Colloquium Colloque des sciences mathématiques du Québec -- Colloque de statistique

12/08/2015 - 15:30
12/08/2015 - 16:30
Speaker: 
Nicolai Meinshausen (ETH Zürich)
Location: 
UdeM, Pav. Roger-Gaudry, Salle S-116
Last edited by on Wed, 12/02/2015 - 11:14

Combinatorial aspects of the Painlevé-I recurrence for maps on surfaces, and Voronoï tessellations of Brownian maps.

12/08/2015 - 15:30
12/08/2015 - 16:30
Speaker: 
Guillaume Chapuy (CNRS, CRM et Université Paris Diderot)
Location: 
CRM, UdeM, Pavillon André-Aisenstadt, 2920, ch. de la Tour, Salle 4336
Abstract: 

The number m_g(n) of maps (fat graphs) with n edges on the g-torus satisfies an asymptotic of the form m_g(n) ~ t_g n^{5(g-1)/2} 12^n when g is fixed and n tends to infinity. The universal constants t_g can be computed thanks to a non-linear recurrence formula of Painlevé-I type (related to the "double-scaling limit" of the one-matrix model).

Although the mathematical integrability underlying this fact is well understood, this simple "topological" recurrence appeals for a direct combinatorial interpretation which is, so far, not known. The purpose of the talk is to relate this question to properties of continuum random surfaces of genus g (a.k.a. "Brownian maps", conjecturally linked to "Liouville quantum gravity"). Namely, we give a simple recursive slicing procedure of the surfaces that gives rise to a non-linear recurrence for the numbers t_g. This recurrence features unknown constants that are naturally interpreted as observables of nearest-neighbour "Voronoï tessellations" around random vertices in Brownian surfaces. By comparing with the Painlevé-I equation, we identify these moments, which are unexpectedly simple and suggest that our (my) understanding of high-genus random surfaces is still very incomplete.

Last edited by on Wed, 12/02/2015 - 11:12

Seminar Geometric Group Theory -- Order-detected slopes and cable knots

12/08/2015 - 11:00
12/08/2015 - 12:00
Speaker: 
Adam Clay (University of Manitoba)
Location: 
Burnside Hall, Room 1214 (McGill University) (Note the unusual time and place)
Abstract: 

In this talk I will discuss how left-orderings of fundamental groups of knot complements behave with respect to the operation of cabling. This work relates closely to the conjectured relationship between left-orderability of fundamental groups, L-spaces and foliations. In particular, what I will discuss is a left-orderability analogue of some recent L-space results of Sarah Dean Rasmussen.

Last edited by on Wed, 12/02/2015 - 11:03

Colloquium DIRO -- Pavages et réseaux appliqués à la structure des protéines

12/04/2015 - 11:00
12/04/2015 - 12:00
Speaker: 
Laurent Vuillon (Université de Savoie)
Location: 
Université de Montréal, Pavillon André-Aisenstadt, 2920 ch. de la Tour, Salle 1409
Abstract: 

Les protéines oligomériques sont faites à partir de l'association de chaînes protéiques à l'aide d'interfaces formées par des interactions entre acides aminés. Dans cet exposé, nous proposons des concepts de mathématiques discrètes et d'informatique théorique pour étudier les contraintes qui régissent les interfaces protéiques et la structure 3D des protéines afin de former des protéines oligomériques. Nous allons dans un premier temps considérer les groupes de symétries dans l'espace et des contraintes de pavages du plan par translation de formes discrètes pour bien comprendre la notion d'interface. En utilisant le théorème de Beauquier-Nivat, nous allons montrer que dans le plan les pavés qui pavent le plan par translation sont soit des pseudo-carrés soit des pseudo-hexagones. Dans un deuxième temps, nous utilisons ces résultats pour paver de façon théorique des cylindres et nous discutons de l'utilisation de ces pavages pour construire des fibres biologiques réelles. Nous donnons ensuite des exemples biologiques de telles constructions. Dans une troisième partie, nous allons construire des réseaux d'adjacences atomiques afin de comprendre la structure 3D des protéines. Et nous allons montrer que certaines mutations d'acides aminés peuvent donner de mauvaises conformations 3D et donc mener à des maladies du type cancer. Nous présentons une mesure sur les graphes d'adjacences atomiques qui permet de tester la robustesse ou la plasticité des acides aminés d'une protéine donnée. L'idée sera de comprendre comment un changement local (une mutation d'un acide aminé par un autre) peut induire un changement global qui déstabilise la protéine tout entière.

Last edited by on Wed, 12/02/2015 - 10:55

Working Seminar Mathematical Physics -- The amplituhedron above tree-level

12/03/2015 - 16:00
12/03/2015 - 17:30
Speaker: 
Hugh Thomas (UQAM / CRM)
Location: 
Concordia University, Math Help Center, Room 912.00 / Library Building, 1400 de Maisonneuve Blvd. West, Montreal
Abstract: 

Arkani-Hamed and Trnka introduced the amplituhedron as a device for calculating scattering amplitudes of N=4 SYM.  Amplituhedra have several discrete parameters: n, the number of particles, k, which encodes the helicities, and L, the loop level.  I will explain the definition of the amplituhedron with particular reference to the case L>0.  I will also explain progress towards a more concrete understanding of the higher loop level amplituhedra, in joint work with Arkani-Hamed and Trnka.  This will be something of a sequel to my talk on tree-level amplituhedra on Tuesday, December 1, in the CRM mathematical physics seminar, but I will try to keep both as self-contained as possible.   

