Seminars and Colloquia by Series

Thursday, October 4, 2012 - 15:05 , Location: Skiles 006 , J.-C. Breton , Institut de Recherche Mathématique de Rennes , Organizer:
 Cramér's theorem from 1936 states  that the sum of two independent random variables is Gaussian if and only  if these random variables are Gaussian. Since then, this property has  been explored in different directions, such as  for other  distributions or non-commutative random variables. In this talk, we will investigate recent results in Gaussian chaoses and free chaoses. In  particular, we will give a first positive Cramér type result in a free  probability context. 
Thursday, September 27, 2012 - 15:05 , Location: Skiles 006 , Ionel Popescu , School of Mathematics, Georgia Tech , Organizer:
 Ricci flow is  a sort of (nonlinear) heat problem under which the metric on a given manifold is evolving.  There is a deep connection between probability and heat equation.  We try to setup a probabilistic approach in the framework of a stochastic target problem. A major result in the Ricci flow is that the normalized flow (the one in which the area is preserved) exists for all positive times and it converges to a metric of constant curvature.  We reprove this convergence result in the case of surfaces of non-positive Euler characteristic using coupling ideas from probability.   At certain point we need to estimate the second derivative of the Ricci flow and for that we introduce a coupling of three particles.     This is joint work with Rob Neel. 
Thursday, September 20, 2012 - 15:05 , Location: Skyles 006 , Karim Lounici , Georgia Institute of Technology , , Organizer:
In the context of a linear model with a sparse coefficient vector, sharp oracle inequalities have been established for the exponential weights concerning the prediction problem. We show that such methods also succeed at variable selection and estimation under near minimum condition on the design matrix, instead of much stronger assumptions required by other methods such as the Lasso or the Dantzig Selector. The same analysis yields consistency results for Bayesian methods and BIC-type variable selection under similar conditions. Joint work with Ery Arias-Castro
Thursday, September 6, 2012 - 15:05 , Location: Skiles 006 , Yuri Bakhtin , Georgia Tech , Organizer:
 The Burgers equation is a basic hydrodynamic model describing the evolution of the velocity field of sticky dust particles. When supplied with random forcing it turns into an infinite-dimensional random dynamical system that has been studied since late 1990's. The variational approach to Burgers equation allows to study the system by analyzing optimal paths in the random landscape generated by the random force potential. Therefore, this is essentially a random media problem. For a long time only compact cases of Burgers dynamics on the circle or a torus were understood well. In this talk I discuss the Burgers dynamics on the entire real line with no compactness or periodicity assumption. The main result is the description of the ergodic components and One Force One Solution principle on each component.  Joint work with Eric Cator and Kostya Khanin. 
Tuesday, April 24, 2012 - 16:05 , Location: skyles 006 , Zongming Ma , The Wharton School, Department of Statistics, University of Pennsylvania , Organizer:
Singular value decomposition is a widely used tool for dimension reduction in multivariate analysis. However, when used for statistical estimation in high-dimensional low rank matrix models, singular vectors of the noise-corrupted matrix are inconsistent for their counterparts of the true mean matrix. In this talk, we suppose the true singular vectors have sparse representations in a certain basis. We propose an iterative thresholding algorithm that can estimate the subspaces spanned by leading left and right singular vectors and also the true mean matrix optimally under Gaussian assumption. We further turn the algorithm into a practical methodology that is fast, data-driven and robust to heavy-tailed noises. Simulations and a real data example further show its competitive performance. This is a joint work with Andreas Buja and Dan Yang.
Tuesday, April 24, 2012 - 11:00 , Location: Skiles 005 , F. Benaych-Georges , Universite Pierre et Marie Curie , Organizer: Christian Houdre
Many of the asymptotic spectral characteristics of a symmetric random matrix with i.i.d. entries (such a matrix is called a "Wigner matrix") are said to be "universal": they depend on the exact distribution of the entries only via its first moments (in the same way that the CLT gives the asymptotic fluctuations of the empirical mean of i.i.d. variables as a function of their second moment only). For example, the empirical spectral law of the eigenvalues of a Wigner matrix converges to the semi-circle law if the entries have variance 1, and the extreme eigenvalues converge to -2 and 2 if the entries have a finite fourth moment. This talk will be devoted to a "universality result" for the eigenvectors of such a matrix. We shall prove that the asymptotic global fluctuations of these eigenvectors depend essentially on the moments with orders 1, 2 and 4 of the entries of the Wigner matrix, the third moment having surprisingly no influence.
Thursday, April 19, 2012 - 15:05 , Location: Skyles 006 , Ionel Popescu , Georgia Institute of Technology, School of Mathematics , Organizer:
This is obtained as a limit from the classical Poincar\'e on large random matrices. In the classical case Poincare is obtained in a rather easy way from other functional inequalities as for instance Log-Sobolev and transportation. In the free case, the same story becomes more intricate. This is joint work with Michel Ledoux.
Tuesday, April 3, 2012 - 16:05 , Location: Skyles 006 , Axel Munk , Institut für Mathematische Stochastik Georg-August-Universität Göttingen , Organizer:
In this talk we will discuss a general concept of statistical multiscale analysis in the context of signal detection and imaging. This provides a large class of fully data driven regularisation methods which can be viewed as a multiscale generalization of the Dantzig selector. We address computational issues as well as the required extreme value theory of the multiscale statistics. Two major example include change point regression and locally adaptive total variation image regularization for deconvolution problems. Our method is applied to problems from ion channel recordings and nanoscale biophotonic cell microscopy.
Thursday, March 29, 2012 - 15:05 , Location: skyles 006 , Alexander Rakhlin , University of Pennsylvania, The Wharton School , Organizer:
The study of prediction within the realm of Statistical Learning Theory is intertwined with the study of the supremum of an empirical process. The supremum can be analyzed with classical tools:Vapnik-Chervonenkis and scale-sensitive combinatorial dimensions, covering and packing numbers, and Rademacher averages. Consistency of empirical risk minimization is known to be closely related to theuniform Law of Large Numbers for function classes.In contrast to the i.i.d. scenario, in the sequential prediction framework we are faced with an individual sequence of data on which weplace no probabilistic assumptions. The problem of universal prediction of such deterministic sequences has been studied withinStatistics, Information Theory, Game Theory, and Computer Science. However, general tools for analysis have been lacking, and mostresults have been obtained on a case-by-case basis.In this talk, we show that the study of sequential prediction is closely related to the study of the supremum of a certain dyadic martingale process on trees. We develop analogues of the Rademacher complexity, covering numbers and scale-sensitive dimensions, which canbe seen as temporal generalizations of the classical results. The complexities we define also ensure uniform convergence for non-i.i.d. data, extending the Glivenko-Cantelli type results. Analogues of local Rademacher complexities can be employed for obtaining fast rates anddeveloping adaptive procedures. Our understanding of the inherent complexity of sequential prediction is complemented by a recipe that can be used for developing new algorithms.
Thursday, March 15, 2012 - 15:05 , Location: skyles 006 , Vygantas Paulauskas , Vilnius University, Lithuania , Organizer:
In the talk we demonstrate the usefulness of the so-called Beveridge-Nelson decomposition in asymptotic analysis of sums of values of linear processes and fields. We consider several generalizations of this decomposition and discuss advantages and shortcomings of this approach which can be considered as one of possible methods to deal with sums of dependent random variables. This decomposition is derived for linear processes and fields with the continuous time (space) argument. The talk is based on several papers, among them [V. Paulauskas, J. Multivar. Anal. 101, (2010), 621-639] and [Yu. Davydov and V. Paulauskas, Teor. Verojat. Primenen., (2012), to appear]