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Spring Semester 2011

Date / Time Speaker Title Location
2 March 2011
15:45-16:45 		Michah Sageev
Technion, Israel
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Geometry Seminar

Title Boundaries of CAT(0) cube complexes
Speaker, Affiliation Michah Sageev, Technion, Israel
Date, Time 2 March 2011, 15:45-16:45
Location HG G 43
Boundaries of CAT(0) cube complexes
HG G 43
9 March 2011
15:45-16:45
Details

Geometry Seminar

Title Title T.B.A.
Speaker, Affiliation
Date, Time 9 March 2011, 15:45-16:45
Location HG G 43
Title T.B.A.
HG G 43
16 March 2011
15:45-16:45 		Francois Maucaurant
Université de Rennes 1, France
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Geometry Seminar

Title Orbit distribution on the plane of fuchsian groups
Speaker, Affiliation Francois Maucaurant, Université de Rennes 1, France
Date, Time 16 March 2011, 15:45-16:45
Location HG G 43
Abstract Following Ledrappier and Nogueira, one investigates the statistical properties of the images of a vector in the plane by elements of norm less than T of a fuchsian group.
Orbit distribution on the plane of fuchsian groupsread_more
HG G 43
23 March 2011
15:45-16:45 		Michelle Bucher-Karlsson
Université de Genève
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Geometry Seminar

Title Norms in cohomology
Speaker, Affiliation Michelle Bucher-Karlsson, Université de Genève
Date, Time 23 March 2011, 15:45-16:45
Location HG G 43
Abstract Norms of cohomology classes were introduced by Gromov and have important applications since they give a priori-bounds for characteristic numbers. Unfortunately, these norms turn out to be difficult to compute explicitly, and there are only few norms known, including the Kähler class for Hermitian symmetric space (in degree 2) and the volume form for hyperbolic n-spaces. One of Gromov's motivations was to give a topological definition for the Riemannian volume of hyperbolic manifolds, and this led him to a new proof of Mostow Rigidity. I will present a reformulation and simplified version of this proof. Finally, I will discuss the norm of the Euler class of flat bundles. Sullivan and Smillie showed that it is bounded. Together with Nicolas Monod, we proved that the Sullivan-Smillie bound is sharp for the norm of the universal Euler class of flat vector bundles in degree n. This was previously known for n=2 by work of Milnor.
Norms in cohomologyread_more
HG G 43
30 March 2011
15:45-16:45 		Andrea Sambusetti
Università degli Studi di Roma "La Sapienza", Italy
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Geometry Seminar

Title On the horoboundary and the geometry of rays of negatively curved manifolds
Speaker, Affiliation Andrea Sambusetti, Università degli Studi di Roma "La Sapienza", Italy
Date, Time 30 March 2011, 15:45-16:45
Location HG G 43
Abstract (joint work with F. Dal'bo and M. Peigne') The notion of Busemann function was originally introduced by Herbert Busemann in the fifties as a tool to develop a theory of parallels on geodesic spaces (e.g. complete Riemannian manifolds). The Busemann functions captures the idea of "angle at infinity" between infinite geodesic rays, and this idea played an important role in the study of the topology complete noncompact Riemannian manifolds. In particular this notion has a special place in the geometry of Hadamard spaces (simply connected manifolds with nonpositive curvature) and in the dynamics of Kleinian groups. For a Hadamard manifold X, the Busemann functions yield a useful compactification of the space, which was originally introduced by M. Gromov; this compactification has the topology of a sphere and is easily understood in terms of rays. This nice picture breaks down for non-simply connected manifolds. The aim of the talk is to explain the main differences between the "visual" description of the Gromov compactification for Hadamard spaces and the non-simply connected case. I will give some examples of the main interesting pathologies in the non-simply connected case, such as: -- divergent rays having the same Busemann functions; -- points on the Gromov boundary which are not Busemann functions of any ray; -- discontinuity of the Busemann functions with respect to the initial conditions. I will also explain that, restricting to geometrically finite manifolds (the simplest class of non-simply connected, negatively curved manifolds) all the pathologies disappear and we recover a simple description of the Gromov boundary.
On the horoboundary and the geometry of rays of negatively curved manifoldsread_more
HG G 43
6 April 2011
15:45-16:45 		Pierre-Emmanuel Caprace
Université catholique de Louvain, Belgium
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Geometry Seminar

Title Simple locally compact groups, branching and property (T)
Speaker, Affiliation Pierre-Emmanuel Caprace, Université catholique de Louvain, Belgium
Date, Time 6 April 2011, 15:45-16:45
Location HG G 43
Abstract Connected simple locally compact groups are all Lie groups. In the disconnected case, the situation is far more intricate, even if one focuses on non-discrete groups which are compactly generated. For example, a simple locally compact group can have open subgroups which split non-trivially as direct products, as illustrated by the group of type-preserving automorphisms of a regular locally finite tree. In this talk, I shall explain how the existence of such an open subgroup is related to branching phenomena. The latter can then be used to highlight local obstructions to property (T) for the ambient simple group.
Simple locally compact groups, branching and property (T)read_more
HG G 43
13 April 2011
15:45-16:45 		Tullia Dymarz
Université Paris-Sud 11, Orsay, France
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Geometry Seminar

