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Tetsuya Hosaka

Publications and source records attributed to Tetsuya Hosaka.

At least 19 recordsLinked to original sources
arXiv2023

On rigidity of Coxeter systems up to finite twists and separations of Coxeter generating sets

In this paper, we study the twist-conjecture for Coxeter systems and rigidity of Coxeter systems up to finite twists. For Coxeter systems $(W,R)$ and $(W,S)$, under the untangle-condition for conjugate subsets, we investigate separations and type(I) and type(II) subsets of $R$ and $S$ and give an equivalent condition of $R$ and $S$ that are conjugate up to finite twists. We provide one direction of approach to solving the twist-conjecture and the isomorphism problem for Coxeter groups of finite ranks.

math.GR↗
arXiv2021

The Reconstruction Conjecture for finite simple graphs and associated directed graphs

In this paper, we study the Reconstruction Conjecture for finite simple graphs. Let $Γ$ and $Γ'$ be finite simple graphs with at least three vertices such that there exists a bijective map $f:V(Γ) \rightarrow V(Γ')$ and for any $v\in V(Γ)$, there exists an isomorphism $ϕ_v:Γ-v \to Γ'-f(v)$. Then we define the associated directed graph $\widetildeΓ=\widetildeΓ(Γ,Γ',f,\{ϕ_v\}_{v\in V(Γ)})$ with two kinds of arrows from the graphs $Γ$ and $Γ'$, the bijective map $f$ and the isomorphisms $\{ϕ_v\}_{v\in V(Γ)}$. By investigating the associated directed graph $\widetildeΓ$, we study when are the two graphs $Γ$ and $Γ'$ isomorphic.

math.CO↗
arXiv2016

On the semi-direct product structure of CAT(0) groups

In this paper, we investigate finitely generated groups of isometries of CAT(0) spaces containing some central hyperbolic isometry, and study CAT(0) groups. We show that every CAT(0) group $Γ$ has the semi-direct product structure $Γ=(\cdots(((Γ'\rtimes\langleδ_{n}\rangle)\rtimes\langleδ_{n-1}\rangle)\rtimes\langleδ_{n-2}\rangle)\cdots)\rtimes\langleδ_{1}\rangle$ where $Γ'$ is a CAT(0) group with finite center and $δ_i\in Γ$ for $i=1,\dots,n$, and $Γ$ contains a finite-index subgroup $Γ'\times A$ where $A$ is isomorphic to ${\mathbb{Z}}^n$. We introduce some examples and remarks. Also we provide an example of a virtually irreducible CAT(0) group with trivial-center that acts geometrically on some CAT(0) space that splits as a product $T \times {\mathbb{R}}$.

math.GR↗
arXiv2015

Reconstructible graphs, simplicial flag complexes of homology manifolds and associated right-angled Coxeter groups

In this paper, we investigate a relation between finite graphs, simplicial flag complexes and right-angled Coxeter groups, and we provide a class of reconstructible finite graphs. We show that if $Γ$ is a finite graph which is the 1-skeleton of some simplicial flag complex $L$ which is a homology manifold of dimension $n \ge 1$, then the graph $Γ$ is reconstructible.

math.CO↗
arXiv2014

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 boundary by the equivariant homeomorphisms as above. As an application, we introduce some examples of (non-)equivariant rigid CAT(0) groups and we show that if Coxeter groups $W_1$ and $W_2$ are equivariant rigid as reflection groups, then so is $W_1 * W_2$. We also provide a conjecture on non-rigidity of boundaries of some CAT(0) groups.

math.GR↗
arXiv2010

On boundaries of Coxeter groups and topological fractal structures

In this paper, based on research on rank-one isometries by W.Ballmann and M.Brin and recent research on rank-one isometries of Coxeter groups by P.Caprace and K.Fujiwara, we study a topological fractal structure of boundaries of Coxeter groups. We also show that the limit-point set is dense in a boundary of a Coxeter group and introduce some remarks on boundaries of CAT(0) groups with rank-one isometries.

math.GR↗
arXiv2010

On equivariant homeomorphisms of boundaries of CAT(0) 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 to obtain a $G$-equivariant homeomorphism of the two boundaries $\partial X$ and $\partial Y$ as a continuous extension of the quasi-isometry $ϕ:Gx_0\to Gy_0$ defined by $ϕ(gx_0)=gy_0$, where $x_0\in X$ and $y_0\in Y$.

