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Thesis (Ph.D.)--University of Nebraska-Lincoln, 2006. / Title from title screen (site viewed on Sep. 12, 2006). PDF text of dissertation: 109 p. : ill. ; 0.71Mb. UMI publication number: AAT 3208081. Includes bibliographical references. Also available in microfilm, microfiche and paper format.

Although it was invented by Mikhail Gromov, in 1987, to describe some family of groups[1], the notion of Gromov hyperbolicity has many applications and interpretations in different fields. It has applications in Biology, Networking, Graph Theory, and many other areas of research. The Gromov hyperbolicity constant of several families of graphs and geometric spaces has been determined. However, so far, the only known algorithm for calculating the Gromov hyperbolicity constant δ of a discrete metric space is the brute force algorithm with running time O (n4) using the four-point condition. In this thesis, we first introduce an approximation algorithm which calculates a O (log n)-approximation of the hyperbolicity constant δ, based on a layering approach, in time O(n2), where n is the number of points in the metric space. We also calculate the fixed base point hyperbolicity constant δr for a fixed point r using a (max, min)−matrix multiplication algorithm by Duan in time O(n2.688)[2]. We use this result to present a 2-approximation algorithm for calculating the hyper-bolicity constant in time O(n2.688). We also provide an exact algorithm to compute the hyperbolicity constant δ in time O(n3.688) for a discrete metric space. We then present some partial results we obtained for designing some approximation algorithms to compute the hyperbolicity constant δ.

In general, a finitely presented group can have very nasty properties, but many of these properties are avoided if the group is assumed to admit a nice action by isometries on a space with a negative curvature property, such as Gromov hyperbolicity. Such groups are surprisingly common: there is a sense in which a random group admits such an action, as do some groups of classical interest, such as fundamental groups of closed Riemannian manifolds with negative sectional curvature. If a group admits an action on a Gromov hyperbolic space then large scale properties of the space give useful invariants of the group. One particularly natural large scale property used in this way is the Gromov boundary. The Gromov boundary of a hyperbolic group is a compact metric space that is, in a sense, approximated by spheres of large radius in the Cayley graph of the group. The technical results contained in this thesis are effective versions of this statement: we see that the presence of a particular topological feature in the boundary of a hyperbolic group is determined by the geometry of balls in the Cayley graph of radius bounded above by some known upper bound, and is therefore algorithmically detectable. Using these technical results one can prove that certain properties of a group can be computed from its presentation. In particular, we show that there are algorithms that, when given a presentation for a one-ended hyperbolic group, compute Bowditch's canonical decomposition of that group and determine whether or not that group is virtually Fuchsian. The final chapter of this thesis studies the problem of detecting Cech cohomological features in boundaries of hyperbolic groups. Epstein asked whether there is an algorithm that computes the Cech cohomology of the boundary of a given hyperbolic group. We answer Epstein's question in the affirmative for a restricted class of hyperbolic groups: those that are fundamental groups of graphs of free groups with cyclic edge groups. We also prove the computability of the Cech cohomology of a space with some similar properties to the boundary of a hyperbolic group: Otal's decomposition space associated to a line pattern in a free group.

This work contains some structural results for von Neumann algebras arising from measure preserving actions by direct products of groups on probability spaces. The technology and the methods we use are a continuation of those used by Chifan and Sinclair in [10]. By employing these methods, we obtain new examples of strongly solid factors as well as von Neumann algebras with unique or no Cartan subalgebra. We show for instance that every II 1 factor associated with a weakly amenable group in the class S of Ozawa is strongly solid [59]. We also obtain a product version of this result: any maximal abelian ∗-subalgebra of any II 1 factor associated with a finite direct product of weakly amenable groups in the class S of Ozawa has an amenable normalizing algebra. Finally, pairing some of these results with Ioana's cocycle superrigidity theorem [36], we prove that compact actions by finite products of lattices in Sp(n, 1), n ≥ 2, are virtually W∗-superrigid. The results presented here are joint work with Ionut Chifan and Thomas Sinclair. They constitute the substance of an article [11] which has already been submitted for publication.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21542号 / 理博第4449号 / 新制||理||1639(附属図書館) / 京都大学大学院理学研究科数学・数理解析専攻 / (主査)教授 熊谷 隆, 准教授 福島 竜輝, 教授 牧野 和久 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Les équivalences sous-linéairement bilipschitziennes ont été introduites par Yves Cornulier afin de décrire les cônes asymptotiques des groupes de Lie. Elles généralisent les quasiisométries. Cette thèse construit des invariants pour l'équivalence sous-linéairement bilipschitzienne entre groupes et espaces hyperboliques au sens de Gromov, en utilisant l'analyse au bord de Gromov. Une classe d'application généralisant les homéomorphismes quasisymétriques, et une dimension conforme associée, sont introduites. Les espaces riemannien de type non-compact et de rang un, ainsi que certains espaces homogènes de courbure strictement négative, sont classifiés à équivalence sous-linéairement bilipschitzienne près. / Sublinearly biLipschitz equivalences have been introduced by Yves Cornulier as a means of describing the asymptotic cones of Lie groups; they include and generalize quasiisometries. This thesis provides invariants for sublinearly biLipschitz equivalence between Gromov-hyperbolic groups and spaces using analysis on the Gromov boundary. A class of applications generalizing quasisymmetric mappings, and a corresponding conformal dimension, are introduced as tools. Riemannian symmetric spaces of noncompact type as well as a subclass of homogeneous negatively curved Riemannian manifolds are classified up to sublinearly biLipschitz equivalence.

