Nonpositive curvature, isometric actions, and dynamics of cocycles
611 May, 2013
Cajón del Maipo, Chile
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The aim of this meeting is to gather together people from different mathematical communities that have been using similar techniques to deal with problems in different contexts and exploit further use of these methods. In this spirit, besides normal talks, we plan to have open problem sessions as well as four minicourses of 4 hours each:
PierreEmmanuel Caprace, Univ. Catholique de Louvain.
– Affine isometric actions.
Alain Valette, Univ. de Neuchâtel.
– A crash course on SL(2,R) and Schrödinger cocycles.
Raphaël Krikorian, École Polytechnique.
– Livsic theory and cocycles, including cocycles of diffeomorphisms.
Rafael de la Llave, Georgia Inst. of Technology.
Organizing Committee:
Daniel Coronel, UNAB
Mickaël Crampon, USACH
Andrés Navas, USACH
Mario Ponce, PUCChile
Contact: nonpositivecocycles@usach.cl
MINICOURSES (4 hours each)
– On the structure of group/actions on CAT(0) spaces.
PierreEmmanuel Caprace, Univ. Catholique de Louvain.
The goal of this course is to provide an introduction to the geometry of CAT(0) spaces and the properties of their isometry groups. Starting with a description of prominent families of examples, namely symmetric spaces and Euclidean buildings, we will develop the basics of a theory showing that proper CAT(0) spaces are subjected, at a high level of generality, to rigidity phenomena similar to the familiar properties of those basic examples. The main tool in establishing those results is the study of the full isometry group, which is naturally endowed with a locally compact group topology. Building upon some general considerations on the possibly nondiscrete full isometry group, we will illustrate this approach by deducing various properties of discrete groups of isometries: existence of free abelian subgroups, algebraic structure of amenable groups, rigidity of lattices.
The course plan is the following:
– Definitions and key examples
– Geometric density
– The full isometry group
– Applications to discrete subgroups.
– A crash course on SL(2,R) and Schrödinger cocycles.
Raphaël Krikorian, University Pierre et Marie Curie (Paris 6)
The aim of the minicourse is to study the spectral properties of quasiperiodic 1D Schrödinger operators. An important tool in this approach is the study of the dynamics of the related Schrödinger cocycles. This approach proved to be very useful these last 20 years.
Program:
1) Spectral theory of bounded and symmetric operators, Schrödinger operators, Berezansky´s theorem, dynamically defined Schrödinger operators, spectral measures and the integrated density of states.
2) Schrödinger cocycles, facts from ergodic theory, rotation number and Lyapunov exponents, mfunctions, uniform and nonuniform hyperbolicity, Oseledec theorem.
3) Links between the spectral and dynamical aspects, spectrum/nonuniform hyperbolicity, density of states/rotation number, Thouless formula.
4) Reducibility of qp cocycles, KAM theory, DinaburgSinai and Eliasson´s reducibility theorems. Links with the absolutely continuous spectrum.
5) Anderson localization and its link with nonuniform hyperbolicity.
6) AubryAndré duality, application to the AlmostMathieu operator.
Prerequisites: Hilbert space, basics of complex analysis (holomorphic, harmonic and subharmonic functions), Fourier analysis, measure theory and ergodicity.
Bibliography:
R. Carmona, J. Lacroix}. Spectral theory of random Schrödinger operators. Birkhauser
Th. Ransford. Potential theory in the complex plane. LMS
P. Walters. An introduction to ergodic theory. Springer
– Livsic theory and cocycles, including cocycles of diffeomorphisms.
Rafael de la Llave, Georgia Inst. of Technology.
The central problem will be the problem of triviality of a cocycle over an Anosov system.
More precisely, we consider an Anosov system f in a manifold M and a mapping phi: M —> G, where G is a group and we consider the problem of whether one can find another mapping eta: M —> G in such a way that eta(x) = phi(x) eta(f x).
Clearly, there are obstrutions. If p is periodic of period N, we obtain that it is necessary that
phi(p) phi(f(p)) … phi(f^{N1}(p)) = Id.
Remarkably, when f is an Anosov diffeomorphism, phi is Hölder and G is a finite dimensional group, this is the only obstruction (the case of commutative, compact or nilpotent groups was obtained by Livsic in the early 70´s; the general case was obtained by Kalinin in 2010).
We plan to explore several ideas around these topics:
1) Motivation. Origins of the problem
2) The basic Livsic argument
3) Regularity of solutions in the finite dimensional case
4) Some variations: conformal methods, hyperbolic sets.
5) The slow variation argument.
6) Infinite dimensional problems.
– Affine isometric actions.
Alain Valette, Univ. de Neuchâtel.
We will define affine isometric actions on Hilbert spaces (together with the relevant mild cohomological formalism) and give examples from geometry. We plan to give a proof of the following results:
– a group is nonamenable if and only if, whenever an action with linear part the regular representation almost has fixed points, it has fixed points (Guichardet);
