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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Compactness Theorems for The Spaces of Distance Measure Spaces and Riemann Surface Laminations

Divakaran, D January 2014 (has links) (PDF)
Gromov’s compactness theorem for metric spaces, a compactness theorem for the space of compact metric spaces equipped with the Gromov-Hausdorff distance, is a theorem with many applications. In this thesis, we give a generalisation of this landmark result, more precisely, we give a compactness theorem for the space of distance measure spaces equipped with the generalised Gromov-Hausdorff-Levi-Prokhorov distance. A distance measure space is a triple (X, d,µ), where (X, d) forms a distance space (a generalisation of a metric space where, we allow the distance between two points to be infinity) and µ is a finite Borel measure. Using this result we prove that the Deligne-Mumford compactification is the completion of the moduli space of Riemann surfaces under the generalised Gromov-Hausdorff-Levi-Prokhorov distance. The Deligne-Mumford compactification, a compactification of the moduli space of Riemann surfaces with explicit description of the limit points, and the closely related Gromov compactness theorem for J-holomorphic curves in symplectic manifolds (in particular curves in an algebraic variety) are important results for many areas of mathematics. While Gromov compactness theorem for J-holomorphic curves in symplectic manifolds, is an important tool in symplectic topology, its applicability is limited by the lack of general methods to construct pseudo-holomorphic curves. One hopes that considering a more general class of objects in place of pseudo-holomorphic curves will be useful. Generalising the domain of pseudo-holomorphic curves from Riemann surfaces to Riemann surface laminations is a natural choice. Theorems such as the uniformisation theorem for surface laminations by Alberto Candel (which is a partial generalisation of the uniformisation theorem for surfaces), generalisations of the Gauss-Bonnet theorem proved for some special cases, and topological classification of “almost all" leaves using harmonic measures reinforces the usefulness of this line on enquiry. Also, the success of essential laminations, as generalised incompressible surfaces, in the study of 3-manifolds suggests that a similar approach may be useful in symplectic topology. With this motivation, we prove a compactness theorem analogous to the Deligne-Mumford compactification for the space of Riemann surface laminations.
2

Géométrie des surfaces singulières / Geometry of singular surfaces

Debin, Clément 09 December 2016 (has links)
La recherche d'une compactification de l'ensemble des métriques Riemanniennes à singularités coniques sur une surface amène naturellement à l'étude des "surfaces à Courbure Intégrale Bornée au sens d'Alexandrov". Il s'agit d'une géométrie singulière, développée par A. Alexandrov et l'école de Leningrad dans les années 1970, et dont la caractéristique principale est de posséder une notion naturelle de courbure, qui est une mesure. Cette large classe géométrique contient toutes les surfaces "raisonnables" que l'on peut imaginer.Le résultat principal de cette thèse est un théorème de compacité pour des métriques d'Alexandrov sur une surface ; un corollaire immédiat concerne les métriques Riemanniennes à singularités coniques. On décrit dans ce manuscrit trois hypothèses adaptées aux surfaces d'Alexandrov, à la manière du théorème de compacité de Cheeger-Gromov qui concerne les variétés Riemanniennes à courbure bornée, rayon d'injectivité minoré et volume majoré. On introduit notamment la notion de rayon de contractibilité, qui joue le rôle du rayon d'injectivité dans ce cadre singulier.On s'est également attachés à étudier l'espace (de module) des métriques d'Alexandrov sur la sphère, à courbure positive le long d'une courbe fermée. Un sous-ensemble intéressant est constitué des convexes compacts du plan, recollés le long de leurs bords. A la manière de W. Thurston, C. Bavard et E. Ghys, qui ont considéré l'espace de module des polyèdres et polygones (convexes) à angles fixés, on montre que l'identification d'un convexe à sa fonction de support fait naturellement apparaître une géométrie hyperbolique de dimension infinie, dont on étudie les premières propriétés. / If we look for a compactification of the space of Riemannian metrics with conical singularities on a surface, we are naturally led to study the "surfaces with Bounded Integral Curvature in the Alexandrov sense". It is a singular geometry, developed by A. Alexandrov and the Leningrad's school in the 70's, and whose main feature is to have a natural notion of curvature, which is a measure. This large geometric class contains any "reasonable" surface we may imagine.The main result of this thesis is a compactness theorem for Alexandrov metrics on a surface ; a straightforward corollary concerns Riemannian metrics with conical singularities. We describe here three hypothesis which pair with the Alexandrov surfaces, following Cheeger-Gromov's compactness theorem, which deals with Riemannian manifolds with bounded curvature, injectivity radius bounded by below and volume bounded by above. Among other things, we introduce the new notion of contractibility radius, which plays the role of the injectivity radius in this singular setting.We also study the (moduli) space of Alexandrov metrics on the sphere, with non-negative curvature along a closed curve. An interesting subset is the set of compact convex sets, glued along their boundaries. Following W. Thurston, C. Bavard and E. Ghys, who considered the moduli space of (convex) polyhedra and polygons with fixed angles, we show that the identification between a convex set and its support function give rise to an infinite dimensional hyperbolic geometry, for which we study the first properties.

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