Analysis Seminar

Coordinators: Jalal Shatah & Nader Masmoudi

The analysis seminar covers a wide range of topics in analysis with particular emphasis on partial differential equations. Many of the speakers are Courant Institute visitors and postdocs. A seminar talk may cover original research or report on an interesting paper. The seminar meets on Thursdays at 11:00 am in room 1302 of Warren Weaver Hall at 251 Mercer Street, New York unless specified otherwise. Talks generally last an hour. A few special analysis seminars may be held at other times and locations.

The most reliable and inclusive list of weekly seminars and events is to be found in the weekly bulletin that is posted on a day-by-day basis on the CIMS home page.

May 4, 2017
Connor Mooney, ETH Zurich
Counterexamples to Sobolev regularity for degenerate Monge-Ampere equations

W^{2,1} estimates for the Monge-Ampere equation det D^2u = f in R^n were obtained by De Philippis and Figalli in the case that f is bounded between positive constants. Motivated by applications to the semigeostrophic equation, we consider the case that f is allowed to be zero on some set. In this case there are simple counterexamples to W^{2,1} regularity in dimension n >= 3 that have a Lipschitz singularity.

In contrast, if n = 2 then a classical result of Alexandrov on the propagation of Lipschitz singularities implies that solutions are C^1. We will discuss a counterexample to W^{2,1} regularity in two dimensions whose second derivatives have nontrivial Cantor part, and a related result on the propagation of Lipschitz/Log singularities which is optimal by example.

May 2, 2017 (special Tuesday seminar at 2:30p in WWH 101)
Zineb Hassainia, CIMS
Relative equilibria for planar Euler equations

Vortex relative equilibria are configurations of vortices that maintain their basic shapes for all time, while rotating or translating rigidly in space.  We shall discuss in this talk the immersion of these patterns where the underlying dynamics are governed by the two-dimensional Euler equations. The main concern is to investigate the existence of the di-polar and multi-polar V-states via the contour dynamics equations combined with a desingularization of the point vortex and the application of the implicit function theorem.
April 27, 2017
Tsukasa Iwabuchi, Tohoku University
Ill-posedness for the compressible Navier-Stokes equations

We study the ill-posedness for the compressible Navier-Stokes equations under the barotropic condition by showing the discontinuity of solutions on initial data. The norm inflation is shown in the scaling invariant Besov spaces. The density and velocity are considered in $\dot B^{1/2}_{2n,1}$ and $\dot B^{-1/2}_{2n,1}$, respectively. We will discuss on which nonlinear terms would have the worst regularity and on its estimates investigating the second iterate.

April 20, 2017
Kevin Payne, University of Milano
Dirichlet problems for mixed type equations

I will present some recent progress on Dirichlet problems for PDEs of mixed elliptic-hyperbolic type. This progress concerns weak well posedness, elements of spectral theory and variational characterizations. Global energy methods, developed for problems related to transonic potential flow and the possible heating in cold plasmas,  will be extended to a class of geometric equations involving the Laplace-Beltrami operator for a metric of mixed Riemannian-Lorentzian signature. The results have been obtained in collaboration with Daniela Lupo, Dario Monticelli and Cathleen Morawetz.

April 13, 2017
Anne-Laure Dalibard, UPMC Paris 6
Models for boundary layer separation: the Prandtl equation and beyond?

In this talk, I will present recent results for fluid boundary layers. The largest part of the talk will be devoted to a work in collaboration with Nader Masmoudi, in which we prove separation for the stationary Prandtl equation and justify Goldstein's singularity. I will then present some instability results for alternative boundary layer models: Prandtl equation with prescribed displacement thickness and interactive boundary layer models. This is an ongoing work with Helge Dietert, David Gérard-Varet and Frédéric Marbach.

April 6, 2017 (special noon seminar)
Chongchun Zeng, Georgia Tech
Instability, index theorems, and exponential dichotomy of Hamiltonian PDEs

Motivated by the stability/instability analysis of coherent states (standing waves, traveling waves, etc.) in nonlinear Hamiltonian PDEs such as BBM, GP, and 2-D Euler equations, we consider a general linear Hamiltonian system $u_t = JL u$ in a real Hilbert space $X$ -- the energy space. The main assumption is that the energy functional $\frac 12 \langle Lu, u\rangle$ has only finitely many negative dimensions -- $n^-(L) < \infty$. Our first result is an $L$-orthogonal decomposition of $X$ into closed subspaces so that $JL$ has a nice structure. Consequently, we obtain an index theorem which relates $n^-(L)$ and the dimensions of subspaces of generalized eigenvectors of some eigenvalues of $JL$, along with some information on such subspaces. Our third result is the linear exponential trichotomy of the group $e^{tJL}$. This includes the nonexistence of exponential growth in the finite co-dimensional invariant center subspace and the optimal bounds on the algebraic growth rate there. Next we consider the robustness of the stability/instability under small Hamiltonian perturbations. In particular, we give a necessary and sufficient condition on whether a purely imaginary eigenvalues may become hyperbolic under small perturbations.  Finally we revisit some nonlinear Hamiltonian PDEs. This is a joint work with Zhiwu Lin.

April 6, 2017
Edriss Titi, Texas A&M
An Algorithm for Advancing Slow Features in Fast-Slow Systems without Scale Separation - A Young Measure Approach

In the first part of the talk, and in order to set the stage, we will offer a multi-scale and averaging strategy to compute the solution of a singularly perturbed system when the fast dynamics oscillates rapidly; namely, the fast dynamics forms cycle-like limits which advance along with the slow dynamics. We describe the limit as a Young measure with values being supported on the limit cycles, averaging with respect to which induces the equation for the slow dynamics. In particular, computing the tube of the limit cycles establishes a good approximation for arbitrarily small singular parameters. We will demonstrate this by exhibiting concrete numerical examples. In the second part of the talk we will examine singularly perturbed systems which may not possess a natural split into fast and slow state variables. Once again, our approach depicts the limit behavior as a Young measure with values being invariant measure of the fast contribution to the flow. These invariant measures are drifted by the slow contribution to the value. We keep track of this drift via slowly evolving observables. Averaging equations for the latter lead to computation of characteristic features of the motion and the location the invariant measures. To demonstrate our ideas computationally, we will present some numerical experiments involving a system derived from a spatial discretization of a Korteweg-de Vries-Burgers type equation, with fast dispersion and slow diffusion.

This is a joint work with Z. Artstein, W. Gear, I. Kevrekidis, J. Linshiz and M. Slemrod.

March 30, 2017 (special noon seminar in WWH 1302)
Zaher Hani, Georgia Tech
Scattering of the Gross-Pitaevskii equation in the 3D radial energy space

We consider the 3D Gross-Pitaevskii (GP) equation, or the nonlinear Schrodinger (NLS) equation with non-vanishing constant amplitude at spatial infinity. We are interested in the asymptotic stability of the plane wave solutions. This was first answered positively by Gustafson, Nakanishi, and Tsai assuming that the perturbation belongs to a weighted Sobolev space. Such a control is not provided by the conserved energy of the system, so the natural question is whether this stability holds for perturbations in the energy space. Such a result is crucial to addressing the large-data theory. We give a positive answer under the assumption of radial symmetry (or angular regularity), and prove small-data scattering to appropriate free states. The interaction with the stationary plane wave is long-range, which makes scattering for GP much harder than that for NLS (i.e. for data vanishing at spatial infinity). The proof relies on: A) a normal forms transformation, which is a modification of that used by Gustafson, Nakanishi, and Tsai, that introduces additional null structures to the problem, and b) improved Strichartz estimates for the linear evolution assuming angular regularity. As mentioned above, this result gives access to the large-data scattering problem in the radial setting, which features a rather intriguing phenomenon: We know that such large-data scattering cannot happen in all energy space due to the presence of traveling wave solutions; however, such solutions are far from being radial. So, one might expect that scattering should happen in the radial setting without any obstruction.

This is joint work with Zihua Guo and Kenji Nakanishi.

March 30, 2017
Alexander Ionescu, Princeton Univ.
Bilinear estimates in quasilinear evolution equations

March 23, 2017
Nam Le, Indiana
The Monge-Ampere eigenvalue problem and global smoothness of the eigenfunctions

In this talk, I will first introduce the Monge-Ampere eigenvalue problem on general bounded convex domains and related analysis.
Then I will discuss the recent resolution, in joint work with Ovidiu Savin, of global smoothness of the eigenfunctions of the Monge-Ampere operator on smooth, bounded and uniformly convex domains in all dimensions. A key ingredient in our analysis is boundary Schauder estimates for certain degenerate Monge-Ampere equations.

March 9, 2017
Semyon Klevtsov, Cologne
Geometry and large N asymptotics in Laughlin states.

Laughlin states are N-particle wave functions, which successfully describe fractional quantum Hall effect (QHE) for plateaux with simple fractions. It was understood early on, that much can be learned about QHE when Laughlin states are considered on a Riemann surface. I will define the Laughlin states on a compact oriented Riemann surface of arbitrary genus and talk about recent progress in understanding their geometric properties and relation to physics. Mathematically, it is interesting to know how do L.s. depend on an arbitrary Riemannian metric, magnetic potential function, complex structure moduli, singularities -- for a large number of particles N. I will review (both from math and physics perspective) the results, conjectures and further questions in this area, and relation to topics such as Coulomb gases/beta-ensembles, Bergman kernels for holomorphic line bundles, Quillen metric, zeta determinants. The talk will be friendly to nonspecialists.

March 2, 2017
Zhongwei Shen, Univ. Kentucky

Boundary Layers in Periodic Homogenization of Neumann Problems

This talk is concerned with a family of second-order elliptic systems in divergence form with rapidly oscillating periodic coefficients. We initiate the study of homogenization and boundary layers for Neumann problems with first-order oscillating boundary data. The investigation is motivated by the study of higher-order convergence rates for Neumann problems with non-oscillating data. We identify the homogenized system and establish the sharp rate of convergence in L^2. Sharp regularity estimates are also obtained for the homogenized boundary data in both Dirichlet and Neumann problems. Our results as well as the approaches used build on the work of D. Gerard-Varet - N. Masmoudi and S.N. Armstrong - T. Kuusi - J.C. Mourrat - C. Prange for the Dirichlet problem. This is a joint work with Jinping Zhuge.

February 23, 2017
Xavier Ros-Oton, Univ. Texas
Regularity of free boundaries in obstacle problems

We present a brief overview of the regularity theory for free boundaries in different obstacle problems. We describe how a monotonicity formula of Almgren plays a central role in the study of the regularity of the free boundary in some of these problems. Finally, we explain new strategies which we have recently developed to deal with cases in which monotonicity formulas are not available.

February 16, 2017
Sam Punshon-Smith, Maryland
Renormalized martingale solutions to the Boltzmann equation with stochastic forcing

The Boltzmann equation is often used to describe the non-equilibrium behavior of a collisional gas in a low density regime. Frequently an external forcing term is included to account for the influence of an external field interacting with the gas. In this talk we will study the Boltzmann equation in the presence of a white in time stochastic forcing. Such a forcing can be thought of as a kinetic analogue of stochastic forcing commonly included in fluid mechanical models of turbulence. Under certain spatial coloring assumptions on the noise, we obtain existence of global in time, probabilistically weak, renormalized (in the sense of DiPerna/Lions) solutions to this equation for a general class of L^1 initial data. Among the topics discussed will be the renormalization theory of stochastic transport equations, velocity averaging of stochastic kinetic equations and various stochastic fluid limits.

February 9, 2017
Pierre Cardaliaguet, Universite Paris-Dauphine
The master equation in mean field games

In mean field game theory, the master equation is a kind of nonlinear, nonlocal transport equation stated on the space of measure. Based on joint works with Delarue, Lasry, Lions and Porretta, I will discussed the well-posedness of the equation and its applications to mean field games.

February 2, 2017
Ping Zhang, AMSS
Large time behavior of solutions to 3-D MHD system with initial data near equilibrium

Given initial data $(b_0,u_0)$ close enough to the equilibrium state $(e_3,0),$ we prove that the 3-D incompressible MHD system without magnetic diffusion has a unique global solution $(b,u).$ Moreover,we prove that $(b(t)-e_3,u(t))$ decay to zero with rates in both $L^\infty$ and $L^2$ norm. (This is a joint work with Wen Deng).
December 13, 2016 (special Tuesday seminar)
Sergei Kuksin, Univ. Paris 7

December 8, 2016
Walter Craig, McMaster Univ.
Hamiltonians and normal forms for water waves

It was shown by VE Zakharov that the equations for water waves can be posed as a Hamiltonian dynamical system, and that the equilibrium solution is an elliptic stationary point. This talk will discuss two aspects of the water wave equations in this context. Firstly, we generalize the formulation of Zakharov to include overturning wave profiles, answering a question posed by T. Nishida. Secondly, we will discuss the question of Birkhoff normal forms for the water waves equations in the setting of spatially periodic solutions, including the function space mapping properties of these transformations.

December 6, 2016 (special Tuesday seminar in WWH 524 at 11a)
Helge Dietert, Univ. Paris 7
Landau damping in the Kuramoto model

The Kuramoto model consists of globally coupled oscillators. Like the Vlasov equation it is a mean-field model. Both models show the remarkable stability mechanism through phase-mixing, which is also called Landau damping. In this talk, I will discuss this stability mechanism for partially locked states in the Kuramoto model, which are inhomogeneous and irregular equilibria. In particular, I will discuss our work (i) establishing an explicit criterion for spectral stability, (ii) showing nonlinear stability for analytic perturbations, and (iii) extending the result to Sobolev regular perturbations. The items (i) and (ii) have been done in collaboration with Bastien Fernandez and David Gerard-Varet and are available at arXiv:1606.04470.

December 1, 2016
Carlos Fernandez-Granda, CIMS
Demixing Sines and Spikes: Spectral Super-resolution in the Presence of Outliers

In this talk we consider the problem of super-resolving the line spectrum of a multisinusoidal signal from a finite number of samples, some of which may be completely corrupted. Measurements of this form can be modeled as an additive mixture of a sinusoidal and a sparse component. We propose to demix the two components and super-resolve the spectrum of the multisinusoidal signal by solving a convex program. Our main theoretical result is that-- up to logarithmic factors-- this approach is guaranteed to be successful with high probability for a number of spectral lines that is linear in the number of measurements, even if a constant fraction of the data are outliers. We show that the method can be implemented via semidefinite programming and explain how to adapt it in the presence of dense perturbations, as well as exploring its connection to atomic-norm denoising. In addition, we propose a fast greedy demixing method which provides good empirical results when coupled with a local nonconvex-optimization step.

November 21, 2016 (special Monday seminar at 11a in WWH 1314)
Tim Laux, Max Planck Institute, Leipzig
Convergence of phase-field models and thresholding schemes to multi-phase mean-curvature flow

The thresholding scheme -- a time discretization for mean curvature flow --  was introduced by Merriman, Bence and Osher. In this talk I'll present new convergence results for modern variants of the scheme, in particular in the multi-phase case with arbitrary surface tensions. The main result establishes convergence to a weak formulation of (multi-phase) mean curvature flow in the BV-framework of sets of finite perimeter. The methods are then extended to incorporate external forces and a volume constraint. Furthermore, I will present a similar result for the vector-valued Allen-Cahn Equation. This talk encompasses joint work with Felix Otto, Thilo Simon, and Drew Swartz (Booz Allen Hamilton).