Deux groupes sont dits élémentairement équivalents s'ils satisfont les mêmes énoncés du premier ordre dans le langage des groupes. Aux environs de l'année 1945, Tarski posa la question suivante, connue désormais comme le problème de Tarski : les groupes libres non abéliens sont-ils élémentairement équivalents ? Une réponse positive à cette fameuse question fut apportée plus d'un demi-siècle plus tard par Sela, et en parallèle par Kharlampovich et Myasnikov, comme le point d'orgue de deux volumineuses séries de travaux. Dans la foulée, Sela généralisa aux groupes hyperboliques sans torsion, dont les groupes libres sont des représentants emblématiques, les méthodes de nature géométrique qu'il avait précédemment introduites à l'occasion de son travail sur le problème de Tarski. Les résultats rassemblés ici s'inscrivent dans cette lignée, en s'en démarquant toutefois dans la mesure où ils traitent des théories du premier ordre des groupes hyperboliques en présence de torsion. Dans un premier chapitre, on démontre, entre autres, que tout groupe de type fini qui est élémentairement équivalent à un groupe hyperbolique est lui-même hyperbolique. On démontre ensuite que les groupes virtuellement libres sont presque homogènes, ce qui signifie que deux éléments qui sont indiscernables du point de vue de la logique du premier ordre sont dans la même orbite sous l'action du groupes des automorphismes du groupe ambiant, à une indétermination finie près. Enfin, on donne une classification complète des groupes virtuellement libres de type fini du point de l'équivalence élémentaire à deux quantificateurs. / Two groups are said to be elementarily equivalent if they satisfy the same first-order sentences in the language of groups, that is the same mathematical statements whose variables are only interpreted as elements of a group. Around 1945, Tarski asked the following question : are non-abelian free groups elementarily equivalent? An affirmative answer to this famous Tarski's problem was given in 2006 by Sela and independently by Kharlampovich and Myasnikov, as the culmination of two voluminous series of papers. Then, Sela gave a classification of all finitely generated groups that are elementarily equivalent to a given torsion-free hyperbolic group. The results contained in the present thesis fall into this context and deal with first-order theories of hyperbolic groups with torsion. In the first chapter, we prove that any finitely generated group that is elementarily equivalent to a hyperbolic group is itself a hyperbolic group. Then, we prove that virtually free groups are almost homogeneous, meaning that elements are almost determined up to automorphism by their type, i.e. the first-order formulas they satisfy. In the last chapter, we give a complete classification of finitely generated virtually free groups up to elementary equivalence with two quantifiers.

Markov partitions provide a 'good' mechanism of symbolic dynamics for uniformly hyperbolic systems, forming the classical foundation for the thermodynamic formalism in this setting, and remaining useful in the modern theory. Usually, however, one takes Bowen's 1970's general construction for granted, or restricts to cases with simpler geometry (as on surfaces) or more algebraic structure. This thesis examines several questions on the algorithmic content of (topological) Markov partitions, starting with the pointwise, entropy-like, topological conjugacy invariant known as orbit complexity. The relation between orbit complexity de nitions of Brudno and Galatolo is examined in general compact spaces, and used in Theorem 2.0.9 to bound the decrease in some of these quantities under semiconjugacy. A corollary, and a pointwise analogue of facts about metric entropy, is that any Markov partition produces symbolic dynamics matching the original orbit complexity at each point. A Lebesgue-typical value for orbit complexity near a hyperbolic attractor is also established (with some use of Brin-Katok local entropy), and is technically distinct from typicality statements discussed by Galatolo, Bonanno and their co-authors. Both our results are proved adapting classical arguments of Bowen for entropy. Chapters 3 and onwards consider the axiomatisation and computable construction of Markov partitions. We propose a framework of 'abstract local product structures'

[pt] Neste trabalho provamos que o fluxo geodésico de uma variedade Finsler de dimensão n compacta, sem pontos conjugados e que é uma variedade de visibilidade uniforme é transitivo. Para isso, introduzimos versões Finsler dos conceitos de hiperbolicidade de Gromov e visibilidade de Eberlein e estudamos suas consequências. Como aplicação da transitividade, provamos que superfícies Finsler k-básicas compactas de gênero maior que um, sem pontos conjugados e com fibrados de Green contínuos são Riemannianas. / [en] In this work we prove that the geodesic flow of a compact, n-dimensional Finsler manifold without conjugate points and which is an uniform visibility manifold is transitive. For this, we introduce Finsler versions of Gromov s hyperbolicity and Eberlein s visibility concepts and study its consequences. As an application of the transitivity, we prove that compact, k-basic Finsler surfaces without conjugate points, with genus greater than one and with continuous Green bundles are Riemannian.