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Automorphisms of two-generator free groups and spaces of isometric actions on the hyperbolic plane  [PDF]
William Goldman,Greg McShane,George Stantchev,Ser Peow Tan
Mathematics , 2015,
Abstract: The automorphisms of a two-generator free group acting on the space of orientation-preserving isometric actions of on hyperbolic 3-space defines a dynamical system. Those actions which preserve a hyperbolic plane but not an orientation on that plane is an invariant subsystem, which reduces to an action on R^3 by polynomial automorphisms preserving the cubic polynomial and an area form on the level surfaces. We describe the dynamical decomposition of this action. The domain of discontinuity of this action corresponds to geometric structures: either complete hyperbolic structures on the 2-holed cross-surface (projective plane) with cusps and funnels, or complete hyperbolic structures on a one-holed Klein bottle, or hyperbolic structures on a Klein bottle with one conical singularity. The action is ergodic on the complement of the orbit of the Fricke space of the 2-holed cross-surface, and we show that the orbit of the generalized Fricke space of the one-holed Klein bottle is open and dense.
Groups acting on hyperbolic $Λ$-metric spaces  [PDF]
Andrei-Paul Grecianu,Alexei Kvaschuk,Alexei Myasnikov,Denis Serbin
Mathematics , 2013,
Abstract: In this paper we study group actions on hyperbolic $\Lambda$-metric spaces, where $\Lambda$ is an ordered abelian group. $\Lambda$-metric spaces were first introduced by Morgan and Shalen in their study of hyperbolic structures and then Chiswell, following Gromov's ideas, introduced the notion of hyperbolicty for such spaces. Only the case of 0-hyperbolic $\Lambda$-metric spaces (that is, $\Lambda$-trees) was systematically studied, while the theory of general hyperbolic $\Lambda$-metric spaces was not developed at all. Hence, one of the goals of the present paper was to fill this gap and translate basic notions and results from the theory of group actions on hyperbolic (in the usual sense) spaces to the case of $\Lambda$-metric spaces for an arbitrary $\Lambda$. The other goal was to show some principal difficulties which arise in this generalization and the ways to deal with them.
Groups acting simply transitively on hyperbolic buildings  [PDF]
Lisa Carbone,Riikka Kangaslampi,Alina Vdovina
Mathematics , 2011, DOI: 10.1112/S1461157012000083
Abstract: We construct and classify all groups, given by triangular presentations associated to the smallest thick generalized quadrangle, that act simply transitively on the vertices of hyperbolic triangular buildings of the smallest non-trivial thickness. Our classification shows 23 non-isomorphic torsion free groups (obtained in an earlier work) and 168 non-isomorphic torsion groups acting on one of two possible buildings with the smallest thick generalized quadrangle as the link of each vertex. In analogy with the Euclidean case, we find both torsion and torsion free groups acting on the same building.
Freely indecomposable groups acting on hyperbolic spaces  [PDF]
Ilya Kapovich,Richard Weidmann
Mathematics , 2002,
Abstract: We obtain a number of finiteness results for groups acting on Gromov-hyperbolic spaces. In particular we show that a torsion-free locally quasiconvex hyperbolic group has only finitely many conjugacy classes of $n$-generated one-ended subgroups. We also show that the rank problem is solvable for the class of torsion-free locally quasiconvex hyperbolic groups (even though it is unsolvable for the class of all torsion-free hyperbolic groups). We apply our results to 3-manifold groups. Namely, suppose $G$ is the fundamental group of a closed hyperbolic 3-manifold fibering over a circle and suppose that all finitely generated subgroups of $G$ are topologically tame. We prove that for any $k\ge 2$ the group $G$ has only finitely many conjugacy classes of non-elementary freely indecomposable $k$-generated subgroups of infinite index in $G$.
On `A characterization of R-Fuchsian groups acting on the complex hyperbolic plane'  [PDF]
Sasha Anan'in
Mathematics , 2011,
Abstract: We indicate a C-Fuchsian counter-example to the result with the above title announced at http://www.maths.dur.ac.uk/events/Meetings/LMS/2011/GAL11/program.pdf and prove a stronger statement.
Invariant random subgroups of groups acting on hyperbolic spaces  [PDF]
D. Osin
Mathematics , 2015,
Abstract: 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 $(X,\mu)$ either has finite stabilizers $\mu$-almost surely or otherwise the stabilizers are very large (in particular, acylindrically hyperbolic) $\mu$-almost surely.
Nielsen methods and groups acting on hyperbolic spaces  [PDF]
Ilya Kapovich,Richard Weidmann
Mathematics , 2002,
Abstract: We show that for any positive integer $n$ there exists a constant $C(n)>0$ such that any $n$-generated group $G$, which acts by isometries on a $\delta$-hyperbolic space (with $\delta>0$), is either free or has a nontrivial element with translation length at most $\delta C(n)$.
Hyperbolically embedded subgroups and rotating families in groups acting on hyperbolic spaces  [PDF]
F. Dahmani,V. Guirardel,D. Osin
Mathematics , 2011,
Abstract: We introduce and study the notions of hyperbolically embedded and very rotating families of subgroups. The former notion can be thought of as a generalization of the peripheral structure of a relatively hyperbolic group, while the later one provides a natural framework for developing a geometric version of small cancellation theory. Examples of such families naturally occur in groups acting on hyperbolic spaces including hyperbolic and relatively hyperbolic groups, mapping class groups, $Out(F_n)$, and the Cremona group. Other examples can be found among groups acting geometrically on $CAT(0)$ spaces, fundamental groups of graphs of groups, etc. We obtain a number of general results about rotating families and hyperbolically embedded subgroups; although our technique applies to a wide class of groups, it is capable of producing new results even for well-studied particular classes. For instance, we solve two open problems about mapping class groups, and obtain some results which are new even for relatively hyperbolic groups.
Groups acting on tree-graded spaces and splittings of relatively hyperbolic group  [PDF]
Cornelia Drutu,Mark Sapir
Mathematics , 2006,
Abstract: 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.
Growth of quotients of groups acting by isometries on Gromov hyperbolic spaces  [PDF]
Stephane Sabourau
Mathematics , 2012,
Abstract: We show that every non-elementary group $G$ acting properly and cocompactly by isometries on a proper geodesic Gromov hyperbolic space $X$ is growth tight. In other words, the exponential growth rate of $G$ for the geometric (pseudo)-distance induced by $X$ is greater than the exponential growth rate of any of its quotients by an infinite normal subgroup. This result generalizes from a unified framework previous works of Arzhantseva-Lysenok and Sambusetti, and provides an answer to a question of the latter.
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