November 17, 2016

Frederic Bernicot, University of Nantes
Paracontrolled calculus and singular PDES through the heat semigroup

We will present the main idea of the paracontrolled calculus, which was recently introduced in the Euclidean situation by Gubinelli, Imkeller and Perkowski. This gives an alternative approach to Hairer's theory in order to deal with singular (stochastic) PDEs, such as the Parabolic Anderson Model (2D-3D) and Burgers equations. Indeed, it relies on the following fact: the paraproduct (as a singular bilinear operator) is the well-suited analytic tool, in order to isolate the stochastic cancellations in nonlinearities. We will then explain how we can extend it in many various situations, given by a heat semigroup with gradient estimates.

November 10, 2016
Afonso Bandeira, CIMS
On the critical points of optimization problems arising from low rank factorization approaches to semidefinite programming

November 3, 2016
Tuomo Kuusi, Aalto University (Helsinki)
Vectorial nonlinear potential theory

We discuss new pointwise potential estimates obtained for vectorial p-Laplacian involving measure data. The estimates allow to give sharp descriptions of fine properties of solutions which are the exact analog of the ones in classical linear potential theory. For instance, sharp characterizations of Lebesgue points  and optimal regularity criteria for solutions are provided exclusively in terms of potentials. The validity of such estimates in the vectorial setting was an open problem for more than 20 years, and we recently settled this in collaboration with G. Mingione.

October 27, 2016

Toan Nguyen, Penn State Univ.
Prandtl's boundary layers: the good, the bad, and the ugly

I will present several recent results concerning the classical Prandtl's boundary layer asymptotic expansions of Navier-Stokes in the inviscid limit. While the expansion has been proven for analytic data (or even for Gevrey data), the expansion is highly unstable for less regular data in view of the classical linear stability theory of shear flows. I will discuss recent advances on the instability of the Prandtl's expansion.

October 25, 2016 (special Tuesday seminar at 11a in WWH 524)
P. LeFloch, Univ. Paris 6

Global solutions to the Einstein-Klein-Gordon system

I will discuss the global evolution problem for self-gravitating massive matter in the context of Einstein's theory and, more generally, of the f(R)-theory of gravity. In collaboration with Yue Ma (Xian), I have investigated the global existence problem for the Einstein-Klein-Gordon system and established that Minkowski spacetime is globally nonlinearly stable in presence of massive fields. The method proposed by Christodoulou and Klainerman and the more recent proof in wave gauge by Lindblad and Rodnianski only cover vacuum spacetimes or massless fields. Analyzing the time decay of massive waves requires a completely new approach, the Hyperboloidal Foliation Method, which is based on a foliation by asymptotically hyperboloidal hypersurfaces and on investigating the algebraic structure of the Einstein-Klein-Gordon system.

October 20, 2016

Paul Yang, Princeton Univ.
Fourth order equations in conformal geometry

In conformal geometry, the higher order Q-curvature equation in general dimensions did not receive much attention until the basic difficulty of the lack of maximum principle is understood. In this talk, I will outline the recent progress about this equation, dealing with the sign of the Green's function, positivity of mass, and the solutions to the Q-curvature equations.

October 13, 2016

John Ball, University of Oxford
Remarks on incompatible and compatible sets of matrices

The talk will discuss various results (mostly drawn from joint work with R. D. James) concerning compact sets of matrices that are compatible or incompatible for gradient Young measures.

October 6, 2016
Julien Guillod, Princeton Univ.
Two-dimensional steady solutions of the Navier-Stokes equations in unbounded domains

Essentially two methods are known to analyze the stationary Navier-Stokes equations in the plane: the topological method and the perturbation method. However, the problem is supercritical similarly to the three-dimensional Cauchy problem and therefore both methods have limitations, more precisely concerning the behavior at infinity of the solutions. I will present a new method to analyze this problem. The Stokes paradox states that the linearization of the Navier-Stokes equations have no bounded solutions in general. I will explain how the nonlinearity helps to obtain bounded solutions going to zero at infinity for the full nonlinear problem.

September 29, 2016

Vladimir Sverak, University of Minnesota
On the Cauchy problem for vortex rings

We consider the initial-value problem for the 3d Navier-Stokes equation when the initial vorticity is supported on a circle. Such initial datum is in certain function spaces where perturbation theory works for small data, but not for large data, even for short times, and there are good reasons to believe that this is not just a technicality. We prove global existence and uniqueness for large data in the class of axi-symmetrix solutions. The main tools are Nash-type estimates and certain monotone quantities.Uniqueness in the class of solutions which are not necessarily axi-symmetric remains a difficult open problem, which we plan to discuss briefly. Joint work with Thierry Gallay.

September 22, 2016

Jeremy Marzuola, University of North Carolina
The relaxation of a general family of broken bond crystal surface models

In work with Jon Weare (Chicago) that started when he was a Courant Instructor actually, we studied the continuum limit of a family of kinetic Monte Carlo models of crystal surface relaxation that includes both the solid-on-solid and discrete Gaussian models. With computational experiments and theoretical arguments we were able to derive several partial differential equation limits identified (or nearly identified) in previous
studies and to clarify the correct choice of surface tension appearing in the PDE. These have the general form of fourth order nonlinear diffusion-type operators, though in certain cases they are degenerate. We will summarize these results, then discuss some properties of the derived PDEs and highlight some recent progress/interesting developments that have arisen in the study of these models.

September 15, 2016
Sylvia Serfaty, CIMS
Mean Field Limits for Ginzburg-Landau Vortices


May 26, 2016

Alexander Lorz, Univ. Paris 6
Population dynamics and therapeutic resistance: mathematical models

We are interested in the Darwinian evolution of a population structured by a phenotypic trait. In the model, the trait can change by mutations and individuals compete for a common resource e.g. food. Mathematically, this can be described by non-local Lotka-Volterra equations. They have the property that solutions concentrate as Dirac masses in the limit of small diffusion. We review results on long-term behaviour and small mutation limits. A promising application of these models is that they can help to quantitatively understand how resistances against treatment develop. The population of cells is structured by how resistant they are against a therapy. We describe the model, give first results and discuss optimal control problems arising in this context.

May 13, 2016 (special Friday seminar at 1p in WWH 512)
Antoine Gloria, Free University of Brussels
The homogenization commutator

As clear from the mechanical point of view and from the very definition of H-convergence, homogenization of elliptic equations in divergence form is the art of averaging fields and fluxes. At the level of the corrector, this takes the following form: large-scale averages of the flux of the corrector are close to the homogenized coefficients times large-scale averages of the field of the corrector. The defect in this relation is what we call the homogenization commutator. This quantity is key to the structure of fluctuations in stochastic homogenization. On the one hand, when properly rescaled, it converges to Gaussian white noise. On the other hand, it characterizes the fluctuations of the (random variable-coefficients) solution operator in a path-wise sense (in the terminology used for thermal noise). This is based on joint works with Mitia Duerinckx (ULB) and Felix Otto (MPI).

May 12
, 2016
Patrick Gerard, University Paris-Sud

The one dimensional cubic half wave equation: small mass solitons, almost integrability and transition to high frequencies

The one dimensional cubic half wave equation is a simple example of a fractional NLS equation with vanishing dispersion. Its focusing version admits solitons with arbitrarily small mass. I will explain how these solitons are connected to some integrable system as the velocity tends to the speed of light, and how this connection allows to construct two-soliton solutions displaying some transition to high frequencies.

May 5, 2016
Thomas Alazard, ENS Paris

Control and stabilization of the incompressible Euler equation with free surface

The incompressible Euler equation with free surface dictates the dynamics of the interface separating the air from a perfect incompressible fluid. This talk is about the controllability and the stabilization of this equation. The goal is to understand the generation and the absorption of water waves in a wave tank. These two problems are studied by two different methods: microlocal analysis for the controllability, and study of global quantities for the stabilization (multiplier method, Pohozaev identity, Hamiltonian formulation, Luke’s variational principle, conservation laws…).

April 28, 2016

Yanyan Li, Rutgers Univ.
Symmetry, quantitative Liouville theorems and analysis of large solutions of conformally invariant fully nonlinear elliptic equations

We establish blow-up profiles for any blowing-up sequence of solutions of general conformally invariant fully nonlinear elliptic equations on Euclidean domains. We prove that (i) the distance between blow-up points is bounded from below by a universal positive number, (ii) the solutions are very close to a single standard bubble in a universal positive distance around each blow-up point, and (iii) the heights of these bubbles are comparable by a universal factor. As an application of this result, we establish a quantitative Liouville theorem. This is a joint work with Luc Nguyen.

April 21, 2016
Camillo de Lellis, Univ. Zurich

A Nash-Kuiper theorem for $C^{1,1/5}$ isometric immersions of disks
In a joint work with Dominik Inauen and L\'aszl\'o Sz\'ekelyhidi we prove that, given a $C^2$ Riemannian metric $g$ on the  $2$-dimensional disk, any short $C^1$ embedding can be uniformly approximated with 
$C^{1,\alpha}$ isometric embeddings for any $\alpha < \frac{1}{5}$. The same statement with $C^1$ isometric embeddings is a groundbreaking result due to Nash and Kuiper. The previous Hoelder threshold, 1/7, was first announced in the sixties by Borisov. If time allows I will also discuss the connection with a conjecture of Onsager on weak solutions to the Euler equations.

April 14, 2016

Messoud Efendiyev, Helmholtz Zentrum Muenchen
Symmetrization and stabilization of solution of nonlinear elliptic problem: dynamical systems approach

We study the asymptotic behaviour of solutions of nonlinear elliptic equations in asymptotically symmetric unbounded domains. We are interested in how the asymptotic symmetry of domain inherited to the symmetry of
appropriate global attractor. To this end we use dynamical systems approach and new Liouville type results which are of independent interest as well.

April 7, 2016
Jake Solomon, Hebrew University of Jerusalem
The degenerate special Lagrangian equation
I will discuss a degenerate form of the special Lagrangian equation that arises as the geodesic equation for the space of positive Lagrangians. Considering graph Lagrangians in Euclidean space, we obtain a second order fully non-linear PDE for a single real function. I will explain how to prove existence and uniqueness for Lipschitz solutions to the Dirichlet problem on a convex domain times the unit interval. The proof uses the subequation theory of Harvey-Lawson. Existence of solutions on general manifolds with sufficient regularity would imply a version of the strong Arnold conjecture in Hamiltonian dynamics as well as uniqueness for special Lagrangians. This talk is based on joint work with Yanir Rubinstein.

March 24, 2016
Fabio Pusateri, Princeton Univ.

Global regularity for water waves

We will begin by introducing the water waves equations and discuss some of the works done in recent years on the question of global regularity. We will then present our main result, joint with Deng, Ionescu and Pausader, about global existence of smooth solutions for the 3D gravity-capillary water waves system in infinite depth. The main difficulties in this problem are the slow decay of linear solutions and the presence of a large set of resonant interactions.

March 3, 2016
Dana Mendelson, IAS

Symplectic non-squeezing for Hamiltonian PDEs

In this talk, I will discuss symplectic non-squeezing for the nonlinear Klein-Gordon equation (NLKG) which can be (formally) regarded as an infinite dimensional Hamiltonian system. The symplectic phase space for this equation is at the critical regularity, and in this setting there is no global well-posedness nor any uniform control on the local time of existence for arbitrary initial data. We will present several non-squeezing results for the NLKG, including a conditional result which states that uniform bounds on the Strichartz norms of solutions for initial data in bounded subsets of the phase space imply global-in-time non-squeezing. The proofs rely on several approximation results for the flow, which we obtain using a combination of probabilistic and deterministic techniques.

March 1, 2016 (special Tuesday seminar at 11a in WWH 1314)
Yasunori Maekawa, Tohoku University
On viscous incompressible flows around a rotating obstacle in two dimensions

In this talk, we consider the Navier-Stokes equations for viscous incompressible flows in a two-dimensional exterior domain subject to the no-slip boundary condition. Due to the boundary condition the motion of the obstacle (complement of the exterior domain) naturally create a nontrivial flow. The most typical case is that the obstacle is translating with a constant velocity, known as the Oseen problem, and the stationary Navier-Stokes flows in this case were obtained by Finn and Smith in 1960's. Another typical motion of the obstacle is the rotation with a constant angular velocity, however, the existence of the corresponding stationary Navier-Stokes flows (in the reference frame) has been open in the two-dimensional case. Recently, by analyzing the linearized problem, Hishida (2015) revealed that the rotation of the obstacle resolves the Stokes paradox as in the Oseen case. In this talk we show the existence of stationary Navier-Stokes flows by extending Hishida's result for the linearized problem. The asymptotic behavior of solutions at spatial infinity is also clarified. This talk is based on the collaboration with Mitsuo Higaki (Tohoku university) and Yuu Nakahara (Tohoku university).

February 25, 2016
Alex Misiats, CIMS
Invariant measures for stochastic reaction-diffusion equations.

We study the long-time behavior of systems governed by nonlinear reaction-diffusion type equations perturbed by an infinite dimensional nuclear Wiener process. This equation is known to have a uniformly bounded (in time) solution provided the nonlinearity (the reaction term) possesses certain dissipative properties, which are fairly restrictive. The existence of a bounded solution implies, in turn, the existence of an invariant measure for this equation, which is
an important step in establishing the ergodic behavior of the underlying physical system. In my presentation I will talk about a new approach to establishing the existence of a bounded solution, which allows to expand the existing class of nonlinearities, for which the invariant measure exists. Our approach is based on applying an infinite dimensional analog of the classic Ito's formula to a certain, carefully constructed Lyapunov functional of a weak solution. I will also address the question of uniqueness of the stationary solution, and its asymptotic behavior.

February 23, 2016 (special Tuesday seminar at 11a in WWH 1314)
Philippe LeFloch, University of Paris 6 and CNRS
The global nonlinear stability of Minkowski spacetime for self-gravitating massive fields

February 18, 2016

Matthias Hieber, Technical University of Darmstadt
Dynamics of the Ericksen-Leslie Model for Nematic Liquid Crystal Flows with  General Leslie Stress

In this talk, we consider the Ericksen-Leslie model for the flow of nematic liquid crystals in a bounded domain with general Leslie and isotropic Ericksen stress in the iso- and nonisothermal setting. We discuss recent local and global well-posedness results in the strong sense  for this system and describe furthermore the dynamical behaviour of its solutions. Note that for these results no structural conditions on the Leslie coefficients are being imposed and that in particular Parodi's relation is not being assumed.
This is joint work with Jan Pruess.

February 11, 2016
Nicholas Alikakos, University of Athens

On the structure of phase transition maps

The scalar Allen-Cahn  equation models coexistence of two phases, and is  related to Minimal Surfaces. The 1979 De Giorgi conjecture for the scalar problem has been settled, not so recently, in a series of papers
(Ghoussoub and Gui (2d), Ambrosio and Cabre (3d), Savin (up to 8d) and Del Pino, Kowalczyk and Wei (counterexample for 9d and above)).

The vector Allen-Cahn equation models coexistence of three or more phases and is related to Plateau Complexes. These are non-orientable minimal objects with a hierarchical structure. The analog of the De Giorgi question in the vector case is open.

After stating an existence theorem for equivariant solutions under a reflection group, we focus on vector extensions of the Caffarelli-Cordoba Density Estimates.  In particular, we establish lower codimension
density estimates. These are useful for studying the hierarchical structure of vector solutions.

February 4, 2016

Federica Sani, University of Milan
On Trudinger-Moser type inequalities in the whole space

The Trudinger-Moser inequality is a substitute for the well known Sobolev embedding theorem when the limiting case is considered. We discuss Moser type inequalities in the whole space which involve complete and reduced Sobolev norm. Then we investigate the optimal growth rate of the exponential type function both in the first order case and in the higher order case.

