New singular solutions of the critical and supercritical Nonlinear Schrodinger equation (NLS)

Speaker: 

Dr Nir Gavish

Institution: 

Tel Aviv University

Time: 

Monday, October 13, 2008 - 4:00pm

Location: 

RH 306

The study of singular solutions of the NLS goes back to the 1960s, with applications in nonlinear optics and, more recently, in BEC. Asymptotic and numerical studies conducted in the 80s showed that singular solutions of the critical NLS collapse with the Townes (R) profile at a blowup rate known as the loglog law. Recently (2003) Merle and Raphael proved this result rigorously for a large class of initial conditions. Concurrently, it was demonstrated experimentally that the profile of collapsing laser beams is given by the Townes profile. Thus, all the research that was carried out from the eighties until these days leads to the belief that the Townes profile is the only attractor of blowup solutions of the critical NLS.
In this talk I will present new families of singular solutions of the critical and supercritical NLS that collapse with a self-similar ring profile, and whose blowup rate is different from the one of the "old" singular solutions. I will show, experimentally and theoretically, that these new blowup profiles are attractors for large super-Gaussian initial conditions.
In addition, I will present in the talk a semi-static adaptive grid method we have developed for the numerical simulations involved in this study for solutions which focus over 15 orders of magnitude.

Classification of compact ancient solutions of the Curve Shortening flow and the Ricci flow on Surfaces

Speaker: 

Professor Panagiota Daskalopoulos

Institution: 

Columbia University

Time: 

Tuesday, October 14, 2008 - 4:00pm

Location: 

RH 306

We classify compact ancient solutions of the curve
shortening flow and the Ricci flow on Surfaces. We show that these are either a family of contracting circles (contracting spheres in the case of the Ricci flow on surfaces), which is a type I ancient solution,or a family of Angenent ovals (Rosenau solutions in the case of the Ricci flow on surfaces), which corresponds to a type II
solution.

Coherent Vortex Extraction and Coherent Vortex Simulation of Turbulence

Speaker: 

Professor Kai Schneider

Institution: 

Universite de Provence (Aix-Marseille I), France

Time: 

Thursday, December 4, 2008 - 3:00pm

Location: 

RH 340P

Turbulence is characterized by its nonlinear and multiscale behaviour, self-organization into coherent structures and generic randomness. The number of active spatial and temporal scales involved increases with the Reynolds number, therefore it soon becomes prohibitive for direct numerical simulation. However, observations show that for a given flow realization these scales are not homogeneously distributed, neither in space nor in time, which corresponds to the flow intermittency. To be able to benefit from this property, a suitable representation of the flow should reflect the lacunarity of the fine scale activity, in both space and time.

A prominent tool for multiscale decompositions are wavelets. A wavelet is a well localized oscillating smooth function, i.e. a wave packet, which is dilated and translated. The thus obtained wavelet family allows to decompose a flow field into orthogonal scale-space contributions. The flow intermittency is reflected in the sparsity of the wavelet representation, i.e. only few coefficients, the strongest ones, are necessary to represent the dynamically active part of the flow. We will illustrate this by considering different 2D and 3D turbulent flows, either computed by direct numerical simulation (DNS) or measured by particle image velocimetry (PIV).

To compute the evolution of turbulent flows we have proposed the Coherent Vortex Simulation (CVS), which is based on the wavelet filtered Navier-Stokes equations. At each time step the turbulent fluctuations are split into two parts: the first corresponding to the coherent vortices which are kept, and the second to an incoherent background flow corresponding to turbulent dissipation which is discarded. We will present several simulations of 2D and 3D turbulent flows and show that CVS preserves their nonlinear dynamics.

Related publications can be downloaded from the following web pages:

http://wavelets.ens.fr
http://cmi.univ-mrs.fr/~kschneid

Regularity criteria for the 3D Navier-Stokes equations

Speaker: 

Chongshen Cao

Institution: 

Florida International University

Time: 

Thursday, October 2, 2008 - 3:00pm

Location: 

RH 340P

The question of global regularity for the 3D Navier-Stokes equations is a major open problem in applied analysis. It is well-known that the existence and uniqueness of strong solutions could be obtain under suitable additional assumptions. In this talk I will review some old and new results about the sufficient conditions for the global regularity to the 3D Navier-Stokes equations.

TBA

Speaker: 

Chongshen Cao

Institution: 

Florida International University

Location: 

RH 340P

Incompressible soul theory and application to Perelman's collapsing and Geometrization.

Speaker: 

Professor Jianguo Cao

Institution: 

Notre Dame

Time: 

Tuesday, November 4, 2008 - 4:00pm

Location: 

RH 306

In this lecture, we present a new proof of Perelman's collapsing theorem for 3-manifolds with boundary which is needed for his work on Thurston's Geometrization Conjecture. Among other things, we use an observation of Hamilton-Perelman on incompressible tori boundary for Ricci flow with surgery on thick part of a 3-manifold. Starting from incompressible tori boundary of thin part of 3-manifold, we found that there is an injective F-structure in the sense of Cheeger-Gromov. Consequently, the part of a 3-manifold for Ricci flow with surgery becomes an aspherical graph-manifold, Perelman's collapsing theorem for 3-manifolds follows.

Large Margin Hierarchical Classification

Speaker: 

Professor Xiaotong Shen

Institution: 

University of Minnesota

Time: 

Monday, February 2, 2009 - 4:00pm

Location: 

RH 306

In this talk, I will present a large margin method for hierarchical
classification. The main focus here is to utilize the dependency
structure among classes to improve the classification performance of
flat classification. In such a situation, flat classification is
infeasible in the presence of a large number of dependent classes, which
occurs often in gene function discovery. Various hierarchical losses
will be discussed, in addition to an application to
gene function prediction.

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