Last edited by on Wed, 12/02/2015 - 10:54

Colloquium DIRO -- Exploring the complementarity of computer graphics and computer vision

12/03/2015 - 15:30
12/03/2015 - 16:30
Speaker: 
Jean-Charles Bazin (Disney Research, Zürich)
Location: 
Université de Montréal, Pavillon André-Aisenstadt, 2920 ch. de la Tour, Salle 3195
Abstract: 

Computer graphics (CG) and computer vision (CV) nicely complement each other. I am convinced that exploring their complementarity space provides exciting research opportunities and opens a wide range of novel applications. In this talk, I will present some of our projects exploring this complementarity, and hope to convince you about the great potential of combining techniques and concepts of CV and CG.

First, I will discuss about our works on 3D point processing. I will start by presenting our approach on RGBD data acquired by multiple color+depth sensors in the context of free-viewpoint video generation (Eurographics 2014). We used CV techniques (such as camera calibration and image processing) and combined them with point-based surface representation from the CG community that we extended with visual information (point color and image structure). Then I will show one of our exciting follow-up projects on scene-space video processing (SIGGRAPH 2015). Our approach tackles noisy 3D point information by leveraging the high amount of redundant visual data in videos for numerous applications such as video denoising, deblurring, and inpainting, but also computational shutter functions and action shot effects.Then I will show novel original applications also enabled by combining CV and CG concepts. First, I will describe a method for object motion manipulation in real footages (under submission) that associates physics-based simulation from the CG community and object pose estimation, camera calibration and visual constraints from CV. Other examples include music retargeting for video temporal resizing (Eurographics 2013), as well as synchronization and blending of facial performance videos (ICCV 2015) for non-linear video story-telling and interactive face editing.Finally I will present video manipulation applications at the boundary of CV and CG: real-time eye contact correction in video calls by depth and facial template manipulation (SIGGRAPH Asia 2012), and video stabilization for virtual visit in the context of cultural preservation (MVA 2011, CVPR 2012).The presentation will be given in French.

Last edited by on Wed, 12/02/2015 - 10:49

Seminar Geometric Group Theory -- L^p-metrics and autonomous flows

12/02/2015 - 15:00
12/02/2015 - 16:00
Speaker: 
Michael Brandenbursky (Ben Gurion University)
Location: 
Burnside Hall Room 920 (McGill University)
Abstract: 

I will discuss a number of results on the interrelation between the L^p -metric on the group of Hamiltonian diffeomorphisms of surfaces and the subset A of autonomous Hamiltonian diffeomorphisms. In particular, I will show that there are Hamiltonian diffeomorphisms of all surfaces of genus g ≥ 2 or g = 0 lying arbitrarily L^p -far from the subset A, and the diameter of this group equipped with the autonomous metric is infinite. This is a joint work with Egor Shelukhin.

Last edited by on Wed, 12/02/2015 - 10:47

Lecture Club mathématique UdeM -- Les points fixes et leurs applications

12/02/2015 - 12:30
12/02/2015 - 13:30
Speaker: 
François Larivière (Université de Montréal)
Location: 
Université de Montréal, Pavillon Claire-McNicoll, Salle Z-255
Abstract: 

Qu'ont en commun l'analyse numérique, la théorie des jeux et la théorie des équations différentielles? La réponse est bien entendu la théorie des points fixes! Dans cet exposé, je montrerai trois résultats importants des domaines mentionnés ci-haut qui reposent sur des théorèmes de points fixes. J'en ferai un point d'honneur de fixer les idées par des exemples pertinents.

Last edited by on Wed, 12/02/2015 - 10:45

Geometric Analysis Seminar -- Schrodinger-Poisson systems in closed manifolds

12/02/2015 - 13:30
12/02/2015 - 14:30
Speaker: 
Pierre-Damien Thizy (University of Cergy-Pontoise)
Location: 
Burnside Hall, Rm. 920 (McGill University)
Abstract: 

The Schrodinger-Poisson system we investigate in this talk arises when we look for standing waves solutions of the full (time dependent) Schrodinger-Maxwell system in the electrostatic case. After a short introduction, we will give recent stability/instability results of the set of the solutions to this system with respect to small perturbations of the "phase" (temporal frequency). In the process of this talk, we will make the connection of these results with some existence, nonexistence, uniqueness and multiplicity results concerning the positive solutions to this system.

Last edited by on Thu, 11/26/2015 - 10:13

CRM Applied Math Seminar -- On the study of singularities of mathematical models for liquid crystals

11/30/2015 - 15:00
11/30/2015 - 16:00
Speaker: 
Lia Bronsard (McMaster University)
Location: 
Burnside Hall, Rm. 920 (McGill University)
Abstract: 

The mathematical analysis of liquid crystal models is difficult, as can be seen by their close relationship to the study of singularities for harmonic maps. In this talk, I will present several mathematical models used to study liquid crystals, as well as the connection with classical results on harmonic maps. I will then present new results on energy minimizing configurations of a nematic liquid crystal around a spherical colloid particle in the context of the Landau de-Gennes energy. The Landau de-Gennes model allows for a greater variety of singularities than is allowed in the Oseen-Frank model, which is related to harmonic maps with values in the unit sphere. 

 

 

Last edited by on Wed, 11/25/2015 - 11:57