FALL 2015

December 22, 2015 (11am in WWH 1314)
Slim Ibrahim, University of Victoria
Time periodic solutions of the Navier-Stokes-Maxwell system: existence and stability

We considered the Navier-Stokes system coupled with the full Maxwell's equations (NSM). When the whole system is forced by a small and time periodic external force, we construct periodic in time solution, and then we show its long time stability.

This is a joint work with P. G. Lemarie & N. Masmoudi.

December 15, 2015 (10am in WWH 102)
Massimiliano Berti, SISSA
Quasi-periodic standing wave solutions of gravity-capillary water waves

We prove the existence and the linear stability of Cantor families of small amplitude time quasi-periodic standing wave solutions (i.e. periodic and even in the space variable x ) of a 2-dimensional ocean with infinite depth under the action of gravity and surface tension. Joint work with Riccardo Montalto.

December 10, 2015

Yehuda Pinchover, Technion
Optimal Hardy-type inequality for nonnegative second-order elliptic operator: an answer to a problem of Shmuel Agmon

We give a general answer to the following fundamental problem posed by Shmuel Agmon 30 years ago:

Given a (symmetric) linear elliptic operator P of second-order in R^n,
find a nonnegative weight function W which is "as large as possible",
such that for some neighborhood of infinity G the following inequality holds

           (P - W) \geq 0   in the sense of the associated
            quadratic form on C_0^\infty(G).

We construct, for a general subcritical second-order elliptic operator P on a domain D in R^n (or on a noncompact manifold), a Hardy-type weight W which is optimal in the following natural sense sense:

1.  For any \lambda \leq 1, the operator (P - \lambda W) \geq 0
  on C_0^\infty(D),
2.  For \lambda = 1, the operator (P - \lambda W) is null-critical in D,
3.  For any \lambda > 1, and any neighborhood of infinity G of D, the
operator (P - \lambda W) is not nonnegative  on C_0^\infty(G).
4. If P is symmetric and W>0, then the spectrum and the essential
spectrum of the operator W^{-1}P are equal to [1,\infty).

Our method is based on the theory of positive solutions and applies to both symmetric and nonsymmetric operators on a general domain D or on a noncompact manifold. Moreover, the results can be generalized to certain
p-Laplacian type operators. The constructed weight W is given by an explicit simple formula involving two positive solutions of the equation Pu=0.

This is a joint work with Baptiste Devyver and Martin Fraas.

December 4, 2015
Benjamin Harrop-Griffiths, Courant
Asymptotic behavior of the mKdV

The Korteweg-de Vries (KdV) equation arises as an asymptotic limit of numerous dispersive systems and together with its generalizations has a wide range of physical applications including fluid mechanics, plasma physics and nonlinear optics. In this talk we will discuss the modified KdV (mKdV) that provides a model for the dispersive part of the KdV. We will present some robust techniques using wave packets that allow us to study both the modified scattering and final state problems without relying on the completely integrable structure. In particular, these techniques may be applied to short range perturbations of the mKdV as well as numerous other non-integrable equat

December 2, 2015 (CANCELLED. This talk has been moved to December 4).
Benjamin Harrop-Griffiths, Courant

November 24, 2015 (special Tuesday seminar in WWH 412)   
Bedros Afeyan, Polymath Research
Kinetic, Nonlinear, Non-Stationary, Multimode, Self-Sustaining Structures in Vlasov Plasmas

Beyond the single mode assumption of common theoretical practice, and much before any stationary state might be reached, we show that it is possible to excite and sustain nonlinear kinetic structures in Vlasov-Poisson plasmas. Beyond linear Landau Damping or its finite, yet small amplitude generalizations ala Villani, beyond echoes and when particle trapping is in full gear, we find multimode structures that essentially reconstitute themselves as coherent waves. They live atop mixed integrable and chaotic orbit particle trajectories. While trapping-unwrapping-retrapping oscillations abound, in certain critical regions of phase space, the overall generally multimode wave structure is preserved, breathing, and surviving undamped.

Not BGK modes, not Landau Damping, not single mode theory, nor stationary; not echoes, not weak turbulence, nor strong, these are large scale vortical structures that must be taken into account in any full picture of the nonlinear evolution of one or more species plasmas with energy injection. These waves have no linear limit in the sense that they are not resonances falling on dispersion curves in the infinitesimal amplitude case. In that limit, they would be strongly damped modes. They also have no fluid limit since they require severe modifications of the distribution function in a spatially inhomogeneous way, self-adjusting and not settling down in the time-independent sense of BGK modes and similar findings.

We will show the example of kinetic electrostatic electron nonlinear (KEEN) waves and their pair plasma generalization also involving positrons instead of a massive stationary ion background. We call these (KEEPN) Waves. The stratification of phase space into segregated regions with their own particle orbit statistics, thus their own phase-space-fluid dynamics, will be demonstrated. The multimode character, the particle orbit statistics in different partitions, and the nonlocal interaction between KEEN wave and linear resonant structures such as electron plasma waves (EPW) will also be shown.

An especially interesting prospect is that these hitherto unknown structures in the 2D Euler equation shear flow setting may connect this work to that of Bedrossian and Masmoudi on Couette flow, for instance. How such nonlinear non-stationary structures are created and how they sustain themselves, will be discussed.

Work supported by the AFOSR. Previously, by the DOE NNSA-FES Joint Program on HEDP.

November 19, 2015
Pin Yu, YMSC, Tsinghua University & CMSA, Harvard Univesity
On the global dynamics of three dimensional incompressible magnetohydrodynamics

We construct and study global solutions for the 3-dimensional incompressible MHD systems with arbitrary small viscosity. In particular, we provide a rigorous justification for the following dynamical phenomenon
observed in many contexts: the solution at the beginning behave like non-dispersive waves and the shape of the solution persists for a very long time (proportional to the Reynolds number); thereafter, the solution
will be damped due to the long-time accumulation of the diffusive effects; eventually, the total energy of the system becomes extremely small compared to the viscosity so that the diffusion takes over and the
solution afterwards decays fast in time. We do not assume any symmetry condition. The size of data and the a priori estimates do not depend on viscosity. The proof is built upon a novel use of the basic energy identity and a geometric study of the characteristic hypersurfaces.  The approach is partly inspired by Christodoulou-Klainerman's proof of the nonlinear stability of Minkowski space in general relativity.

This is a joint work with Ling-Bing HE (Tsinghua University) and Li XU (Chinese Academy of Sciences).

November 18, 2015 (special Wednesday seminar at 11am)
Svitlana Mayboroda, Univ. Minnesota

Absolute continuity and rectifiability of harmonic measure

The properties of harmonic measure (most importantly, absolute continuity and rectifiability) are key to many problems in Analysis, Probability, Geometric Measure Theory, as well as PDEs. In this talk we will establish precise connections between the structure of the harmonic measure, geometry of the set, and well-posedness of the underlying boundary problems. The central results to be presented are as follows.

(1) We prove that for any open connected set $\Omega\subset R^{n+1}$, and any $E\subset \partial \Omega$ with $0<H^n(E)<\infty$ absolute continuity of the harmonic measure $\omega$ with respect to the Hausdorff measure on $E$ implies that $\omega|_E$ is rectifiable.

(2) We prove that for any elliptic operator $-div A \nabla$ with $t$-independent bounded measurable coefficients elliptic measure is absolutely continuous with respect to the Lebesgue measure on Lipschitz domains.

November 17, 2015
(special Tuesday seminar at 2pm in WWH 1314)
Chang-Shou Lin, National Taiwan University

The program toward computing the topological degree of Toda system of rank 2

It has been already shown that the generating function of the topological degree for the mean field equation is a rational function. This fact motivates us to study the nonlinear PDE from the aspects of integrable system. The theory developed from this point of view has a rich connection with hyper-elliptic curves, modular forms and Painleve VI equations. Naturally we would like to study the corresponding theory of higher rank, that is,the nonlinear elliptic PDE which are known as Toda system. Without singularity, the Toda system appears in the literature as Plucker formula in algebraic geometry. The aim of this talk is to initiate the program of calculating the topological degree for solutions of Toda systems of rank 2. The program presents many challenging analytical questions, in particular the new bobbling phenomena which could not concentrate the mass. So it leads to revisit an old theory of Brezis and Merle.

November 12, 2015
Ioannis Angelopoulos, Princeton University
Linear and nonlinear waves on spherically symmetric spacetimes I will present several new results on linear and nonlinear waves on spherically symmetric spacetimes. I will focus on the behaviour of linear waves close to the horizon of spherically symmetric extremal black hole spacetimes, and
on the behaviour of linear waves close to null infinity of general asymptotically flat and spherically symmetric spacetimes. I will also present applications of the aforementioned results to nonlinear problems in both settings.

November 6, 2015 (special Friday seminar at 10am in WWH 101)
Edriss Titi,
Texas A&M University and Weizmann Institute of Science
Continuous Data Assimilation for Geophysical Models Employing Coarse Mesh Observables

In this talk we will implement the notion of nite number of determining parameters for the long-time dynamics of the Navier-Stokes equations (NSE), such as determining modes, nodes, volume elements, and other
determining interpolants, to design nite-dimensional feedback control for stabilizing their solutions. The same approach is found to be applicable for data assimilation of weather prediction. In addition, we will show
that the long-time dynamics of the NSE can be imbedded in an in nite-dimensional dynamical system that is induced by an ordinary di erential equations, named determining form, which is governed by a globally
Lipschitz vector eld. The NSE are used as an illustrative example, and all the above mentioned results equally hold to other dissipative evolution PDEs.

November 5, 2015
Vadim Kaloshin, University of Maryland
Stochastic Arnold diffusion of deterministic systems In 1964, V. Arnold constructed an example of a nearly integrable deterministic system exhibiting instabilities. In the 1970s, physicist B. Chirikov coined the term for this phenomenon “Arnold diffusion”, where
diffusion refers to stochastic nature of instability. One of the most famous examples of stochastic instabilities for nearly integrable systems is dynamics of Asteroids in Kirkwood gaps in the Asteroid belt.
They were discovered numerically by astronomer J. Wisdom. During the talk we describe a class of nearly integrable deterministic systems, where we prove stochastic diffusive behaviour. Namely, we show
that distributions given by deterministic evolution of certain random initial conditions weakly converge to a diffusion process. This result is conceptually different from known mathematical results, where
existence of “diffusing orbits” is shown.
This work is based on joint papers with O. Castejon, M. Guardia, J. Zhang, and K. Zhang.

October 29, 2015
Zineb Hassainia, CIMS
On the existence of the V-states for some transport models In this lecture, we shall discuss some recent results on rotating patches (also called V-states) for two dimensional transport models such as Euler equations and the generalized surface quasi-geostrophic equations. We shall first
focus on the simply connected case and prove the existence of such structures in a neighborhood of Rankine vortices by using the bifurcation theory. In the second part we will deal with the doubly connected case where the
patches admit only one hole. More precisely, close to a given annulus we describe this family by countable branches bifurcating from this annulus at some angular velocity related to Bessel functions.
The lecture is based on joint works with de-La Hoz, Hmidi and Mateu.

October 22, 2015

Tristan Buckmaster, CIMS

A New Analytic Approach to Wave Turbulence

In this talk we discuss improvements to a new approach to wave turbulence instigated by Erwan Faou, Pierre Germain and Zaher Hani. This approach will combine techniques from analytic number theory and dispersive PDE theory to study an example of discrete turbulence, for which dynamics is dominated by the exact resonances of the equation. Specifically we will study the large box limit of the Nonlinear Schr\"odinger Equation in the weakly nonlinear regime. This is joint work with Pierre Germain, Zaher Hani and Jalal Shatah.

October 15, 2015
Hoai-Minh Nguyen, EPFL
On the Helmholtz equations with sign changing coefficients
This talk is devoted to various properties and applications of the Helmholtz equations with sign changing coefficients. These equations are used to model negative index materials which are artificial structures whose refractive index are negative over some frequency range.  These materials were first 
investigated theoretically by Veselago in 1964 and their existence was confirmed experimentally by Shelby et al. in 2001. The study of these equations faces two difficulties. First the ellipticity and the compactness are lost in general due to the changing sign coefficients. Second, the localized resonance, i.e., the fields blow up in some regions and remain bounded in some others as the loss (the viscosity) goes to 0, might occur. In this talk, I will present several remarkable properties of these equations and their applications such as cloaking and superlensing using complementary media and cloaking a source and an object via anomalous localized resonance. Various conditions under which the equations are stable are also mentioned.

October 8, 2015
Qing Han, University of Notre Dame
Boundary expansions for the Loewner-Nirenberg problem

Loewner and Nirenberg discussed complete metrics conformal to the Euclidean metric and with a constant scalar curvature in bounded domains in the Euclidean space. The conformal factors blow up on boundary. In this talk, we discuss the optimal boundary expansions for the conformal factors in the context of the finite regularity. We also discuss the convergence of such expansions.

September 17, 2015
Costante Bellettini
Compactness questions for triholomorphic maps

A triholomorphic map u between hyperKahler manifolds solves the "quaternion del-bar" equation du=I du i + J du j + K du k. Such a map turns out, under suitable assumptions, to be stationary harmonic. We focus on compactness issues regarding the quantization of the Dirichlet energy and the structure of the blow-up set. We can relax the assumptions on the manifolds, in particular we can take the domain to be merely "quaternion Kahler": this leads to the weaker notion of "almost-stationarity", without however affecting our compactness results. This is a joint work with G. Tian (Princeton).


May 21, 2015
Jacob Bedrossian, Maryland
Dynamics near the subcritical transition of the 3D Couette flow

We discuss the dynamics of small perturbations of the plane, periodic Couette flow in the 3D incompressible Navier-Stokes equations at high Reynolds number. For sufficiently regular initial data, we determine the stability threshold for small perturbations and characterize the long time dynamics of solutions below this threshold. The primary stability mechanisms are an anisotropic enhanced dissipation effect and an inviscid damping effect of the velocity component normal to the shear, both a result of the mixing caused by the large mean shear. After detailing these linear effects, we will discuss some of the important steps in the proof, such as the analysis of the weakly nonlinear (potential) instabilities connected to the non-normal nature of the linearization. Joint work with Pierre Germain and Nader Masmoudi.

May 14, 2015

Anne-Sophie De Suzzoni, Paris 13

An equation on random variables related to fermionic systems of particles

I will present an equation on random variables which is an extension of the cubic defocusing Schrödinger equation on density operators. I will then compute solutions of this equation whose laws are invariant both on the sphere and the Euclidean space. I will then give results of well-posedness and explain the consequences at the level of density operators.

April 30, 2015

Jameson Graber, University of Texas at Dallas
Variational methods for mean field games with degenerate diffusion
Mean field games have attracted lots of attention recently due to their applications in the social sciences  and systems theory. In this study we develop a theory of existence and uniqueness a solutions to a
coupled system of nonlinear PDEs describing mean field Nash equilibrium. It turns out that the system can be characterized as an optimality condition for the optimal control of the Fokker-Planck equation, where the adjoint state satisfies a Hamilton-Jacobi equation. Our variational methods are of particular interest when the PDEs are of degenerate parabolic type, for which traditional fixed-point methods break down due to lack of regularity.

April 29, 2015 (WWH 1302 at 12p, continuation of seminar from April 28)
Hatem Zaag (CNRS and University Paris 13)
Energy methods and blow-up rate for semilinear wave equations

In a series of papers with Mohamed Ali Hamza (University of Tunis el Manar), we consider the semilinear wave equations with power nonlinearity.