Title Bilipschitz equivalence is not equivalent to quasi-isometric equivalence for finitely generated groups
Speaker, Affiliation Tullia Dymarz, Université Paris-Sud 11, Orsay, France
Date, Time 13 April 2011, 15:45-16:45
Location HG G 43
Abstract We give an example of two finitely generated groups that are quasi-isometric (even commensurable) but not bilipschitz equivalent. The proof involves work of Eskin-Fisher-Whyte on the structure of quasi-isometries of certain solvable groups, uniformly finite homology and analysis of bilipschitz maps of the n-adics.
Bilipschitz equivalence is not equivalent to quasi-isometric equivalence for finitely generated groupsread_more
HG G 43
20 April 2011
15:45-16:45
Details

Geometry Seminar

Title Title T.B.A.
Speaker, Affiliation
Date, Time 20 April 2011, 15:45-16:45
Location HG G 43
Title T.B.A.
HG G 43
4 May 2011
15:45-16:45 		Andrei Agrachev
SISSA, Italy
Details

Geometry Seminar

Title The Curvature Revisited
Speaker, Affiliation Andrei Agrachev, SISSA, Italy
Date, Time 4 May 2011, 15:45-16:45
Location HG G 43
Abstract I'll explain how to define the curvature of variational problems including those with nonholonomic constraints. Classical Riemannian curvature comes as a very special case. I'll give more details and some applications for the 3D contact sub-Riemannian problem that is the length minimization problem on the space of Legendre curves.
The Curvature Revisitedread_more
HG G 43
11 May 2011
15:45-16:45 		Marc Bourdon
Université Lille 1, France
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Geometry Seminar

Title Quasiconformal geometry and Coxeter groups (joint work with Bruce Kleiner)
Speaker, Affiliation Marc Bourdon, Université Lille 1, France
Date, Time 11 May 2011, 15:45-16:45
Location HG G 43
Abstract Every hyperbolic group has a canonical action on its boundary at infinity; with respect to any visual metric, this action is by uniformly quasi-Moebius homeomorphisms. This structure has a central role in the proofs of Mostow's rigidity theorem and numerous others results in the same vein, which are based on the analytic theory of quasiconformal homeomorphisms of the boundary. With the aim of extending these rigidity results to a larger class of hyperbolic groups, we study a quasi-Moebius invariant property of the boundary, called the Combinatorial Loewner Property.
Quasiconformal geometry and Coxeter groups (joint work with Bruce Kleiner)read_more
HG G 43
18 May 2011
15:45-16:45 		Jayadev Athreya
University of Illinois at Urbana-Champaign, USA
Details

Geometry Seminar

Title Ergodic theory of the BCZ map
Speaker, Affiliation Jayadev Athreya, University of Illinois at Urbana-Champaign, USA
Date, Time 18 May 2011, 15:45-16:45
Location HG G 43
Abstract In their study of Farey fracitons, Boca-Cobeli-Zaharescu introduced a piecewise-linear Lebesgue measure preserving map of the Farey triangle x, y: 0 < x, y <= 1, x+y >1. They posed the question of ergodicity of this map. In joint work with Yitwah Cheung, we show that the map is ergodic with respect to Lebesgue measure by displaying it as a first return map for horocycle flow on the modular surface. We obtain corollaries on the distribution of gaps for Farey fractions which had previously been known using analytic number theory methods. We also obtain results on cusp excursions for horocycles.
Ergodic theory of the BCZ mapread_more
HG G 43
* 25 May 2011
13:00-14:00 		Anton Petrunin
Penn State University, USA
Details

Geometry Seminar

Title Telescopic actions
Speaker, Affiliation Anton Petrunin, Penn State University, USA
Date, Time 25 May 2011, 13:00-14:00
Location HG G 43
Abstract A group action H on X is called "telescopic" if for any finitely presented group G, there exists a subgroup H' in H such that G is isomorphic to the fundamental group of X/H'. We construct some examples of telescopic actions. As an application we give an alternative proof of Taubes' theorem: "For every finitely presented group G there exists a smooth compact complex 3-manifold with fundamental group isomorphic to G." This is joint work with Dmitri Panov.
Telescopic actionsread_more
HG G 43
1 June 2011
15:45-16:45 		Tobias Hartnick
Details

Geometry Seminar

Title Kac-Moody groups over local fields and their topological twin buildings
Speaker, Affiliation Tobias Hartnick,
Date, Time 1 June 2011, 15:45-16:45
Location HG G 43
Kac-Moody groups over local fields and their topological twin buildings
HG G 43
8 June 2011
15:45-16:45 		Alessandro Ottazzi
Universitá di Milano Bicocca, Italy
Details

Geometry Seminar

Title Ultrarigid tangents of sub-Riemannian nilpotent groups
Speaker, Affiliation Alessandro Ottazzi, Universitá di Milano Bicocca, Italy
Date, Time 8 June 2011, 15:45-16:45
Location HG G 43
Documents Abstract of the two talksfile_download
Ultrarigid tangents of sub-Riemannian nilpotent groupsread_more
HG G 43

Notes: events marked with an asterisk (*) indicate that the time and/or location are different from the usual time and/or location.

Organisers: Marc Burger, Manfred Einsiedler, Alessandra Iozzi, Urs Lang, Enrico Le Donne, Viktor Schröder

Archive: AS 24 SS 24 AS 23 SS 23 AS 22 SS 22 AS 21 AS 20 SS 20 AS 19 SS 19 AS 18 SS 18 AS 17 SS 17 AS 16 SS 16 AS 15 SS 15 AS 14 SS 14 AS 13 SS 13 AS 12 SS 12 AS 11 SS 11 AS 10 SS 10 AS 09 SS 09

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