Spring School
Discrete Ricci curvature
Paris, Institut Henri Poincaré (I.H.P.), May 1822, 2015
This Spring School will consist in two courses given by professors Jürgen Jost and Christian Leonard on discrete Ricci curvature. The aim is to bring researchers from different communities (Geometry, Probability, Analysis) on the common topic of the Ricci curvature and its consequences. Follow the abstracts of the courses, practical informations and registration.
Jürgen Jost (Max Plank Institute, Germany)  Curvature concepts for metric spaces and their implications and applications. 

The concept of curvature was originally introduced to express the deviation of a curve from being straight, or of a surface in space from being planar, that is, flat. Gauss then discovered that curvature had both an extrinsic aspect, expressing how a surface curves in space, and an intrinsic one, how quickly geodesics emanating from the same point converge or diverge. Positive Gauss curvature leads to convergence, as on a sphere, negative curvature to divergence as in the hyperbolic plane. Riemann then developed the general theory of manifolds equipped with a metric tensor. The curvature tensor of such a Riemannian manifold encodes its geometric invariants and expresses the deviation from Euclidean, that is, flat space. In the 20th century, it was found that such a deviation from being Euclidean could also be quantified for more general metric spaces, and various notions of gneralized sectional curvature inequalities were proposed. In Riemannian geometry, sectional curvature is evaluated on tangent planes, that is, twodimensional spaces, and generalized sectional curvature therefore refers to the distance between pairs of geodesics. It is quantified by convexity properties of that distance function between geodesics. Another important notion of curvature in Riemannian geometry is that of Ricci curvature, the average of the sectional curvatures of all tangent planes containing some given direction. In order to extend the concept of Ricci curvature to more general metric spaces, one therefore needs a measure in order to perform some suitable analogue of such an averaging. An important aspect of Ricci curvature is that it can control the eigenvalues of the Laplace operator.
In my lectures, building upon the notions of curvature in Riemannian geometry as just sketched, I shall develop such abstract notions of curvature for metric spaces and other mathematical objects. The guiding principle will always be to express and quantify some deviation from a flat reference situation. This could be the divergence or convergence of geodesics, as already sketched above; the transportation cost of the measure of some distance ball to another ball, compared with the distance between their centers; the probability that two independent stochastic motions get closer to each other as they evolve; or eigenvalue bounds for Laplace operators.
Perhaps somewhat surprisingly, such notions of curvature are even meaningful for discrete structures like graphs. In fact, graphs are ideally suited for explaining the meaning of curvature and the consequences of curvature inequalities in the simplest possible setting, without analytical complications.
Christian Leonard (Paris Ouest Nanterre La Défense University, France)  Some aspects of lower bounded curvature of metric spaces with applications to random walks on graphs. 

The aim of the LottSturmVillani (LSV) theory is to extend from Riemannian manifolds to geodesic spaces the notion of lower bounded Ricci curvature as well as several consequences in terms of concentration of measure and convergence to equilibrium of heat flows. Optimal transport plays a crucial role. In particular, it leads to the notions of displacement interpolation and gradient flow on the space of probability measures which are fundamental tools for this approach.
The lectures will introduce the bases of the LSV theory on a geodesic space. Then, several recent attempts to extend the LSV theory to the framework of discrete metric graphs will be developed. The main approaches that will be considered are based on displacement interpolations and gradient flows on sets of probability measures. Representations in terms of stochastic processes (Brownian motion on a manifold and random walk on a graph) will be emphasized. This will lead us to some entropic inequalities and their consequences in terms of convergence to equilibrium of random walks.
Further details: Several connections between probability theory, analysis and the notion of curvature are known for a long time. Geometers consider the LaplaceBeltrami operator as a fundamental tool for deriving global properties of Riemannian manifolds, analysts look at it as the infinitesimal generator of the heat semigroup leading to the heat equation: a parabolic partial differential equation, while the Laplace operator is associated with the Brownian motion, a fundamental probabilistic notion.
Let us sketch a typical consequence of these interesting connections. Since the diameter of a Riemannian manifold with positive curvature depends decreasingly on the curvature (BonnetMyers theorem), it is not surprising that the speed of exploration of the manifold by a Brownian motion increases with the curvature. Conversely, estimating the speed of convergence to the volume measure of a Brownian motion allows to derive some upper bounds for the diameter.
Besides the BonnetMyers theorem, one can also derive functional inequalities such as Poincaré or logarithmic Sobolev inequalities which encode the speed of convergence of the heat flow to the volume measure as well as the concentration profile of this measure. The aim of the LottSturmVillani (LSV) theory is to extend this type of results from Riemannian manifolds to geodesic spaces. The basic ingredient of this approach is the quadratic optimal transport. It allows for defining the Wasserstein distance (of order 2) on the space P(X) of probability measures on the geodesic state space X which in turn allows for defining relevant notions of geodesic paths (McCann displacement interpolations) and gradient flows on P(X). When the state space X is a Riemannian manifold, some dynamical properties of these geodesic motions on P(X) are intimately connected to lower bounds of the Ricci curvature of X. This suggests the definition of a natural extension of lower bounded Ricci curvature on geodesic spaces which leads to results on the concentration of measure or the convergence to equilibrium of heat flows.
As a discrete space is not geodesic, the LSV theory doest not apply to the discrete setting. The aim of these lectures is to present several variants of the LSV strategy designed to obtain similar results for discrete metric graphs.