In the subconformal and the conformal case, we consider perturbations with lower order terms and modify the Lyapunov functional Antonini and Merle designed for the unperturbed case. We also find a blow-up criterion for the equation. As a consequence, we bound the Lyapunov functional. Thanks to interpolations in Sobolev spaces and a Gagliardo-Nirenberg inequality, we bound the solution in the self-similar variable, which gives a sharp bound on the blow-up rate.

Surprisingly, our approach works in the superconformal case (still Sobolev subcritical), leading to a new bound on the blow-up rate, which improves the bound of Killip, Stoval and Visan.

April 28, 2015 (special Tuesday seminar)
Hatem Zaag (CNRS and University Paris 13)
Energy methods and blow-up rate for semilinear wave equations

In a series of papers with Mohamed Ali Hamza (University of Tunis el Manar), we consider the semilinear wave equations with power nonlinearity.

In the subconformal and the conformal case, we consider perturbations with lower order terms and modify the Lyapunov functional Antonini and Merle designed for the unperturbed case. We also find a blow-up criterion for the equation. As a consequence, we bound the Lyapunov functional. Thanks to interpolations in Sobolev spaces and a Gagliardo-Nirenberg inequality, we bound the solution in the self-similar variable, which gives a sharp bound on the blow-up rate.

Surprisingly, our approach works in the superconformal case (still Sobolev subcritical), leading to a new bound on the blow-up rate, which improves the bound of Killip, Stoval and Visan.

April 24, 2015 (special seminar at 11am in WWH 605)
David Lannes, University of Bordeaux
Water Waves with vorticity and asymptotic models

Motivated by the study of nonlinear wave-current interactions (such as rip-currents) we study the influence of vorticity on surface water waves. We first derive a generalization of the classical Hamiltonian Zakharov-Craig-Sulem formulation of irrotational water waves that takes into account the effects of the vorticity. The canonical variables for this formally Hamiltonian generalization are the surface elevation, the "gradient component" of the horizontal component of the tangential vector field, and the vorticity. It allows therefore to keep track of the influence of vorticity on the flow. We show the local well-posedness of these equations, and establish their stability with respect to shallow water limits. Based on the bounds thus obtained, we turn to derive shallow water asymptotic models. The big difference with irrotational flows is that the dynamics of the vorticity is fully three dimensional, while shallow water models are typically two-dimensional (through vertical averaging). We show however that the vorticity contribution can be reduced to two-dimensional equations; the idea, based on an analogy with turbulence theory, is that the vorticity contributes to the averaged momentum equation through a Reynolds-like tensor. A cascade of equations is then derived for this tensor, but contrary to standard turbulence theory, closure of the equations is obtained after a finite number of steps.

April 23, 2015 (special afternoon seminar at 2pm)
Cyril Tintarev, Uppsala University and University of California, Irvine (emeritus)
Analysis of concentration in Banach spaces

Concentration compactness principle says that when compactness of an imbedding between two normed vector spaces is lost due to invariance of the norms with respect to action of a non-compact group, it can be partially restored by accounting to "blowups" by the group action. The formal expression of it is the profile decomposition: every bounded sequence has a subsequence that converges (in a stronger than weak topology) up to a sum of the blowup terms. We give a general profile decomposition result for uniformly convex Banach spaces.

Surprisingly, the underlying convergence is not the weak convergence, but Delta-convergence of T.-C.Lim, known to experts in the fixed point theory, and related to the notion of asymptotic centers. In Hilbert spaces Delta convergence and weak convergence coincide. We discuss properties of Delta-convergence (including Delta-compactness) and its relation to weak convergence. We also give an analog of Brezis-Lieb Lemma where the condition of a.e. convergence is replaced by Delta- and weak convergence to the same limit. This is a joint work with Sergio Solimini.

April 23, 2015
Stefan Steinerberger, Yale University
Lower bounds for nodal sets of Laplacian eigenfunctions using heat flow arguments
If u is a Laplacian eigenfunction on a manifold, then there are easy heuristic arguments predicting the size of {x:u(x) = 0}  (in two dimensions, this set can be created using a metal plate, sand and a violin bow and has already amazed Napoleon). These heuristic arguments, though almost certainly accurate, are difficult to make rigorous. We survey existing results and give a new proof for the currently best known lower bound (proved independently by Colding & Minicozzi as well as Sogge & Zelditch). Our argument exploits the fact that the heat equation with the  eigenfunction as initial value can (a) be explicitely solved for trivial reasons and (b) studied using the stochastic interpretation based on Brownian motion. We also describe some other applications of our argument.
April 21, 2015 (special Tuesday seminar)
Vuk Milisic, Universite Paris 13
Mathematical modelling of cell adhesion forces: from delay to friction, an instantaneous limit

In this talk we present the starting mechanical model of the lamellipodial actin-cytoskeleton meshwork. The model is derived starting from the microscopic description of mechanical properties of filaments and cross-links and also of the life-cycle of cross-linker molecules. We introduce a simplified system of equations that accounts for adhesions created by a single point on which we apply a force. We present the adimensionalisation that led to a singular limit that motivated our mathematical study. Then we explain the mathematical setting and results already published. In the last part we present the latest developments: we give results for the fully coupled system with unbounded non-linear off-rates. This leads to two possible regimes : under certain hypotheses on the data there is global existence, out of this range we are able to prove blow-up in finite time.

April 16, 2015
Daniele Cassani, University of Insubria (Italy)
Supercritical systems of Schroedinger equations in dimension two

We present recent results on existence of solitary waves solutions for semilinear elliptic systems in the whole plane. We assume nonlinearities may exhibit supercritical growth with respect to the critical growth prescribed by the Pohozaev-Trudinger-Moser inequality. A suitable variational framework provides linking type solitons which have finite energy in a Sobolev-Lorentz function space setting.

April 9, 2015
Laurent Stolovitch, University of Nice
Quantum complete integrability

We consider families of commuting quantum Hamiltonian which are pertubations of constant Hamiltonians on a torus, which may have a dense pure point spectrum. We show that some of these families are unitary equivalent to a quantum Birkhoff normal form. As a consequence, we show that the Rayleigh-Schrödinger series are convergent in a neighborhood of each eigenvalue of the unperturbed Hamiltonians and that the spectrum of the perturbation is pure point. The results are uniform in the Planck constant. This a joint work with Thierry Paul.

March 26, 2015 (special 10am seminar)
Christof Sparber, U. Illinois - Chicago
On Wigner and Bohmian measures in semiclassical quantum dynamics

We report on a series of recent mathematical results in the description of semiclassical quantum dynamics, jointly obtained (in different parts) with: P. Markowich, C. Klein, A. Figalli, and T. Paul. We consider a class of phase-space measures, which naturally arise from the Bohmian interpretation of quantum mechanics. These, so-called, Bohmian measures describe the time-evolution of the quantum mechanical position and velocity densities and are given by the push-forward under the Bohmian flow on phase space. The latter can be seen as a perturbation of the classical Hamiltonian flow. We study the classical limit of this flow and of the associated Bohmian measure, whenever the dimensionless Planck's constant tends to zero. Connections to the, by now, classical theory of Wigner measures are also discussed.

March 26, 2015
Diego Cordoba, Madrid University
Global existence solutions and geometric properties of the SGQ sharp front

A particular kind of weak solutions for a 2D active scalar are the so called “sharp fronts”, i.e., solutions for which the scalar is a step function. The evolution of such distribution is completely determined by the evolution of the boundary, allowing the problem to be treated as a non-local one dimensional equation for the contour. In this setting we will present several analytical results for the surface quasi-geostrophic equation (SQG): the existence of convex $C^{\infinity}$ global rotating solutions, elliptical shapes are not rotating solutions (as opposed to 2D Euler equations) and the existence of convex solutions that lose their convexity in finite time.

March 12, 2015

Guido de Phillipis, University of Lyon
Faber-Krahn inequalities in sharp quantitative form

In this talk we present a sharp quantitative improvement of the celebrated Faber-Krahn inequality. The latter asserts that balls uniquely minimize the first eigenvalue of the Dirichlet-Laplacian, among sets with given volume. We prove that indeed more can be said: the difference between the first eigenvalue λ(Ω) of a set Ω and that of a ball of the same volume controls the deviation from spherical symmetry of Ω. Moreover, such a control is the sharpest possible. This settles a conjecture by Bhattacharya, Nadirashvili and Weitsman.

March 10, 2015 (special Tuesday seminar)
Takahisa Inui, Kyoto University
Scattering and Blow-up for the focusing nonlinear Klein-Gordon equation with complex valued data below the standing waves

We consider the focusing cubic nonlinear Klein-Gordon equation in 3-d space. It is well known that this initial value problem is locally well posed. So we are concerned with what initial values cause scattering or blow-up. Ibrahim, Masmoudi and Nakanishi gave a sufficient and necessary condition for the real valued solution with energy below the ground state to scatter or blow up. We will extend their result to complex valued solutions below the standing waves by using the charge. If time permits, we will consider an extension of the classification by the momentum.

March 5, 2015

Yu Deng, Princeton University
The Euler-Maxwell system in 2D

We study the Euler-Maxwell electron system in 2D and prove global stability of the constant equilibrium. The proof combines modified energy estimates, spacetime resonance analysis, vector field method, and a crucial linear $L^2$ lemma.

February 26, 2015
Juergen Jost, Max Planck Institute (Leipzig)
Convexity properties of Grassmannians and the Bernstein problem

February 24, 2015
Frederic Rousset, Universite Paris-Orsay
Quasineutral limit in plasma physics

February 19, 2015

Antonella Marini, Yeshiva Univ.
Gauge-invariant boundary conditions with possible applications to a millennium problem

A brief history of elliptic boundary value problems arising in Yang-Mills theory will be given, as well as their application to on-going work in collaboration with Moncrief (Yale) and Maitra (Wenthworth Institute of Technology) on a new method for Euclidean-signature semi-classical Yang-Mills fields, directed at a possible new approach to the mass gap problem.

The review will include the Dirichlet, Neumann, and generalized Neumann boundary conditions for Yang-Mills in dimension 4, as well as the proof of existence of a very rich structure for the space of solutions, including non-minimal solutions, for the Dirichlet problem with small boundary data (joint work with Isobe, Tokyo Institute of Technology).

The difficulties inherent in these natural boundary value problems and their main points of interest will be discussed, as well as the extension to Yang-Mills fields of the modified semi-classical method developed for the analysis of finite dimensional, nonlinear quantum oscillations systems (with Moncrief and Maitra).

February 12, 2015
Gaoyong Zhang, NYU Polytechnic School of Engineering
The logarithmic Minkowski problem

The logarithmic Minkowski problem, like the classical Minkowski problem, is a fundamental 
question in the affine geometry of convex bodies. For symmetric convex bodies, it asks what are the necessary and sufficient conditions for a measure in (n-1)-dimensional projective space to be the cone-volume measure of the unit ball in an n-dimensional normed space. Solving this problem is equivalent to establishing existenceof a solution to a Monge-Ampere equation. This talk outlines a complete solution to the symmetric logarithmic Minkowski problem and will present related open problems.

February 5, 2015
Jon Keating, University of Bristol
Spectral density and eigenstate entanglement in quantum spin chains

I will describe some recent results with Noah Linden and Huw Wells concerning the density of states in quantum spin chains, the entanglement of most of the eigenstates in these systems, and an associated random-matrix model.

February 5, 2015 *special seminar at 1:30p*
Irene Gamba, Univ. Texas - Austin
The Boltzmann equation in all space and the effects of dispersion and dissipation

We analyze the long time behavior of solutions of the Boltzmann equation in the vicinity of global Maxwellian initial data, and show the existence of a scattering regime that leads to the construction of eternal solutions that do not coincide with a global Maxwellian. This long time behavior is a consequence of decay conditions in space, showing that dispersion takes over the collisional dissipative effect by increasing the rarefaction effect.

This is work in collaboration with C. Bardos, F. Golse and C.D. Levermore.

January 29, 2015
Slim Ibrahim, University of Victoria
Rigorous asymptotic derivation of the Magneto-Hydro-Dynamic system from Navier-Stokes-Maxwell

The incompressible Magneto-Hydro-Dynamic (MHD) system is a classical and fundamental model in plasma physics. Although well known, its derivation from Navier-Stokes type equations has been so far formal. In this talk and after reviewing the results about the well-posedness, I show how an asymptotic analysis of such equations can rigorously lead to a such a derivation. The key points is a precise study of the weak stability in the Lorentz.

This is a joint work with D. Arsenio (Paris 7) & N. Masmoudi (Courant)

January 22, 2015

André Lisibach, ETH Zurich
Shock Development in Spherical Symmetry

The general problem of shock formation in three space dimensions was solved by Christodoulou in 2007. In his work also a complete description of the maximal development of the initial data is provided. This description sets up the problem of continuing the solution beyond the point where the solution ceases to be regular. This problem is called the shock development problem. It belongs to the category of free boundary problems but possesses the additional property of having singular initial data due to the behavior of the solution at the blowup surface. In my talk I will present the solution to this problem in the case of spherical symmetry. This is joint work with Demetrios Christodoulou.

January 20, 2015 (special Analysis Seminar)
Manuel Victor Gnann, University of Michigan
Well-posedness and regularity for a class of thin-film free boundary problems

We investigate a free boundary problem for a thin-film equation with quadratic mobility and a zero contact angle condition at the triple point where air, liquid, and solid meet. This problem can be derived by a lubrication approximation from the Navier-Stokes system with a Navier-slip condition at the substrate. By treating the model problem of source-type solutions, we motivate why general solutions to this problem are generically singular. The method for proving well-posedness therefore requires to subtract the leading-order singular expansion at the free boundary in the maximal regularity estimates for the linearized evolution. We also discuss the regularizing effect of the degenerate-parabolic operator to arbitrary orders of the singular expansion. Many of the presented results are joint with Lorenzo Giacomelli, Hans Knüpfer, and Felix Otto.
FALL 2014

December 18, 2014
Mohamed Majdoub, Faculte de Sciences de Tunis

Title: Existence, uniqueness and regularity of solutions for the thin-film equation

Abstract: We show the uniqueness of strong solution for the thin-film equation: $u_t + (u u_{xxx})_x =0,  t>0$ with initial data $u(0)=m\delta, m>0$ 
where $\delta$ is the Dirac mass at the origin.  We also  investigate the boundary regularity of source-type self-similar solutions to the thin-film equation with gravity:
$h_t=-(h^nh_{xxx})_x+(h^{n+3})_{xx},$ $ t>0, h(0)= m \delta$ where $n\in (3/2,3)$. The existence of these solutions has been established by E. Beretta. It is also shown that the leading order expansion near the edge of the support coincides with that of a travelling-wave solution for the standard TFE: $h_t=-(h^n h_{xxx})_x$. We sharpen this result  by proving that the higher order corrections are analytic with respect to three variables: the first one is just the spacial variable, whereas the second and third (except for $n = 2$) are irrational powers of it. We also prove the uniqueness of solutions. This is a joint work with Nader Masmoudi and Slim Tayachi.

December 11, 2014
Cyrill Muratov, NJIT
A non-local variational problem arising from studies of nonlinear charge screening in graphene monolayers

This talk is concerned with energy minimizers in an orbital-free density functional theory that models the response of massless fermions in a graphene monolayer to an out-of-plane external charge. The considered energy functional generalizes the Thomas-Fermi energy for the charge carriers in graphene layers by incorporating a von-Weizsaecker-like term that penalizes gradients of the charge density. Contrary to the conventional theory, however, the presence of the Dirac cone in the energy spectrum implies that this term should involve a fractional Sobolev norm of the square root of the charge density. We formulate a variational setting in which the proposed energy functional admits minimizers in the presence of an out-of-plane point charge. The associated Euler- Lagrange equation for the charge density is also obtained, and uniqueness, regularity and decay of the minimizers are proved under general conditions. In addition, a bifurcation from zero to non-zero response at a finite threshold value of the external charge is proved. This is joint work with J. Lu and V. Moroz.