math.GT↗
arXiv2010

Parabolic subgroups of Coxeter groups acting by reflections on CAT(0) spaces

We consider a cocompact discrete reflection group $W$ of a CAT(0) space $X$. Then $W$ becomes a Coxeter group. In this paper, we study an analogy between the Davis-Moussong complex $Σ(W,S)$ and the CAT(0) space $X$, and show several analogous results about the limit set of a parabolic subgroup of the Coxeter group $W$.

math.GR↗
arXiv2010

On splitting theorems for CAT(0) spaces and compact geodesic spaces of non-positive curvature

In this paper, we show some splitting theorems for CAT(0) spaces on which a product group acts geometrically and we obtain a splitting theorem for compact geodesic spaces of non-positive curvature. A CAT(0) group $Γ$ is said to be {\it rigid}, if $Γ$ determines the boundary up to homeomorphisms of a CAT(0) space on which $Γ$ acts geometrically. C.Croke and B.Kleiner have constructed a non-rigid CAT(0) group. As an application of the splitting theorems for CAT(0) spaces, we obtain that if $Γ_1$ and $Γ_2$ are rigid CAT(0) groups then so is $Γ_1\times Γ_2$.

math.GR↗
arXiv2008

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) $\liminf\{d_{\partial X}(gα,gβ) | g\in G\}=0$. We investigate when are boundaries of CAT(0) groups (and Coxeter groups) scrambled sets.

math.GR↗
arXiv2006

Minimality of the boundary of a right-angled Coxeter system

In this paper, we show that the boundary $\partialΣ(W,S)$ of a right-angled Coxeter system $(W,S)$ is minimal if and only if $W_{\tilde{S}}$ is irreducible, where $W_{\tilde{S}}$ is the minimum parabolic subgroup of finite index in $W$. We also provide several applications and remarks. In particular, we obtain that for a right-angled Coxeter system $(W,S)$, the set $\{w^{\infty} | w\in W, o(w)=\infty\}$ is dense in the boundary $\partialΣ(W,S)$.

math.GR↗
arXiv2005

On fixed-point sets in the boundary of a CAT(0) space

In this paper, we investigate the fixed-point set of an element of a CAT(0) group in its boundary. Suppose that a group $G$ acts geometrically on a CAT(0) space $X$. Let $g\in G$ and let $\mathcal{F}_g$ be the fixed-point set of $g$ in the boundary $\partial X$. Then we show that $\mathcal{F}_g=L(Z_g)$, where $Z_g$ is the centralizer of $g$ (i.e. $Z_g=\{v\in G| gv=vg\}$) and $L(Z_g)$ is the limit set of $Z_g$ in $\partial X$. Thus we obtain that $\mathcal{F}_g\neq \emptyset$ if and only if the set $Z_g$ is infinite. We also show that if $g$ is a hyperbolic isometry, then $\mathcal{F}_g=\partial\Min(g)$, where $\partial\Min(g)$ is the boundary of the minimal set $\Min(g)$ of $g$. This implies that the fixed-point set $\mathcal{F}_g$ and the periodic-point set $\mathcal{P}_g$ of $g$ in $\partial X$ have suspension forms.

math.GR↗
arXiv2005

On the center of a Coxeter group

In this paper, we show that the center of every Coxeter group is finite and isomorphic to $(\Z_2)^n$ for some $n\ge 0$. Moreover, for a Coxeter system $(W,S)$, we prove that $Z(W)=Z(W_{S\setminus\tilde{S}})$ and $Z(W_{\tilde{S}})=1$, where $Z(W)$ is the center of the Coxeter group $W$ and $\tilde{S}$ is the subset of $S$ such that the parabolic subgroup $W_{\tilde{S}}$ is the {\it essential parabolic subgroup} of $(W,S)$ (i.e.\ $W_{\tilde{S}}$ is the minimum parabolic subgroup of finite index in $(W,S)$). The finiteness of the center of a Coxeter group implies that a splitting theorem holds for Coxeter groups.

math.GR↗
arXiv2005

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.\ $\partial X$ is minimal) and the set $\{g^{\infty} | g\in G, o(g)=\infty\}$ is also dense in the boundary $\partial X$. We obtain an application for dense subsets on the boundary of a Coxeter system.

math.GR↗