Constructing geometrically infinite groups on boundaries of deformation spaces

直角 Coxeter 群边界的局部连通性

Local Connectivity of Right-angled Coxeter group boundaries

本文研究直角 Coxeter 群所作用 CAT(0) 空间边界的局部连通性，给出由定义图刻画的充分条件，并对已发表版本进行证明精简与插图补充。

Boundaries of groups with isolated flats are path connected

A seminal result in geometric group theory is that a 1-ended hyperbolic group has a locally connected visual boundary. As a consequence, a 1-ended hyperbolic group also has a path connected visual boundary. In this paper, we study when this phenomenon occurs for CAT(0) groups. We show if a 1-ended CAT(0) group with isolated flats acts geometrically on a CAT(0) space, then the visual boundary of the space is path connected. As a corollary, we prove all CAT(0) groups with isolated flats are semistable at infinity.

Right-angled Coxeter groups with totally disconnected Morse boundaries

This paper introduces a new class of right-angled Coxeter groups with totally disconnected Morse boundaries. We construct this class recursively by examining how the Morse boundary of a right-angled Coxeter group changes if we glue a graph to its defining graph. More generally, we present a method to construct amalgamated free products of CAT(0) groups with totally disconnected Morse boundaries that act geometrically on CAT(0) spaces that have a treelike block decomposition.

CAT(0) groups and Coxeter groups whose boundaries are scrambled sets

In this paper, we study CAT(0) groups and Coxeter groups whose boundaries are scrambled sets. Suppose that a group $G$ acts geometrically (i.e. properly and cocompactly by isometries) on a CAT(0) space $X$. (Such group $G$ is called a {\it CAT(0) group}.) Then the group $G$ acts by homeomorphisms on the boundary $\partial X$ of $X$ and we can define a metric $d_{\partial X}$ on the boundary $\partial X$. The boundary $\partial X$ is called a {\it scrambled set} if for any $α,β\in\partial X$ with $α\neqβ$, (1) $\limsup\{d_{\partial X}(gα,gβ) | g\in G\}>0$ and (2) $\liminf\{d_{\partial X}(gα,gβ)…

Proper homotopy types and Z-boundaries of spaces admitting geometric group actions

We extend several techniques and theorems from geometric group theory so that they apply to geometric actions on arbitrary proper metric ARs (absolute retracts). A second way that we generalize earlier results is by eliminating freeness requirements often placed on the group actions. In doing so, we allow for groups with torsion. The main theorems are new in that they generalize results found in the literature, but a significant aim is expository. Toward that end, brief but reasonably comprehensive introductions to the theories of ANRs (absolute neighborhood retracts) and Z-sets are included, …

关于均匀完美Morse边界

On Uniformly Perfect Morse Boundaries

本文研究了proper geodesic metric spaces中均匀完美Morse边界的概念，并证明了非平凡群的Morse边界在非空时的均匀完美性，同时建立了相关边界的同胚刚性定理。

Dense subsets of boundaries of CAT(0) groups

In this paper, we study dense subsets of boundaries of CAT(0) groups. Suppose that a group $G$ acts geometrically on a CAT(0) space $X$ and suppose that there exists an element $g_0\in G$ such that (1) $Z_{g_0}$ is finite, (2) $X\setminus F_{g_0}$ is not connnected, and (3) each component of $X\setminus F_{g_0}$ is convex and not $g_0$-invariant, where $Z_{g_0}$ is the centralizer of $g_0$ and $F_{g_0}$ is the fixed-point set of $g_0$ in $X$ (that is, $Z_{g_0}=\{h\in G| g_0h=hg_0\}$ and $F_{g_0}=\{x\in X| g_0x=x\}$). Then we show that each orbit $G α$ is dense in the boundary $\partial X$ (i.e…

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…

某些Coxeter群Bowditch边界的共形维数界

Conformal dimension bounds for certain Coxeter group Bowditch boundaries

针对定义图为完全图且边标签≥3的Coxeter群，通过拟等距嵌入圆树和CAT(-1)空间的几何有限作用给出其Bowditch边界共形维数的上下界，揭示相关群族存在无限拟等距类并得到共形维数的稠密性结果。

On equivariant homeomorphisms of boundaries of CAT(0) groups and Coxeter groups

In this paper, we investigate an equivariant homeomorphism of the boundaries $\partial X$ and $\partial Y$ of two proper CAT(0) spaces $X$ and $Y$ on which a CAT(0) group $G$ acts geometrically. We provide a sufficient condition and an equivalent condition to obtain a $G$-equivariant homeomorphism of the boundaries $\partial X$ and $\partial Y$ as a continuous extension of the quasi-isometry $ϕ:Gx_0\rightarrow Gy_0$ defined by $ϕ(gx_0)=gy_0$, where $x_0\in X$ and $y_0\in Y$. In this paper, we say that a CAT(0) group $G$ is {\it equivariant (boundary) rigid}, if $G$ determines its ideal boundar…

Gromov boundaries as Markov compacta

We prove that the Gromov boundary of every hyperbolic group is homeomorphic to some Markov compactum. Our reasoning is based on constructing a sequence of covers of $\partial G$, which is quasi-$G$-invariant wrt. the ball $N$-type (defined by Cannon) for $N$ sufficiently large. We also ensure certain additional properties for the inverse system representing $\partial G$, leading to a finite description which defines it uniquely. By defining a natural metric on the inverse limit $\lim K_n$ and proving it to be bi-Lipschitz equivalent to an accordingly chosen visual metric on $\partial G$, we pr…

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…

Comparing the Roller and B(X) boundaries of CAT(0) cube complexes

The Roller boundary is a well-known compactification of a CAT(0) cube complex X. When X is locally finite, essential, irreducible, non-Euclidean and admits a cocompact action by a group G, Nevo-Sageev show that a subset, B(X), of the Roller boundary is the realization of the Poisson boundary and that the action of G on B(X) is minimal and strongly proximal. Additionally, these authors show B(X) satisfies many other desirable dynamical and topological properties. In this article we give several equivalent characterizations for when B(X) is equal to the entire Roller boundary. As an application …

非球面群上的几何实现

Geometric realisation over aspherical groups

本文证明关系模标量扩张直和可实现为有限2-复形的π_2，并构造(10,15)环面纽结群及Baumslag–Solitar群上的相关非自由模例子。

Constructing a quantum field theory from spacetime

The paper shows deep connections between exotic smoothings of a small R^4 (the spacetime), the leaf space of codimension-1 foliations (related to noncommutative algebras) and quantization. At first we relate a small exotic R^4 to codimension-1 foliations of the 3-sphere unique up to foliated cobordisms and characterized by the real-valued Godbillon-Vey invariant. Special care is taken for the integer case which is related to flat PSL(2,R)-$bundles. Then we discuss the leaf space of the foliation using noncommutative geometry. This leaf space contains the hyperfinite III_1 factor of Araki and W…