December 4, 2014
Abbas Bahri, Rutgers Univ.

Revisiting Contact and Conformal Geometry

We will revisit, using our techniques, Contact Form Geometry and we will describe the results that we can derive using these techniques. Time permitting, we will also discuss some open problems in Conformal Geometry. which come naturally to our understanding.

November 13, 2014

Philippe LeFloch, Univ. Paris 6 and CNRS

Weakly regular Einstein spacetimes with symmetry

November 12, 2014 *special seminar taking place at 10am in WWH 805*
Yannick Sire, Aix-Marseille University

Regularity results for general nonlocal equations of quasilinear type

I will consider an equation which can be seen as a fractional p-laplacian, with measure data. I will introduce a method to prove the existence of suitable classes of solutions. More importantly, I will prove some estimates relating the solution to the associated Wolff potentials of the measure.

November 6, 2014 *special seminar taking place at 2pm in WWH 1302*

Benjamin Harrop-Griffiths, Univ. of California - Berkeley
The lifespan of small data solutions to the KP-I

We show that for small, localized initial data there exists a global solution to the KP-I equation in a Galilean-invariant space using the method of testing by wave packets. This is joint work with Mihaela Ifrim and Daniel Tataru.

November 6, 2014
Mircea Petrache, ETH Zurich
The Yang-Mills Plateau problem in supercritical dimensions

The celebrated results of K. K. Uhlenbeck furnished the bases for a complete variational study of the Yang-Mills functional in dimension 4, leading to the definition of new differential invariants of 4-manifolds by S. K. Donaldson. In this setting the objects of study were Sobolev connections over smooth bundles. In the first part of the talk I will explain how this setting works, and show why it is not sufficient for studying the Yang-Mills functional in dimensions higher than 4.

I will then define a space of weak connections introduced in collaboration with Tristan Riviere, and present our weak closure and approximability results in that setting. These results lead to the an existence-and-regularity theory for Yang-Mills minimizers in dimensions greater than 4. This bears a strong analogy to the work of Federer-Fleming on the Plateau problem. The existence of minimizers requires the definition of new weak connection spaces and we have a proof that they correspond to smooth connections over classical bundles, up to a codimension 5 singular set. This gives a first step for connecting the variational theory to the algebraic geometry framework which motivated some conjectures of Gang Tian. The optimality of the codimension 5 is shown by an explicit example of a minimizer, and the proof of its minimality uses a new combinatorial technique together with a decomposition theorem for vector fields by S. Smirnov.

October 30, 2014
Mark Wilkinson, CIMS

October 23, 2014
Michael Jenkinson, Columbia U.
On-Site and Off-Site Solitary Waves of the Discrete Nonlinear Schrödinger Equation in Multiple Dimensions

We construct several families of symmetric localized standing waves (solitons) to the one-, two-, and three-dimensional discrete nonlinear Schrödinger equation (DNLS) with cubic nonlinearity using bifurcation methods about the continuum limit. Such waves and their energy differences play a role in the propagation of localized states of DNLS across the lattice. The energy differences, which we prove to exponentially small in a natural parameter, are related to the "Peierls-Nabarro Barrier" in discrete systems, first investigated by M. Peyrard and M.D. Kruskal (1984). These results may be generalized to different lattice geometries and inter-site coupling parameters. This is joint work with Michael I. Weinstein.

October 16, 2014
Tom Trogdon, CIMS
Gibbs-like behavior of dispersive PDEs

The classical Gibbs phenomenon is an artifact of non-uniform convergence.  More precisely, it arises from the approximation of a discontinuous function with an analytic partial sum of the Fourier series.  It is known from the work of DiFranco and McLaughlin (2005) that a similar phenomenon occurs when a box initial condition is taken for the free Schrödinger equation in the short-time limit.  This talk is focused extending the linear theory of this work in two ways.  First, we establish sufficient conditions for the classical smoothness of the solutions of linear dispersive equations for positive times.  Second, we derive a highly-oscillatory and computable short-time asymptotic expansion of the solution of general linear dispersive PDEs with a large class of discontinuous initial data.  Boundary-value problems can also be treated.

October 9, 2014
Shouhong Wang, U. of Indiana
Field Theory for Four Fundamental Interactions

First, we present two basic principles, the principle of interaction dynamics (PID) and the principle of representation invariance (PRI). Intuitively, PID takes the variation of the action functional under energy-momentum conservation constraint. PID offers a completely different and natural way of introducing Higgs fields. For gravity, we show that PID is the direct consequence of Einstein’s principle of general relativity and the presence of dark matter and dark energy. PRI requires that the SU(N) gauge theory be independent of representations of SU(N). PRI has remarkably rich physical consequences.

Second, we briefly introduce a field theory coupling the four fundamental interactions, based only on a few fundamental principles and symmetries. This theory leads to insights to the understanding of such challenging problems as the dark matter and dark energy and quark confinement. This is joint work with Tian Ma.

October 2, 2014
Yuri Latushkin, U. of Missouri
The Morse and Maslov indices for multidimensional Schroedinger operators with matrix-valued potentials

We study the Schroedinger operator $L=-\Delta+V$ on a star-shaped domain in $\bbR^d$ with Lipschitz boundary. The operator is equipped with quite general Dirichlet- or Robin-type boundary conditions induced by operators between $H^{1/2}(\dOm)$ and $H^{-1/2}(\dOm)$, and the potential takes values in the set of symmetric $N\times N$ matrices. By shrinking the domain and rescaling the operator we obtain a path in the Fredholm--Lagrangian--Grassmannian of the subspace of $H^{1/2}(\dOm)\times H^{-1/2}(\dOm)$ corresponding to the given boundary condition. The path is formed by computing the Dirichlet and Neumann traces of weak solutions to the rescaled eigenvalue equation. We prove a formula relating the number of negative eigenvalues of $L$ (the Morse index), the signed crossings of the path (the Maslov index), the number of negative eigenvalues of the potential matrix evaluated at the center of the domain, and the number of negative squares of a bilinear form related to the boundary operator. This is a joint work with G. Cox, C. Jones, and A. Sukhtayev.

September 25, 2014
Miles Wheeler, CIMS

Solitary water waves on shear flows and with surface pressure

In this talk we will consider steady traveling solitary water waves which have "large amplitude" in the sense that they are not well-described by traveling wave solutions of KdV. After proving lower and upper bounds on the wave speed and some qualitative properties of the surface profile, we will construct exact solutions using a degree-theoretic continuation argument. Here the novelty will be in the inclusion of an arbitrary distribution of vorticity in the mean flow and/or a prescribed non-constant pressure on the surface.

September 18, 2014
Tristan Buckmaster, CIMS
Onsager's Conjecture

In 1949, Lars Onsager in his famous note on statistical hydrodynamics conjectured that weak solutions to the Euler equation belonging to Hölder spaces with Hölder exponent greater than 1/3 conserve energy; conversely, he conjectured the existence of solutions belonging to any Hölder space with exponent less than 1/3 which dissipate energy.  

The first part of this conjecture has since been confirmed (cf. Eyink 1994, Constantin, E and Titi 1994).  During this talk we will discuss recent work by Camillo De Lellis, László Székelyhidi Jr., Phil Isett and myself related to resolving the second component of Onsager's conjecture. In particular, we will discuss the construction of weak non-conservative solutions to the Euler equations whose Hölder $1/3-\epsilon$ norm is Lebesgue integrable in time.


January 16, 2014
Slim Tayachi , University Tunis El Manar
The nonlinear heat equation with high order mixed derivatives of the Dirac delta as initial values and Applications

In this talk we prove local existence of solutions for the nonlinear heat equation $$u_t = \Delta u + |u|^\alpha u, \; t\in(0,T),\; x\in \R^N,\; \alpha>0,$$  with initial values in high order negative Sobolev spaces. In particular, we consider high order mixed derivatives of the Dirac Delta as initial values.

As an application, we prove the existence of initial values $u_0 = \lambda f$ for which the resulting solution blows up in finite time if  $\lambda>0$ is sufficiently small and $\alpha<2/(N+m).$ Here, $f$ satisfies in particular $f\in C_0(\R^N)\cap L^1(\R^N)$ and is anti-symmetric with respect to $x_1,\; x_2,\; \cdots,\; x_m,\; 1\leq
m \leq N,$ where $x:=(x_1,x_2,\cdots,x_N)\in \R^N.$ Moreover we require, $\int_{\R^N} x_1\cdots x_mf(x) dx\not=0$. This extends known ``small lambda" blowup results.

If $f$ is also in $H^1(\R^N)$, then by standard energy arguments, the solution with initial value $u_0 = \lambda f$ blows up in finite time if $\lambda > 0$ is sufficiently large. We prove the existence of a function $f_0$ for which, the solution with initial value  $u_0 = \lambda f_0,\; \alpha<2/(N+m)$ would blow up in finite time  for large and small $\lambda > 0$, but would be global for $\lambda = 1.$

We show also that the condition $\alpha<2/(N+m)$ is sharp. If $\alpha>2/(N+m)$, then all initial data $u_0 = \lambda f$ anti-symmetric in $x_1,x_2,\cdots, x_m,$
$\lambda>0$ is sufficiently small and $f$ satisfying  the same conditions as above produce solutions which are global in time.

 This is a joint work with Fred B. Weissler.

January 30, 2014
Sergiu Klainerman, Princeton University
Are Black Holes Real ?

February 4, 2014
Igor Kukavica   (USC)
On the well-posedness of an interface damped free boundary fluid-structure model

We address a fluid-structure system which consists of the incompressible Navier-Stokes equations and a damped linear wave equation defined on two dynamic domains. The equations are coupled through transmission boundary conditions and additional boundary stabilization effects imposed on the free moving interface separating the two domains.  We will discuss local existence and uniqueness of solutions and establish global existence for small initial data. This is a joint work with M. Ignatova, I. Lasiecka, and A. Tuffaha.

February 6, 2014
Eric Carlen, Rutgers
A quantitative Brunn-Minkowski inequality and estimates on the the remainder in the Riesz rearrangement inequality

We prove a quantitative Brunn-Minkowski inequality for sets E and K, one of which, K,  is assumed convex, but without assumption on the other set. We are
primarily interested in the case in which K is a ball. We use this to prove an estimate on the remainder in the Reisz rearrangement inequality under certain conditions on the three functions involved that are relevant to a problem arising in statistical mechanics:  This is joint work with Franceso Maggi.

February 13, 2014 
Thierry Goudon, Inria, France
Models for “mixtures”, multifluid flows

February 20, 2014 
Thomas H. Otway, Yeshiva University
A weak Dirichlet problem for the cold plasma model

The open Dirichlet problem for elliptic—hyperbolic equations, in which data are prescribed on a proper subset of the boundary, has been studied for decades and arises, for example, in nozzle flow.  But the closed Dirichlet problem for such equations, in which data are prescribed on the entire boundary, is not well known although it also arises naturally, e.g., in transonic flow about a profile and in plasma physics. Recently, Lupo, Morawetz, and Payne showed the existence of weak solutions to a closed Dirichlet problem for a class of equations having strong regularity in comparison to other elliptic—hyperbolic equations. We consider the same problem for an elliptic—hyperbolic equation, introduced by Weitzner in 1984, for which regularity is more problematic.

Andrea Nahmod
, University of Massachusetts, Amherst   *** Please note unusual time and location for this event: 2:30 pm in room WWH 517
Almost sure well-posedness for the periodic 3D quintic NLS below the energy space

In this talk we first review recent progress in the study of certain evolution equations from a non-deterministic point of view (e.g. the random data Cauchy problem) which stems from incorporating to the deterministic toolbox, powerful but still classical tools from probability as well. We will explain some of these ideas and describe in more detail recent work, joint with Gigliola Staffilani on the almost sure well-posedness for  the periodic 3D quintic nonlinear Schrodinger equation in the supercritical regime; that is, below the critical space $H^1(\mathbb T^3)$.

February 27, 2014 
Pierre Germain, CIMS
KdV limit for geometric Schrodinger equations with potential

It is well known that the KdV equation arises as a long wave limit of the Gross-Pitaevskii equation. In this talk, I will present a generalization of this derivation to geometric Schrodinger equations (wave maps). This has applications to various physical situtations : spin models, coupled nonlinear Schrodinger equations.

March 4, 2014 ***** 11 am in room 1302
Erwan Faou, INRIA Rennes and ENS Paris
Landau damping in Sobolev spaces for the Vlasov-HMF model

We consider the Vlasov-HMF (Hamiltonian Mean-Field) model. We consider solutions starting in a small Sobolev neighborhood of a spatially homogeneous state satisfying a linearized stability criterion (Penrose criterion). We prove that these solutions exhibit a scattering behavior to a modified state, which implies a nonlinear Landau damping effect with polynomial rate of damping

March 6, 2014
Tej-eddine Ghoul, CIMS
Stability of infinite time aggregation for Keller-Seguel equation

In this talk we give a sharp description and we proof the stability of the collapse in infinite time for the parabolic elliptic Patlak-Keller-Seguel equation when the mass of the solution is critical equals to 8\pi.

March 20, 2014
Valeria Banica, Universite d'Evry
Various dynamics for several vortex filaments

We consider the Schrödinger system with point vortex-type interactions that was derived by R. Klein, A. Majda and K. Damodaran and by V. Zakharov to modelize the dynamics of N nearly parallel vortex filaments in a 3-dimensional homogeneous incompressible fluid. The known large time existence results are due to C. Kenig, G. Ponce and L. Vega and concern the case of same circulations for two filaments and for a class of configurations of three filaments. We prove large time existence results for particular configurations of four nearly parallel filaments and for a class of configurations of N filaments for any N larger or equal to 2. We also show the existence of travelling wave type dynamics, and we describe configurations leading to collision for N larger or equal to 3. Finally we consider the problem of collisions for perturbations of antiparallel translating pairs of filaments. This are joint works with Erwan Faou and Evelyne Miot.

March 27, 2014
Scott Armstrong

Higher regularity in stochastic homogenization for uniformly elliptic equations

We study uniformly elliptic equations with random coefficients satisfying a finite range of dependence. We show that, with overwhelming probability, solutions possess much higher regularity than can be expected in general for equations with rapidly coefficients. The statements take the form of a priori estimates, except that in place of universal constants are random variables which, while not almost surely bounded, but have quite good integrability in the probability space. These results can be thought of as stochastic, quantitative analogues of the results Avellaneda-Lin developed in the late 80s with compactness arguments. In the stochastic setting, we don't have compactness, but we do have concentration inequalities.

April 1, 2014
Bob Strichartz, Cornell
Spectral asymptotics for "April Fools" wave equations on compact spacetimes that pretend to be elliptic. The Weyl asymptotic law tells you about the spectrum of an elliptic pde on a compact manifold. You would not ordinarily think it would tell you anything about wave equations. Nevertheless, for certain compact spacetimes, such as the product of a circle and a 2-sphere, there is an asymptotic law for the spectrum (separately for the positive and negative part). However, "April Fools!", it gets the dimension wrong. The gist of the argument is elementary number theory (distictions between even and odd numbers). This is joint work with Jonathan Fox, and I will bring along some nice graphics that he has made.
April 10, 2014
Erik Wahlen, University of Lund
Solitary water waves in three dimensions

I will discuss some existence results for solitary waves with surface tension on a three-dimensional layer of water of finite depth. The waves are fully localized in the sense that they converge to the undisturbed state of the water in every horizontal direction. The existence proofs are of variational nature and different methods are used depending on whether the surface tension is weak or strong. In the case of strong surface tension, the existence proof also gives some information about the stability of the waves. The solutions are to leading order described by the Kadomtsev-Petviashvili equation (for strong surface tension) or the Davey-Stewartson equation (for weak surface tension). These model equations play an important role in the theory.