The school is part of the the A.N.R. project GeMeCoD.
Participation of postdocs and PhD students is strongly encouraged.
Lectures will be held in the main amphitheater (Hermite) of the I.H.P.
Coffee/tea will be available before each slot.
Slot\Day  Mon 18  Tue 19  Wed 20  Thu 21  Fri 22 

10:0010:50  Jost  Jost  Jost  Jost  Jost 
11:0012:00  Jost  Jost  Jost  Jost  Jost 
12:0014:00  Break  Break  Break  Break  Break 
14:0014:50  Leonard  Leonard  Leonard  Leonard  Leonard 
15:0016:00  Leonard  Leonard  Leonard  Leonard  Leonard 
Nathaël Gozlan (ParisEst), Cyril Roberto (ParisOuest), Pascal Romon (ParisEst), PaulMarie Samson (ParisEst)
With the crucial support of Audrey Patout and Christiane Lafargue.
Contact: springschool2015ihp@gmail.com
Institut Henri Poincaré (IHP), A.N.R. projet GeMeCoD, Laboratoire d'Analyse et de Mathématiques Appliquées (LAMA)  Université de ParisEstMarnelaVallée (UPEMLV), Institut Universitaire de France (IUF), Labex Bézout.
Participants  Affiliation 

Gozlan, Nathaël  Université ParisEst Marne la Vallée 
Roberto, Cyril  Université ParisOuest Nanterre la Défense 
Romon, Pascal  Université ParisEst Marne la Vallée 
Samson, PaulMarie  Université ParisEst Marne la Vallée 
Fathi, Max  UPMC 
Fradelizi, Matthieu  Université ParisEst MarnelaVallée 
Han, Bangxian  Université ParisDauphine 
Liu, Shiping  Durham University, UK 
Melbourne, James  University of Minnesota 
10  Won, Yong Sul  Imperial College London 
Calderon, Eran  Technion (institute of technology, Israel) 
Tetali, Prasad  School of Mathematics, Georgia Institute of Technology, Atlanta, GA, U.S.A. 
Paseka, Olga  Lomonosov Moscow State University, Russia 
Ralli, Peter  Georgia Institute of Technology 
Kaur, Gursharn  Indian Statistical Institute, New Delhi, India 
Baird, Paul  Université de Bretagne Occidentale 
Papilin, Sergey  Lomonosov Moscow State University, Russian Federation 
Salamon, Joe  University of California, San Diego 
Tran, Tat Dat  Max Planck Institute for Mathematics in the Sciences, Leipzig 
20 Wang, Jian  Max Planck Institute for Mathematics in the Sciences 
Jolany, Hassan  University of Lille1 
Skorokhod, Natalia  Technion, Israel 
Joharinad, Parvaneh  Institution for Advanced Studies in Basic Sciences, Zanjan, Iran 
Chang, Yinshan  Max Planck Institute for Mathematics in the Sciences, Leipzig 
Huou, Benoit  IMT, Toulouse 
SHU, Yan  Université ParisOuest Nanterre la Défense 
Xiao, Yunlong  Max Planck Institute for Mathematics in the Sciences, Leipzig 
Zamora, Sergio  Universidad Nacional Autónoma de México 
Li, Anshui  Mathematical Institute, Utrecht University, The Netherlands 
30  Fillastre, François  Université CergyPontoise 
Seppi, Andrea  Università di Pavia, Italy 
Münch, Florentin  Friedrich Schiller Universität Jena 
Conforti, Giovanni  BMS, Potsdam 
Joulin, Aldéric  Institut de Mathématiques de Toulouse 
Kerkyacharian, Gerard  LPMA 
Rossi, Maurizia  University of Rome Tor Vergata 
Monasse, Pascal  Université ParisEst Marne la Vallée 
Rousset, Mathias  INRIA and CERMICS ENPC 
Kahn, Jonas  CNRS 
40  Collins , Emil  Paris 6 
Urquijo, Ramón  Max Planck Institute for Mathematics in the Sciences 
Sosa, Gerardo  Max Planck Institute for Mathematics in the Sciences, Leipzig 
de Buyer, Paul  MODAL'X  Université Paris Ouest 
Debin, Clément  Institut Fourier (Grenoble) 
Giona Veronelli  Paris 13 
ETBER, Ali  BMCE Capital 
Al Reda, Fatima  Laboratoire Mathématiques d'Orsay 
De Marco, Stefano  Ecole Polytechnique 
Tamanini, Luca  Université Paris Ouest 
Kell, Martin  IHES 
Hubard, Alfredo  INRIA/Marne la Vallee 
Financial support: The school may have some funding for (a limited number of) students (essentially PhD student and postdocs). If you wish to ask for a financial support please enter “yes” in the registration form below (notice that this information will not be public). For administrative reason the financial support will be guaranteed only if the participant give official bills (hotel. In particular no handmade letter from your uncle or cousin attesting that you spent a week in Paris at his/her home! Nothing is required for the meals.).
Depending on the sponsor that will make the reimbursement, the rule might be different. Please wait before buying your ticket and/or booking your hotel/lodging. We will put you in contact with the administration very soon.
The amount of money will approximately be of 500 euros.