– every amenable group admits a proper action on a Hilbert space;
– if an amenable group in Shalom´s class (HFD) (which contains polycyclic groups) embeds quasiisometrically into Hilbert space, then it is virtually abelian.
TALKS (45 min each)
– Denseness of domination.
Jairo Bochi, PUCRio de Janeiro.
Among linear cocycles (vector bundle automorphisms), the projectively hyperbolic ones (i.e., those that have a dominated splitting) form an important subclass.
Assume that the base dynamics is minimal, and that the fiber dimension is at least 3. Then I prove the following result: for any homotopy class C of cocycles, the (open) subset of C formed by the cocycles that have a dominated splitting is either dense in C or empty. In other words, obstructions to domination are purely topological, in the sense that they cannot be removed by deforming the cocycle.
As I will explain, the proof has two parts: the first part is about expansion rates (Lyapunov exponents), and the second part is about rotations. Each part reduces to a problem of finding almostinvariant sections for a certain skewproduct dynamics which is isometric on the fibers. In the first part, the fibers are noncompact and of nonpositive curvature, and the problem is solved cleanly using geometrical tools developed with Navas. In the second part, however, the fibers are compact and have positive curvature. In this case, the construction of almost invariant sections is more elaborate and ultimately relies on a littleknown result in quantitative homotopy theory.
If time allows, I will also explain why the corresponding 2dimensional statement is false, and how to correct it.
– Attempts at nonlinear versions of spectral theory.
Anders Karlsson, Univ Geneva.
I will describe some statements and questions on spectral aspects of transformations such as diffeomorphisms of compact manifolds and bounded linear operators. Instead of linear spaces, we will study induced actions on nonlinear ones (like symmetric spaces). Two results that provide motivation are Thurston´s spectral theorem for surface homeomorphisms and Oseledets´ multiplicative ergodic theorem for cocycles of matrices.
– Regularity of the stochastic entropy.
François Ledrappier, Univ. NotreDame.
We consider a closed negatively curved manifold $(M,g)$ and the stochastic (or Kaimanovich) entropy $h(M,g)$. In this talk, we recall the relations with the other growth rates (volume entropy, bottom of the spectrum). We discuss the $C^1$ regularity of $h(M, g) $ along conformal variations of $g$ and applications. This is a joint work with Lin Shu (Peking University).
– Stable transitivity of Heisenberg group extensions of hyperbolic systems.
Viorel Nitica, West Chester Univ.
(Joint work with A.Torok) We show that among C^r extensions (r > 0) of a uniformly hyperbolic dynamical system with fiber the standard real Heisenberg group H_{n} of dimension 2n+1 that avoid an obvious obstruction, those that are topologically transitive are open and dense.
– Fixed point property for groups acting on simplicial complexes.
Izhar Oppenheim, The Ohio State Univ.
Given a group G and a metric space Y, one can ask when does the group G have a fixed point property with respect to Y, (i.e. does every isometric action of G on Y have a fixed point?). When the group G also acts geometrically on a simplicial complex X, one can sometimes prove that G has a fixed point property by «comparing» the local structure of X with the metric structure of Y. The classical example of such theorem is the «Zuk criterion» stating that G has a fixed point property with respect to every Hilbert space if the Laplacian on links of X has a large enough spectral gap. In my talk I want to discuss «generalized Zuk criterion» – i.e. given a metric space Y with certain nice properties (e.g. a reflexive Banach space or a Busemann NPC, uniformly convex metric space), I´ll present a suitable fixed point criterion relying on the interplay between Y and the local structure of X.
– An exotic deformation of the hyperbolic space.
Pierre Py, Univ. de Strasbourg.
I will explain how to construct a continuous family of ´´exotic´´ locally compact CAT(−1) spaces with cocompact isometry groups all isomorphic to the isometry group of the real hyperbolic space H^n. This is joint work with Nicolas Monod.
– A problem involving higherdegree cocycles.
Felipe A. Ramírez, Univ. of Bristol.
I will discuss higherdegree smooth cohomology of higherrank abelian actions, with emphasis on a conjecture of A. and S. Katok generalizing the Livshitz Theorem to Anosov actions of higherrank abelian groups. Specifically, the conjecture predicts that the obstructions to solving the topdegree coboundary equation are only those coming from integration over closed orbits (like in the Livshitz Theorem) and that all of the lower cohomologies trivialize (as they are known to do in degree one, by work of A. Katok and R. Spatzier).
I will outline a representationtheoretic approach to this problem that yields results for certain systems, for example some Weyl chamber flows and also some unipotent actions.
– Random walks on symmetric groups.
Andrzej Zuk, Univ. Paris 7.
Talks will start on Monday morning (May, 6th) and end on Saturday noon (May, 11th).
Here is a tentative schedule:

Monday 
Tuesday 
Wednesday 
Thursday 
Friday 
Saturday 


De la Llave

De la Llave



Caprace

10:3011 

Coffee 
Coffee 
Coffee 
Coffee  Coffee 

Coffee






1212:45 (talks) 




Nitica 
Zuk 

Lunch 
Lunch 
Lunch 
Lunch 
Lunch 

15:4516:45 (courses) 






16:4517:15 
Coffee 
Coffee 
Coffee 
Coffee 

17:1518 (talks) 

Py 


Participants:
Azer Akhmedov,NDSU
Amir Algom, Hebrew Univ. Jerusalem
Andrzej Zuk Univ. Paris 7
Most of them are on this picture, provided by PierreEmmanuel Caprace.
To register to the conference, please send a mail to the organizing committee at nonpositivecocycles@usach.cl with your complete name and your institution.
Local expenses will be covered for all participants.
In case you need further financial support, please send an email to the organizing committee at nonpositivecocycles@usach.cl.
The conference will be held at the lodge «Cascada de las Ánimas» www.cascadadelasanimas.cl on Cajón del Maipo, a beautiful landscape on the hillside of the Andes Mountains, 1 hour away from Santiago de Chile.
The conference organizers will take care of all participants transportation from Santiago´s airport to the place of the conference and back. The lodging of all participants is also covered by the organization, including meals and coffee breaks, as well as a special dinner on tuesday plus a short visit to a small astronomy center on Wednesday night.
Besides this, The lodge counts with activities like rafting $ 18,000 per person (minimum 6 people), canopy $ 16,000pp (minimum 4 people), horse back riding $ 18,000pp; massages $26,000 pp/per hour. ( U$1 = $468 aprox.) Soft drinks and water goes from$ 1,200. Beers $ 1,200 – $ 1,700; natural juices $ 2,500. An average lunch a la carte is approximately $ 15,000 per person. Cascada accepts credit cards, but does not have an ATM, therefore, if you would like to go around and get to know more of Cajón del Maipo, you will need to get some local currency in the airport. Cascada also has a SUV for rent. There is an ATM in the village of San José de Maipo, which is 10 minutes away by car or public transportation.
Weather in May in this part of Chile is cold in the mornings (510°C) and then the temperature goes up 1520°C in the afternoon, so bring your jacket and boots, and maybe an umbrella, because there is always the possibility of rain.
In case of problems upon your arrival, you can contact us to the phone number 56 2 74845109.
Last but not least, if you are entering to Chile with a passport issued by Australia, Canada, México, or USA, you have to pay a Reciprocity Tax at the airport; please check: www.aeropuertosantiago.cl/english/index.php?option=com_content&id=35&task=view&Itemid=51
We are looking forward to see you in May.
For any information about the conference, please send an email to the organizing committee at
nonpositivecocycles@usach.cl
In case of problems upon your arrival, please contact us to the phone number 56 2 74845109.
Center of Dynamical Systems and Related Fields (ACT project 1103 PIACONICYT).
Pontificia Universidad Católica de Chile.
Universidad de Santiago de Chile.