April 11, 2014 ****Please note this talk is at 10am
Mahir Hadzic, Kings College, London

Stability problem in the dust-Einstein system with a positive cosmological constant

The dust-Einstein system models the evolution of a spacetime containing a pressureless fluid, i.e. dust. We will show nonlinear stability of the well-known Friedman-Lemaitre-Robertson-Walker (FLRW) family of solutions to the dust-Einstein system with positive cosmological constant. FLRW solutions represent initially a quiet fluid evolving in a spacetime undergoing accelerated expansion. We work in a harmonic-type coordinate system, inspired by prior works of Rodnianski and Speck on Euler-Einstein system, and Ringstroem's work on the Einstein-scalar-field system. The main new mathematical difficulty is the additional loss of one degree of differentiability of the dust matter. To deal with this degeneracy, we commute the equations with a well-chosen differential operator and derive a family of elliptic estimates to complement the high-order energy estimates. This is joint work with Jared Speck.  
April 17, 2014
Zhouping Xin, Chinese University of Hong Kong
On Multi-Dimensional Compressible Navier-Stokes System with Possible Large Oscillations and Vacuum

In this talk, I will discuss some of recent results on the large time well-posedness of classical solutions to the multi-dimensional compressible Navier-Stokes system with possible large oscillations and vacuum.
The focus will be on finite-time blow-up of classical solutions for the 3-D full compressible Navier-Stokes system, and the global existence of classicla solutions to the isentropic compressible Navier-Stokes system in both 2-D and 3-D in the presence of vacuum and possible large oscillations. Some new estimates on the fast decay of the pressure in the presence of vacuum will be presented,  which are crucial for the well-posedness theory in 2-dimension.

April 24, 2014
Zhen Lei, Fudan
Global well-posedness of incompressible elastodynamics in 2D

In this talk I will report our recent result on the global well posedness of classical solutions to system of incompressible elastodynamics in 2D. The system is revealed to be inherently strong linearly degenerate and automatically satisfies a strong null condition, due to the isotropic nature and the incompressible constraint. 

May 1, 2014
Antoine Gloria

Quantitative central limit theorem for the effective diffusion in iid environments

In this talk I shall present a quantitative central limit theorem for the approximation $A_L$ of the effective diffusion $A_{hom}$ by periodization (on the $L$-torus) in a discrete iid environment in dimension $d\geq 2$. On the one hand, using Stein's method, I shall prove a quantitative estimate on the decay of the Wasserstein distance between $A_L$ minus its expectation rescaled by the square-root of its variance and a normal random variable. On the other hand, I shall prove that the rescaled variance $\sigma_L^2 := L^d \mathrm{var}(A_L)$ has a limit $\sigma^2$ as $L$ goes to infinity, and shall quantify in terms of $L$ the convergence of the expectation of $A_L$ to $A_{hom}$ and the convergence of $\sigma_L^2$ to $\sigma^2$. The proofs of these results are based on a seriesof works in collaboration with Marahrens, Mourrat, Neukamm, and Otto. Combining both types of results then allows us to bound the Wasserstein distance between $\sigma^{-1} L^{d/2} (A_L-A_{hom})$ and a normal random variable by a constant times $L^{-d/2}\log^d L$, which we think is optimal. This is joint work with Jim Nolen (Duke).

May 6, 2014
Reza Pakzad, University of Pittsburgh

May 8, 2014

Marta Lewicka,University of Pittsburgh
On the biharmonic energy with Monge-Ampe`re constraints

May 15, 2014
Miles Wheeler, Brown
Large-amplitude solitary water waves with vorticity 

In this talk, we will construct exact solitary water waves of large amplitude and with an arbitrary distribution of vorticity. Starting from a shear flow with a flat free surface, we use a degree-theoretic continuation argument to construct a global connected set of symmetric solitary waves of elevation, whose profiles decrease monotonically on either side of a central crest. We will also discuss solitary waves generated by a non-constant pressure on the free surface.

May 22, 2014
Philip Rosneau, Tel-Aviv University
Multi-Dimensional Compact Patterns

Solitons, kinks or breathers, are manifestations of weakly nonlinear excitations in, say, anharmonic mass-particle chains. In a strongly anharmonic chains, the tails of the emerging patterns rather than exponentially, decay at a doubly-exponential rate and in the continuum limit collapse into a singular surface with the resulting waves becoming strictly compact, hence their name: compactons. Using the   Z-K, the Sub-linear Complex Klein-Gordon and the Sub-linear NLS equations as examples, we shall show how typical multidimensional compactons emergence and interact. In general, for compact- and hence non-analytical, structures to emerge, the underlying system has to admit a local loss of uniqueness due to, for instance, a degeneracy of the highest order operator or other, singularity inducing, mechanisms. 

June 12, 2014
Messoud Efendiyev Degenerate parabolic equations arising in the modelling of biofilms   

                                                                                                            Fall 2013

September 12
Laurent Thoman, Universite de Nantes
Random weighted Sobolev inequalities

We extend a randomisation method, introduced by Burq-Lebeau on compact manifolds, to the case of the harmonic oscillator.  We construct measures,
under concentration of measure type assumptions, on the support of which we prove optimal weighted Sobolev estimates on R^d. As an application we can prove almost sure global well posedness results for the nonlinear Schrödinger equation with harmonic potential. This is a joint work with Aurélien
Poiret and Didier Robert.

September 19
Zaher Hani, CIMS
Out-of-equilibrium dynamics for the nonlinear Schroedinger equation: From energy cascades to weak turbulence

Out-of-equilibrium dynamics are a characteristic feature of the long-time behavior of nonlinear dispersive equations on bounded domains. This is partly due to the fact that dispersion does not translate into decay in this setting (in contrast to the case of unbounded domains like $R^d$). In this talk, we will take the cubic nonlinear Schroedinger equation as our model and discuss some results and aspects of this out-of-equilibrium dynamics, from energy cascades (i.e. migration of energy from low to high frequencies) to weak turbulence. 

September 26
Vladimir Sverak, University of Minnesota
On invariant measures for 2d Euler and some other models

We will discuss aspects of certain constructions of invariant measures on solutions with finite energy, some of their properties, and connections to the long-time behavior of solutions.

October 3
Patrick Louis Combettes,
Laboratoire Jacques-Louis Lions, Université Paris 6
Splitting two monotone operators goes a long way

A basic problem in applied mathematics is to find a zero of the sum of two monotone operators. The main splitting algorithms for solving this problem were essentially developed in the late 1970s. We show that, by bringing into play duality tools and product formulations, these fundamental splitting principles can be exploited to solve considerably more complex multicomponent composite inclusion problems efficiently. Applications to machine learning, PDEs, and image recovery will be discussed.

October 10
Christophe Lacave, Institut Mathematique de Jussieu
On the motion of a small body immersed in a two dimensional incompressible perfect fluid

In this talk, we consider the motion of a small solid body in a planar ideal fluid, and the limit behavior of the system as the solid body shrinks to a point. We study two regimes:* in the case of a fixed mass, we prove that the solutions converge to a variant of the vortex-wave system where the vortex, placed in the point occupied by the shrunk body, is accelerated by a lift force similar to the Kutta-Joukowski force of the irrotational theory. * in the case of a fixed density, the limit behavior is exactly the vortex-wave system. This work is in collaboration with O. Glass (Paris Dauphine) and F. Sueur (Paris 6-UPMC)

October 17
Jianli Liu
Stability and existence of traveling wave solutions to the timelike extremal surface in Minkowski space

In this talk, firstly we will give the background of global classical solutions to the equation of timelike extremal surface in Minkowski space. Under the appropriate small oscillation assumptions on initial traveling waves, we derive the stability result of the traveling wave solutions.

October 24
Anne-Laure Dalibard
Well-posedness of the Stokes-Coriolis system in the  half-space over a rough surface

This talk is devoted to the well-posedness of the 3d stationary Stokes-Coriolis system set in the half-space over a rough Lipschitz surface. The main issue lies in the fact that we work with solutions of infinite energy: indeed, the Dirichlet data for the Stokes-Coriolis system on the rough boundary does not decrease at infinity. Moreover, we do not assume any kind of spatial structure on the system, i.e. there is no underlying periodicity or stationarity. Following an idea of Gerard-Varet and Masmoudi, the general strategy is to reduce the problem to a bumpy channel bounded in the vertical direction thanks a transparent boundary condition involving  a Dirichlet to Neumann operator. Our analysis emphasizes some strong singularities of the Stokes-Coriolis operator at low horizontal frequencies. One of the main features of our work lies in the definition of a Dirichlet to Neumann operator for the Stokes-Coriolis system with data in the Kato space $H^{1/2}_{uloc}$. This is a joint work with Christophe Prange.
October 31
Hatem Zaag

Blow-up behavior for subconformal semilinear wave equations

November 7
Susan Friedlander
Ill-posedness / Well-posedness Results for a Class of Active Scalar Equations

We discuss a class of active scalar equations where the transport velocities are more singular than the active scalar. There is a significant difference in the well-posedness properties of the problem depending on whether the Fourier multiplier symbol for the velocity is even or odd. The "even" symbol non-diffusive or weakly diffusive equations are ill-posed in Sobolev spaces. However the critically diffusive equations are globally well posed in both the odd and even cases.
Examples of "even" equations are the magnetogeostrophic equation that is a model for the geodynamo and the modified porous media equation.
This is joint work with Francisco Gancedo, Weiran Sun and Vlad Vicol.

November 14
Jacob Bedrossian
A new proof of Landau damping in the nonlinear Vlasov equations

Landau damping is an important mechanism in kinetic descriptions of plasmas, however, mathematical works on the topic have been relatively scarce. We give a simpler proof of Landau damping in the Vlasov equations on the ND torus, originally due to Mouhot and Villani (2011). Moreover, we may take our initial data in Gevrey class smaller than 3, the regularity requirement conjectured in the original work. The proof combines ideas from the original proof with ideas from our recent work on inviscid damping in 2D Euler (joint with N. Masmoudi). The Newton iteration scheme and Lagrangian estimates of the original work are replaced by paraproduct decompositions and the treatment of the plasma echo resonances is simplified by controlled regularity losses in time. Joint work with N. Masmoudi and C. Mouhot.

November 21
Ronen Eldan
A Two-Sided Estimate for the Gaussian Noise Stability Deficit

The Gaussian Noise Stability of a set $A \subset \RR^n$ with parameter $0 <\rho < 1$ is defined as $$ S_\rho(A) = \PP(X,Y \in A) $$ where $X,Y$ are jointly Gaussian random vectors such that X and Y are standard Gaussian vectors and $E X_i Y_j = \delta_{ij} \rho$. Borel's celebrated noise stability inequality states that if $H$ is a half-space whose Gaussian measure is equal to that of $A$, then $S_\rho(H) \geq S_\rho (A)$ for all $0 < \rho < 1$. We will present a novel short proof of this inequality, based on stochastic calculus. Moreover, we prove an almost tight, two-sided, dimension-free robustness estimate for this inequality: by introducing a new metric to measure the distance between the set A and its corresponding half-space H(namely the distance between the two centroids), we show that the deficit between the noise stability of A and H can be controlled from both below and above by essentially the same function of the distance, up to logarithmic factors.As a consequence, we also manage to get the conjectured exponent in the robustness estimate proven by Mossel-Neeman, which uses the total-variationdistance as a metric. Moreover, in the limit $\rho \to 1$, we get an improved dimension free robustness bound for the Gaussian isoperimetricinequality.

Luis Vega
** this talk is at 11:55 directly following the 11:00 talk
The vortex filament equation for a regular polygon

I shall present some recent results obtained in collaboration with V. Banica and F. de la Hoz about the evolution of vortex filaments according to the so-called binormal law. After reviewing the results for filaments with one corner, we will look at the case of general polygons. The dynamics turn out to be quite complex. Among other things numerical evidence on the appearance of multifractals will be given. This multifractal structure turns out to be connected to the one proved by S. Jaffard on Riemann's non-differentiable function.

December 4
Benoit Pausader, Princeton University
Asymptotic behavior for the nonlinear Schrodinger equation with partially periodic data

We consider the NLS equation on quotients of R^d, focusing on the case of RxT^2. The question is to explore the asymptotic behavior of solutions in a more ``compact'' setting. We show how the scattering theory in the quintic case (the equivalent of the mass-critical case) is affected by the ``smaller'' volume and how, in the cubic case theasymptotic behavior is strongly modified by the presence of a secondary dynamics in logarithmic time. In the case of R or RxT (completely integrable case), this secondary dynamics can be explicitely integrated and only causes a phase correction. In the case of RxT^d, d>=2, this dynamics is more complicated and leads to new regimes. In particular, in this case, one can find global solutions which start arbitrarily small in H^s and grow unboundedly with time. This is a joint work with Z. Hani as well as (for the cubic case) N.Tzvetkov and N. Visciglia.

December 12
Remi Schweyer
On the different rates of blow-up for the 1-corotational harmonic heat flow

March 27, 2014
Scott Armstrong

March 20, 2014
Valeria Banica, Universite d'Evry

April 4, 2014
Bob Strichartz
, Cornell
Spectral asymptotics for "April Fools" wave equations on compact >  spacetimes that pretend to be elliptic.
The Weyl asymptotic law tells you about the spectrum of an elliptic pde on a compact manifold. You would not ordinarily think it would tell you anything about wave equations. Nevertheless, for certain compact spacetimes, such as the product of a circle and a 2-sphere, there is an asymptotic law for the spectrum (separately for the positive and negative part). However, "April Fools!", it gets the dimension wrong. The gist of the argument is elementary number theory (distictions > between even and odd numbers). This is joint work with Jonathan Fox

Apr 10, 2014
Erik Wahlen, University of Lund

May 1, 2014
Antoine Gloria

                                                                                                    Spring 2013

January 24
Yin, Z.Y., Sun Yat Sen University

February 7
Phil Isett, Princeton University 
** This seminar is at 10am in room 312

Frederic Rousset,
Construction of multi-solitons solutions for the water waves

February 14
Kenji Nakanishi, Kyoto University
Center-stable manifold of the ground state in the energy space for the critcal wave equation

February 21
No seminar today, Abel Day

February 22
Walter Craig
Vortex filament interactions

February 28
Vlad Vicol, Princeton University
On the inviscid limit for the stochastic Navier-Stokes equations

We discuss recent results on the behavior in the infinite Reynolds number limit of invariant measures for the 2D stochastic Navier-Stokes equations. We prove that the limiting measures are supported on bounded vorticity solutions of the 2D Euler equations. Invariant measures provide a canonical object which can be used to link the fluids equations to the heuristic statistical theories of turbulent flow. Motivated by 2D turbulence considerations we are lead to the problem of well-posedness for the stochastic 2D Euler equations. This is joint work with Nathan Glatt-Holtz and Vladimir Sverak.

