群.如果P是无挠的，那么Out（P to G）是VF类型的，特别是VF呈现的。我们还确定了当G是相对双曲的时，Out（G）是无穷大的。有趣的情况是当G是无穷终结的并且有挠。当G是双曲群时，我们证明了Out（G）是无穷的当且仅当G在一个具有无穷中心的极大虚循环子群上分裂.在一般情况下，我们表明，无穷的出（G）来自于存在一个分裂与无限多的扭曲，或有一个顶点群，这是极大的抛物与无穷多个自同构平凡的事件棱道群。
In this article we prove that the set of torsion-free groups acting by isometries on a hyperbolic metric space whose entropy is bounded above and with a compact quotient is finite. The number of such groups can be estimated in terms of the hyperbolicity constant and of an upper bound of the entropy of the space and of an upper bound of the diameter of its quotient. As a consequence we show that the set of non cyclic torsion-free $δ$-hyperbolic marked groups whose entropy is bounded above by a number $H$ is finite with cardinality depending on $δ$ and $H$ alone. From these results, we draw homo…