March 5 ** This seminar is at 11am in room 1302
Tak Kwong Wong
Local existence and uniqueness of Prandtl equations

The Prandtl equations, which describe the boundary layer behavior of a viscous incompressible fluid near the physical wall, play an important role in the zero-viscosity limit of Navier-Stokes equations. In this talk we will discuss the local-in-time existence and uniqueness for the Prandtl equations in weighted Sobolev spaces under the Oleinik's monotonicity assumption. The proof is based on weighted energy estimates, which come from a new type of nonlinear cancellations between velocity and vorticity.  

March 7
Emmanuel Hebey
Recent advances on Klein-Gordon-Maxwell-Proca systems

Mihai Tohaneanu, Johns Hopkins University ** This seminar is at 12pm
The Strauss conjecture on black holes

The Strauss conjecture for the Minkowski spacetime in three dimensions states that the semilinear equation wave (u) = u^p has a global solution for compactly supported and sufficiently small data if $p> 1+\sqrt 2$. We prove a similar result in the context of Schwarzschild and
Kerr with small angular momentum black holes. This is joint work with H. Lindblad, J. Metcalfe, C. Sogge, and C. Wang.

March 14

Camillo De Lellis, University of Zurich / Princeton University
Regularity theory for area-minimizing currents

It was established by Almgren at the beginning of the eighties that area-minimizing $n$-dimensional currents in Riemannian manifolds are regular up to a singular set of dimension at most $n-2$. To reach this goal Almgren developed an entirely new regularity theory, which occupies a very large monograph, published posthumously.
This talk is based on a series of joint works with Emanuele Spadaro, where we give alternative proofs to all Almgren's main steps, resulting into a much more manageable approach to his entire theory.

March 21
Yannick Sire, University of Marseille
Fractional Ginzburg-Landau equations and boundary harmonic maps

March 28
Thomas Sideris, U.C. Santa Barbara
Almost global existence of small solutions for 2d incompressible Hookean elastodynamics

We will examine the IVP for equations of 2d incompressible isotropic elastodynamics.
We show that classical solutions exist “almost globally" in time, for small initial perturbations of the identity.
We use the energy method with a ghost weight. This is combined with decay estimates, obtained via generalized Sobolev inequalities and weighted L^2 bounds. The argument exploits the inherent null structure of the nonlinearities.
This is joint work with Zhen Lei and Yi Zhou of Fudan University.

April 4
Marius Beceanu, IAS
Strichartz Estimates for Equations with Time-Dependent Potentials

In this talk I present some inequalities valid for Schroedinger's equation and for the wave equation, which hold for a more general class of time-dependent potentials than previously known estimates.

April 18
Yan Yan Li, Rutgers University

A compactness theorem for a fully nonlinear Yamabe problem under a lower Ricci curvature bound

April 23
Fabio Pusateri, Princeton University
Global existence for two-dimensional water waves

April 25
Yoshikazu Giga, University of Tokyo
Analyticity of the Stokes semigroup in space of bounded functions

The Stokes system is a linearized system of the Navier-Stokes equations describing the motion of incompressible viscous fluids. It is believed that the nonstationary problem is very close to the heat equation. (In fact, if one considers the Stokes system in a whole space R^n, the problem is reduced to the heat equation.) The solution operator S(t) of the Stokes system is called the Stokes semigroup. It is well-known that S(t) is analytic in the L^p setting for a large class of domains including bounded and exterior domains with smooth boundaries provided that p is finite and larger than 1. This property is the same as the heat semigroup. Moreover, for the heat semigroup it is analytic even when p equals the infinity.

The corresponding (p=infinity) result for the Stokes semigroup S(t) has been open for more than thirty years even if the domain is bounded. Using a blowup-argument, we have now solved this long-standing problem for a large class of domains, including bounded and exterior domains. A key step is to derive a harmonic pressure gradient estimate by a velocity gradient. We give a sketch of the proof as well as a few possible applications to the Navier-Stokes equations. This is a joint work of my student Ken Abe and the main paper is going to appear in Acta Mathematica.

April 30
Isabelle Gallagher, University Paris 7
The diffusion limit from a hard spheres system, via the linear Boltzmann equation

In this talk we report on a recent result with Laure Saint-Raymond and Thierry Bodineau in which we derive the linear Boltzmann equation from a Newtonian system of hard spheres, following Lanford's proof. This convergence holds for a very long time and enables us to obtain the heat equation in the diffusion limit.

May 3
Edris Titi, UC Irvine

May 9
Philip Rosenau, Tel-Aviv University

On a Well-Tempered Diffusion

The classical transport theory as expressed by, say, the Fokker-Planck equation, lives in an analytical paradise but, in sin. Not only its response to initial datum spreads at once everywhere oblivious of the basic tenets of physics, but it also induces an infinite flux across a sharp interface. Attempting to overcome these difficulties one notices that the moment expansion of any of the micro ensembles of the kind that beget the equations of the classical mathematical physics, say the Chapman-Enskog expansion of Boltzmann Eq., if extended beyond the second moment, yields an ill posed PDE (the Pawla Paradox)!
We shall describe mathematical strategies to overcome these generic difficulties. The resulting flux-limited transport equations are well posed and capture some of the crucial effects of the original ensemble lost in moment expansion. For instance, initial discontinuities do not dissolve at once but persist for a while. There is a critical transition from analytical to discontinuous states with embedded sub-shock(s).

May 16
Neston Guillen, UCLA
Global well-posedness for the homogeneous Landau equation

In joint work with Maria Gualdani we consider the homogeneous Landau equation from plasma physics.  Both global well-posedness and exponential decay to equilibrium are proved assuming only boundedness and spatial decay of the initial distribution. In particular, we can
handle discontinuous initial conditions that might be far from equilibrium. Despite the equation not having a maximum principle the key steps of the proof rely on barrier arguments and parabolic regularity theory.

June 13
Anne-Laure Dalibard, ENS Paris
***Please note this seminar is in room 512
Mathematical study of a degenerate boundary layer

The goal of this talk is to analyze asymptotically an equation stemming from oceanographic models describing the motion of large scale currents. This equation is known to give rise to boundary layers on the east and west coasts of the domain. One of the major issues of our study lies in the fact that the size of these lateral boundary layers becomes very large as one approaches the north and south end points of the domain. In a neighbourhood of these zones, the classical construction of boundary layers must therefore be completely changed. We prove that the north and south boundary layers are the solutions of some evolutionary equation, and that their profile is thus non-intrinsic. We also exhibit discontinuity boundary layers, which penetrate the interior of the domain when the latter has islands, for instance. This is a joint work with Laure Saint-Raymond.


Fall 2012

September 6
Edriss Titi, University of California - Irvine
On the Loss of Regularity for the Three-Dimensional Euler Equations

 A basic example of shear flow was introduced by DiPerna and Majda to study the weak limit of oscillatory solutions of the Euler equations of incompressible ideal fluids. In particular, they proved by means of this example that weak limit of solutions of Euler equations may, in some cases, fail to be a solution of Euler equations. We use this shear flow example to provide non-generic, yet nontrivial, examples concerning the immediate loss of smoothness and ill-posedness of solutions of the three-dimensional Euler equations, for initial data that do not belong to C1; . Moreover, we show by means of this shear  flow example the existence of weak solutions for the three-dimensional Euler equations with vorticity that is having a nontrivial density concentrated on non-smooth surface. This is very diferent from what has been proven for the two-dimensional Kelvin-Helmholtz problem where a minimal regularity implies the real analyticity of the interface. Eventually, we use this shear flow to provide explicit examples of non-regular solutions of the three-dimensional Euler equations that conserve the energy, an issue which is related to the Onsager conjecture. In addition, we will use this shear  flow to provide a nontrivial example for the use of vanishing viscosity limit, of the Navier-Stokes solutions, as a selection principle for uniqueness of weak solutions of the 3D Euler equations.
This is a joint work with Claude Bardos.    

September 20
Philippe LeFloch, University of Paris 6
Finite energy method for compressible fluids

I will discuss the initial value problem for the Euler system of compressible fluid flows governed by a general pressure law, when solutions enjoy a certain symmetry (for instance, plane symmetry) and have solely finite total energy. In a recent series of papers with Pierre Germain (Courant Institute), I have established an existence and compactness theory for this problem and analyzed the vanishing viscosity--capillarity limit in weak solutions with finite energy to the Navier-Stokes-Korteweg system. The proposed approach is referred to as the Finite Energy Method for compressible fluid flows and leads to a generalization of DiPerna's theorem (bounded solutions) and LeFloch-Westdickenberg's theorem (finite energy solutions) to the Euler system of polytropic fluids.

September 27
Andrew Comech, Texas A&M
Linear instability of solitary waves in nonlinear Dirac equation

We study the linear instability of solitary wave solutions  to the nonlinear Dirac equation (known to physicists as the Soler model). That is, we linearize the equation at a  solitary wave and examine the presence of eigenvalues with  positive real part.

We show that the linear instability of the small amplitude  solitary waves is described by the Vakhitov-Kolokolov stability
criterion which was obtained in the context of the nonlinear  Schroedinger equation: small solitary waves are linearly
unstable in dimensions 3, and generically linearly stable in 1D.

A particular question is on the possibility of bifurcations  of eigenvalues from the continuous spectrum; we address it
using the limiting absorption principle and the Hardy-type estimates.

The method is applicable to other systems, such as the  Dirac-Maxwell system.

Some of the results are obtained in collaboration with  Nabile Boussaid, Université de Franche-Comté, and
Stephen Gustafson, University of British Columbia.

October 4
Tej-eddin Gouhl, CIMS

October 11
Arie Israel, CIMS
The Whitney Extension Problem in Sobolev Spaces

October 18
Christophe Lacave  (Paris 7)
Large time behavior for the two-dimensional motion of a disk in a viscous fluid

October 25
Wang Changyou, University of Kentucky
Well-posedness of the nematic liquid crystal flow in  $L3_{uloc}(R3)$ 

In this talk, I will consider a simplified Ericksen-Leslie system modeling the incompressible nematic liquid crystal flow.  For any initial data $(u_0,d_0)$, with $(u_0,\nabla d_0)$ belonging to   $L3_{uloc}(R3)$-- the space of uniformly locally $L3$-integrable functions, we will show the   local well-posedness provided that the $L3_{uloc}(R3)$-norm of $(u_0,\nabla d_0)$ is small.    A characterization of the first singular time for such a solution will also be given. This is a joint  work with Jay Hineman.

November 1
Jeremie Szeftel, ENS, France
Bounded L2 curvature conjecture

November 8
Gilles Francfort, University of Paris 13
Elasto-plasticity: heterogeneity and homogenization

November 15
Sylvia Serfaty, CIMS
Large vorticity local minimizers to the Ginzburg-Landau functional

November 29
Andrew Lawrie, University of Chicago ** This seminar is at 11am
Classification of large energy equivariant wave maps

I will discuss some recent joint work with Raphael Cote, Carlos Kenig, and Wilhelm Schlag. We consider energy critical 1-equivariant wave maps, R1+2 ! S2. This problem admits a unique (up to scaling) harmonic map, Q, given by stereographic projection. We show that every topologically trivial (degree 0) solution with energy less than twice the energy of Q exists globally in time and scatters. Next we establish a classi cation, in the spirit of recent work by Duyckaerts, Kenig, and Merle, of all degree 1 solutions with energy below three times the energy of Q.

Vedran Sohinger, University of Pennsylvania. **This seminar is at 11:55am
The Boltzmann equation, Besov spaces, and optimal decay rates in R^n

In this talk we will study the large-time convergence to the global Maxwellian of perturbative classical solutions to the Boltzmann equation on R^n, for n geq 3, without the angular cut-off assumption. We prove convergence of the k-th order derivatives in the norm L^r_x (L^2_v), for any 2 leq r leq infty, with optimal decay rates, in the sense that they are equal to the rates which one obtains for the corresponding linear equation. The initial data is assumed to lie in a mixed norm space involving the negative homogeneous Besov space of order geq -n/2 in the space variable, without a smallness assumption on the appropriate norm. The space for the initial data is physically relevant since it contains spaces of the type L^p_x (L^2_x), by the use of Besov-Lipschitz space embeddings. Due to the nature of the vector valued spaces, we need to use a vector analogue of the Calderon-Zygmund theory to prove the necessary nonlinear energy estimates. These results hold both in the hard and soft potential case. Furthermore, in the hard potential case, we prove additional optimal decay results if the order of the Besov space belongs to [-(n+2)/2,-n/2). The latter result requires a closer study of the spectrum of the linearized Boltzmann operator for small frequencies dual to the spatial variable. This is a joint work with Robert Strain.

December 4

Frederic Bernicot, University of Nantes  **Please note special day and place of this event 10am room 1314
Transport of BMO-type spaces by a measure-preserving map and applications to 2D Euler equation

We will present some sharp estimates about the transport of BMO-type spaces, via a bi-Lipschitz measure preserving map
in the Euclidean space. More precisely,  we are interested in inequalities of the following type: $$  \| f(\phi) \|_X \lesssim C(\phi) \|f\|_X   $$
where $X$ is a space like BMO, Lipschitz space, Hardy space, Carleson measure spaces .... and $\phi$ is a bi-Lipschitz measure preserving map.
The aim is to prove such inequalities with a sharp constant $C(\phi)$. We want to emphasize how the "measure preserving" property allows us to
get some improved inequalities. Then, we will explain how we can use this argument to describe a new framework for 2D Euler equations. We can define a space strictly containing $L^\infty$ where global well-posedness results can be proved with the vorticity living in this new space.

December 6
Jonathan Luk, Princeton University **Please note this seminar is at 11am
Impulsive Gravitational Waves

We study the problem of the propagation and nonlinear interaction of impulsive gravitational waves for the vacuum Einstein equations. The problem is studied in the context of a characteristic initial value problem with data given on two null hypersurfaces and containing curvature delta singularities. For a single impulsive gravitational wave, we show that in the resulting spacetime, the delta singularity propagates along a 3-dimensional characteristic hypersurface, while away from that hypersurface the spacetime remains smooth. We also construct spacetimes representing interaction of two impulsive gravitational waves in which the curvature delta singularities propagate along two 3-dimensional null hypersurfaces intersecting to the future of the data. To the past of the intersection, the spacetime can be thought of as containing two independent, non-interacting impulsive gravitational waves and the intersection represents the first instance of their nonlinear interaction. Our analysis extends to the region past their first interaction and shows that the spacetime still remains smooth away from the continuing propagating individual waves. This is joint work with Igor Rodnianski.

Remi Schweyer, Toulouse**Please note this seminar is at 11:55am
Stable blow-up dynamics for the parabolic-elliptic Keller-Segel model of chemotaxis

December 13
Clement Mouhot, Cambridge

                                                                                                 Spring 2012                                                                                              

January  19
Tianling Jin (Rutgers)
A fractional Yamabe flow and some applications.

In this talk, we introduce a fractional Yamabe flow involving nonlocal conformally invariant operators on the conformal infinity of asymptotically hyperbolic manifolds, and show that on the conformal spheres $(S^n, [g_{S^n}])$, it converges to the standard sphere up to a Mobius diffeomorphism. This result allows us to obtain extinction profiles of solutions of some fractional porous medium equations. In the end, we use this fractional fast diffusion equation, together with its extinction profile and some estimates of its extinction time, to improve a Sobolev inequality via a quantitative estimate of the remainder term. This is joint work with Jingang Xiong.

January 25  ** Special Wednesday Seminar**
Frederic Bernicot
Differential inclusions describing unilateral constraints

We will present existing and new results about differential inclusions, involving proximal normal cone. This tool aims to encode unilateral constraints in some differential equations of order 1 and 2. Using some convex analysis, we presnet results of global existence for solutions of such equations and we will discuss about the uniqueness of them

January 26
Lazhar Tayeb (University of Tunis)
Approximation by diffusion and homogenization for Fermi-Dirac-Poisson Statistics

We study the diffusion approximation of a  Boltzmann-Poisson system dealing with Fermi-Dirac statistics  in the presence of an extra external  oscillating electrostatic potential. The relative entropy disspation and the two-scale Young  measures are used to prove a two-scale strong convergence leading to a nonlinear Drift-diffusion with a effective potential and coupled to Poisson equation.

February 2
Hideyuki Miura
On fundamental solutions for fractional diffusion equations with divergence free drift

We are concerned with fractional diffusion equations in the presence of a divergence free drift term. By using the Nash approach, we show the existence of fundamental solutions, together with the continuity estimates, under weak regularity assumptions on the drift. Our results give the alternative proof of Caffarelli-Vasseur's theorem on the regularity for the critical 2D dissipative quasi-geostrophic equations. This is a joint work with Yasunori Maekawa.

February 9
Alessio Figalli, UT Austin ** this talk is at 10am**
Di Perna-Lions theory, with application to semiclassical limits for the Schrodinger equation

At the beginning of the '90, DiPerna and Lions studied in detail the connection between transport equations and ordinary differential equations. In particular, by proving an existence and uniqueness result at the level of the transport equation, they obtained (roughly speaking)
existence and uniqueness of solutions for ODEs with Sobolev vector-fields for a.e. initial condition. Ten years later, Ambrosio has been able to extend such a result to BV vector fields. In some recent works we have investigated this theory in a more general setting, which allows us to show the semiclassical convergence of the quantum dynamics to the Liouville dynamics for the linear Schrodinger equations, under very weak regularity assumptions on the potential. In analogy to the classical DiPerna-Lions' theory, the price to pay for allowing singular potential is that the convergence result holds true only for "a.e. initial data", where "a.e." is with respect to a suitable family of reference measures in the space of the initial data. The aim of this talk is to give an overview of these results.

Mahir Hadzic (MIT)
The Classical Stefan problem and the vanishing surface tension limit

We develop a new unified framework for the treatment of well-posedness for the Stefan problem with and without surface tension.  We provide new estimates for the regularity of the moving surface in the absence of surface tension. We conclude by proving that solutions of the Stefan problem with positive surface tension converge to solutions of the
Stefan problem without surface tension. This is joint work with S. Shkoller.

February 16
Paul Feehan (Rutgers)
Degenrate Obstacle Problems

Degenerate elliptic and parabolic obstacle problems arise in mathematical finance when valuing American-style options on an underlying asset modeled by a degenerate diffusion process. We will describe our work on existence, uniqueness, and regularity of solutions to stationary and evolutionary variational inequalities and associated obstacle problems when the underlying asset is modeled by a degenerate diffusion process. This is joint work with Panagiota Daskalopoulos (Department of Mathematics, Columbia University) and Camelia Pop (Department of Mathematics, Rutgers University).

February 23
Vitaly Moroz (Swansea, UK)
Existence and concentration for nonlinear Schroedinger equations with fast decaying potentials

We discuss the existence of positive stationary solutions for a class of nonlinear Schrödinger equations. Amongst other results, we prove the existence of semi-classical solutions which concentrate around a positive local minimum of the potential. The novelty is that no restriction is imposed on the rate of decay of the potential at infinity. In particular, we cover the case where the potential is compactly supported. This is joint work with Jean Van Schaftingen(Louvain-la-Neuve, Belgium)

Erwan Faou

March 1
Frederic Rousset (Rennes)
Uniform regularity and inviscid limit for free surface Navier-Stokes

March 8
Masashi Aiki

March 9 ** this talk is at 1pm in WWH 512
Pengfei Guan (McGill University)
On a uniqueness problem in classical geometry and the maximum principle

March 15
P-E Jabin

March 22
Tuomas Hytonen
How much, or little, is needed for Harmonic Analysis?

One aspect of my recent work has been developing harmonic analysis under minimal assumptions on the space on which the considered functions are defined. Perhaps surprisingly, some classical methods, which at first sight seem to rely heavily on the structure and symmetries of the Euclidean space, can actually be extended to very general settings. On the other hand, some methods developed to tackle with abstract spaces, have shown to be instrumental for getting sharp results for classical inequalities on the Euclidean space.

March 27 ** Special Seminar 3/27 @ 11:30a.m.**
Zhen Lei
On Incompressible Euler and navier-Stokes Equations

In this talk I will report our recent results on finite time finite energy singularities of a 3D incompressible inviscid model of Euler and Navier-Stokes equations and a Liouville theorem for axi-symmetric navier-Stokes equations.

March 29
Victor Lie, Princeton University
Topics in the time-frequency analysis

This talk will be structured as follows: we start by discussing several facts about the history and evolution of the time-frequency area (Fourier Series, Calderon-Zygmund theory, wave-packet theory) and then refer to the general approach/tools for solving a time-frequency problem. Next we present two fundamental topics in this ¯eld: the pointwise con- vergence of the Fourier Series and the boundedness of the Bilinear Hilbert transform. Further we address several aspects of our work including the problem of the boundedness of the Bilinear Hilbert transform on smooth curves and the question regarding the boundedness of the Polynomial Car-leson operator

April 5

Cyrill Muratov
Asymptotic properties of ground states of scalar field equations with vanishing parameter

We study the leading order behavior of positive solutions of the equation
−Δu + εu−|u|p−2u + |u|q−2u = 0, x RN, where N ≥ 3, q > p > 2 and when ε > 0 is a small parameter. We give a complete characterization of all possible asymptotic regimes as a function of p, q and N. The behavior of solutions depends sensitively on whether p is less, equal or bigger than the critical Sobolev exponent p = 2N/(N−2). For p < p the solution asymptotically coincides with the solution of the equation in which the last term is absent. For p > p the solution asymptotically coincides with the solution of the equation with ε = 0. In the most delicate case p = p the asymptotic behavior of the solutions is given by a particular solution of the critical Emden–Fowler equation, whose choice depends on ε in a nontrivial way.

April 10
Eric Lindgren (NTNU)
Fractional eigenvalues
I will discuss a non-local eigenvalue problem that arises as the Euler-Lagrange equation of Rayleigh quotients in the fractional Sobolev spaces. This can be seen as a non-local or fractional version of the eigenvalue problem for the p-Laplacian. In particular, I will talk about the limiting case when p goes to infinity for which the eigenvalues exhibit some strange behaviour that can be seen even in some one-dimensional examples.

April 12

David Gerard-Varet (Paris 7)
Dynamics of a rough body in a viscous fluid

April 19
Marcel Guardia, IAS **please note this seminar is at 10am**
Growth of Sobolev norms for the cubic defocusing nonlinear Schr\"odinger equation in polynomial time

We consider the cubic defocusing nonlinear Schr\"odinger equation in the two dimensional torus. Fix s>1. Colliander, Keel, Staffilani, Tao and Takaoka (2010) proved existence of solutions with s-Sobolev norm growing in time by any given factor R. Refining their methods in several aspects we find solutions with s-Sobolev norm growing in polynomial time in R. This is a joint work with V. Kaloshin.

Dehua Wang (University of Pittsburgh)

April 26
Alberto Bressan (Penn State)
Nash equlibria for a odel of traffic flow

In connection with the Lighthill-Whitham model of traffic flow, a cost functional can be introduced depending on the departure
time and on the arrival time of each driver. Under natural assumptions, there exists a unique globally optimal solution, minimizing the total cost to all drivers. In a realistic situation, however, the actual traffic is better described by the Nash equilibrium solution, where no driver can lower his individual cost by changing his own departure time. A characterization of the Nash solution can be provided, establishing its existence and uniqueness. Extensions to the case of several groups of drivers on a network of roads will also be discussed, together with open problems.

May 1 **Please note special day**
Yifeng Yu (UC Irvine)
G-equations in the modeling of turbulent flame speeds

Predicting turbulent flame speed ($s_T$) is a fundamental
problem in turbulent combustion theory. Several simplified models have been proposed to study $s_T$.  The G-equation (A Hamilton-Jacobi level set equation) is a very popular model in turbulent combustion. Two important projects are (1) establish the theoretical existence of $s_T$ and (2) determine the dependence of turbulent flame speeds on the turbulence intensity (think of the relation between the spreading velocity of wild fire and strength of the wind). In this talk, I will present some theoretical results under the G-equation model. If time permits, I will also compare it with predictions from a model introduced by Majda and
Souganidis.  These are joint works with Jack Xin. May 3
Anne-Sophie de Suzzoni (CERGY)
On statistical description of the flow of dispersive PDEs

May 10
Ben Schweitzer (University of Dortmund)
Homogenization of Maxwell equations in complex geometries: on the counter-intuitive behavior of meta materials

Optically active meta-materials can nowadays be constructed as  physical objects. They can have astonishing properties or lead to  striking effects, the key-words are negative refraction, perfect imaging, and cloaking. I will present the effect of a negative  magnetic permeability of the effective material and perfect light  transmission through small holes in a metallic structure.

Mathematically, we analyze the time-harmonic Maxwell equations in a  heterogeneous medium, the coefficients of the equation can oscillate  on a small spatial scale and the oscillations of the values can be  very large. The heterogeneity of the optical medium is prescribed by  specifying the permittivity, which varies on a small length scale \eta. The electric and magnetic fields are determined by the time-harmonic Maxwell system. We analyze the weak limits of the  electric and magnetic fields as \eta tends to zero, obtaining an "effective equation" that characterizes the limits. The coefficients  of the effective equation describe the behavior of the metamaterial.
This is joint work with G. Bouchitte and with A. Lamacz.

May 17
Benjamin Texier
The onset of instability in quasi-linear systems

May 31
Messoud Efendiev
Finite and infinite dimensional attractors for porous medium equations

June 7
Slim Ibrahim, University of Victoria  ***Please note this talk is at 10:00am
Existence of a ground state and scattering for a nonlinear Schroedinger equation with critical growth

This talk concerns the focusing energy-critical nonlinear Schroedinger equation with a mass-supercritical and energy-subcritical perturbation. In particular, we consider the existence of a ground state and  the scattering problem in the spirit of Kenig-Merle.
This is a joint work with Akahori, Kikushi and Nawa

Adimurth Tifrcam, Bangalore  **Please note this talk is at 11:00 am
Structure Theorem for entropy solutions of Conservation Law

Fall 2011

September 15
Camil Muscalu, Cornell University
Triangular Fourier series and physical reality

September 22
Zaher Hani, CIMS
Long time strong instability and unbounded orbits for some nonlinear Schrodinger equations

Tuesday, September 27, 10:00 a.m. in room 1314
Xavier Cabre, Universitat Politecnica de Catalunya
Minimal surfaces and entire solutions of the Allen-Cahn equation

September 29
Benoit Pausader, CIMS
The energy critical Schrodinger equation on the 3D torus

October 6
George Hagstrom, CIMS
A Krein-Moser Theorem for Vlasov-Poisson

October 13
Federica Sani, CIMS and Univ of Milan
Sharp Adams-type inequalities in R^N

October 20
Jacob Bedrossian, CIMS
Global existence and finite time blow-up for critical Patlak-Keller-Segel models with spatially inhomogeneous diffusion

Special Analysis Seminar Tuesday October 25
2 pm room 1302 WWH Nassif Ghoussoub
University of British Columbia Selfdual polar decomposition

October 27
Vincent Duchene, Columbia University
Asymptotic models for internal waves in the long wave regime, and the dead water phenomenon

November 3
Alex Ionescu, Princeton
On the local extension of Killing vector-fields in Ricci flat manifolds

I will discuss some recent work, joint with S. Klainerman, on the problem of extension of Killing vector-fields in manifolds that satisfy the Einstein vacuum equations. This problem is motivated by the black hole rigidity conjecture, concerning the uniqueness of the Kerr family among regular, stationary black hole solutions of the Einstein vacuum equations.

November 10
Comgming Li, University of Boulder
A degree theory approach for the shooting method

November 17
Christophe Lacave, Universite Paris-Diderot (Paris 7)
Well-posedness for Euler 2D in non-smooth domains

The well-posedness of the Euler system has been of course the matter of many works, but a common point in all the previous studies is that the boundary is at least $C^{1,1}$. In a first part, we will establish the existence of global weak solutions of the 2D incompressible Euler equations for a large class of non-smooth open sets. These open sets are the complements (in a simply connected domain) of a finite number of connected compact sets with positive capacity. Existence of weak solutions with $L^p$ vorticity is deduced from an approximation argument, that relates to the so-called $\gamma$-convergence of domains. In a second part, we will prove the uniqueness if the open set is the interior or the exterior of a simply connected domain, where the boundary has a finite number of corners. Although the velocity blows up near these corners, we will get a similar theorem to the Yudovich's result, in the case of an initial vorticity with definite sign, bounded and compactly supported. The key point for the uniqueness part is to prove by a Liapounov energy that the vorticity never meets the boundary. The existence part is a work in collaboration with David Gerard-Varet.

SPECIAL ANALYSIS SEMINAR, November 22, 11 a.m., room 1314
Shen Zhongwei, University of Kentucky
The Periodic Homogenization of Green's and Neumann Functions

December 1st
Oana Pocovnicu (Imperial College, London)
Effective dynamics of a non-linear wave equation

We consider the non-linear wave equation on the real line iu_t-|D|u=|u|^2u. Its resonant dynamics is given by the Szego equation, which is a completely integrable non-dispersive non-linear equation. We show that the solution of the wave equation can be approximated by that of the resonant dynamics for a long time. The proof uses the renormalization group method introduced by Chen, Goldenfeld, and Oono in the context of theoretical physics. As a consequence, we obtain growth of high Sobolev norms of certain solutions of the non-linear wave equation, since this phenomenon was already exhibited for the Szego equation.

December 8
Aynur Bulut (IAS)
The defocusing Cubic Nonlinear Wave Equation in the Energy Super-critical Regime

In this talk, we will discuss a series of recent works on the global well-posedness and scattering conjecture for the defocusing cubic nonlinear wave equation in the energy super-critical regime, that is dimensions five and higher.  More precisely, using a concentration compactness approach we show that if a solution remains bounded in the critical Sobolev space throughout its maximal interval of existence then it is global and scatters.

December 15
Daniela Tonon (ICERM and SISSA)
Regularity of viscosity solutions for Hamilton-Jacobi equations.

We present two results on the regularity of viscosity solutions of  Hamilton-Jacobi equations obtained in collaboration with Professor Stefano Bianchini. When the Hamiltonian is strictly convex viscosity solutions are semiconcave, hence their gradient is BV. First we prove the SBV regularity of the gradient of a viscosity solution of the Hamilton-Jacobi equation  u_t+ H(t,x,D_x u)=0 in an open set of R^(n+1), under the hypothesis of uniform convexity of the Hamiltonian H in the last variable. Secondly we remove the uniform convexity hypothesis on the Hamiltonian, considering a viscosity solution u of the Hamilton-Jacobi equation  u_t+ H(D_x u)=0  in an open set of R^(n+1) where H is smooth and convex. In this case the viscosity solution is only locally Lipschitz. However when the vector field d(t,x):=H_p(D_xu(t,x)), here H_p is the gradient of H, is BV for all t in [0,T] and suitable hypotheses on the Lagrangian L hold, the divergence of  d(t, ) can have Cantor part only for a countable number of t's in [0,T]. These results extend a result of Bianchini, De Lellis and Robyr for a uniformly convex Hamiltonian  which depends only on the spatial gradient of the solution.