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• The Unconditional Uniqueness for the Energy-critical Nonlinear Schrödinger Equation on T^4

  Speaker: Xuwen Chen (University of Rochester) -
  

  When: Thu, September 24, 2020 - 3:30pm
  Where: Online
  

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  Abstract: We consider the T^4 cubic NLS which is energy-critical. We study the unconditional uniqueness of solution to the NLS via the cubic Gross-Pitaevskii hierarchy, an uncommon method, and does not require the existence of solutions in Strichartz type spaces. We prove U-V multilinear estimates to replace the previously used Sobolev multilinear estimates, which fail on T^4. To incorporate the weaker estimates, we work out new combinatorics from scratch and compute, for the first time, the time integration limits, in the recombined Duhamel-Born expansion. The new combinatorics and the U-V estimates then seamlessly conclude the H^1 unconditional uniqueness for the NLS under the infinite hierarchy framework. This work establishes a unified scheme to prove H^1 uniqueness for the R^3/R^4/T^3/T^4 energy-critical Gross-Pitaevskii hierarchies and thus the corresponding NLS.
  
• Machine learning and partial differential equations (PDEs)

  Speaker: Eric Vanden-Eijnden (Courant Institute (NYU)) -
  

  When: Thu, October 1, 2020 - 3:30pm
  Where: Online
  

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  Abstract: The tools from machine learning (ML) offer exciting new prospects in scientific computing in general, and the numerical solution of PDEs in particular. In this talk, I will discuss some recent results in this context, by focusing on problems with a variational formulation, and discussing three main ingredients of their solution by ML: (i) approximation quality, i.e. how accurate the representation of the solution by a neural network can in principle be; (ii) optimization, i.e. how effective are the methods to train the parameter of the network; and (iii) generalization error, i.e. how much data is necessary to obtain a solution accurate also outside this data set. In particular I will show that this third aspect typically requires using importance sampling methods for data acquisition. These results will be illustrated on eigenvalue problems in high dimension. This is joint work with Grant Rotskoff.
  
  The seminar will be available on zoom at the link https://umd.zoom.us/j/98038768667?pwd=L3lrQ0lZVmQyM1ZQLzA0dkp3OUJXUT09
  
• Linear stability analysis of the homogeneous Couette flow for a planar compressible fluid

  Speaker: Michele Dolce (Imperial College of London) -
  

  When: Thu, October 8, 2020 - 3:30pm
  Where: Online
  

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  Abstract: In this talk, we are interested in studying linear stability properties of perturbations around the Couette flow with constant density for an isentropic compressible fluid. In the inviscid case, we show a generic Lyapunov type instability for the density and the irrotational part of the velocity, namely their L^2 norm grows as t^(1/2). Instead, the solenoidal component of the velocity experience inviscid damping, meaning that it decays to zero even in the absence of viscosity. For a viscous compressible fluid, we show that the perturbations decay exponentially fast on a faster time-scale with respect to the standard diffusive one, a phenomenon also called enhanced dissipation. This is a joint work with P. Antonelli and P. Marcati.  
  
  Zoom link: https://umd.zoom.us/j/98038768667?pwd=L3lrQ0lZVmQyM1ZQLzA0dkp3OUJXUT09
  
• New results on constant Q-curvature metrics

  Speaker: Yannick Sire (Johns Hopkins University) -
  

  When: Thu, October 15, 2020 - 3:30pm
  Where: Online
  

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  Abstract: In the recent years, several major results have been obtained in the problem of finding a constant Q-curvature metric in a given conformal class in dimensions bigger than 5. This talk will cover new results concerning existence and multiplicity of such metrics. I will first present a rather general geometric approach to prove existence and multiplicity of regular metrics, giving several explicit examples. Then I will move to the case of singular metrics, i.e. complete metrics with constant Q-curvature outside of a closed set. This requires to develop several tools to handle 4th order equations (but applicable actually to higher order ones). I will also provide some explicit examples of such metrics and investigate their multiplicity. I will state open problems as well.
  
  Zoom link: https://umd.zoom.us/j/98038768667?pwd=L3lrQ0lZVmQyM1ZQLzA0dkp3OUJXUT09
  
• Global in x Stability of Prandtl's Boundary Layer for 2D, Stationary Navier-Stokes Flows

  Speaker: Sameer Iyer (Princeton) -
  

  When: Thu, October 29, 2020 - 3:30pm
  Where: Online
  

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  Abstract: In this talk I will discuss a recent work which proves stability of Prandtl's boundary layer in the vanishing viscosity limit. The result is an asymptotic stability result of the background profile in two senses: asymptotic as the viscosity tends to zero and asymptotic as x (which acts a time variable) goes to infinity. In particular, this confirms the lack of the "boundary layer separation" in certain regimes which have been predicted to be stable. This is joint work w. Nader Masmoudi (Courant Institute, NYU).
  
  https://umd.zoom.us/j/98038768667?pwd=L3lrQ0lZVmQyM1ZQLzA0dkp3OUJXUT09
  
• Isolated singularities of minimal hypersurfaces

  Speaker: Luca Spolaor (University of California, San Diego) -
  

  When: Thu, November 12, 2020 - 3:30pm
  Where: Online
  

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  Abstract: In this talk I will discuss some old and new results about a class of isolated singularities of minimal surfaces arising from minimization or Min-Max.
  
  Zoom link: https://umd.zoom.us/j/98038768667?pwd=L3lrQ0lZVmQyM1ZQLzA0dkp3OUJXUT09
  
• On the stability of a point charge for the Vlasov-Poisson system

  Speaker: Klaus Widmayer (Ecole Polytechnique Federale de Lausanne) -
  

  When: Thu, November 19, 2020 - 3:30pm
  Where: Online
  

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  Abstract: A point charge is a particularly basic and important equilibrium of the Vlasov-Poisson equations, and the study of its stability has inspired several major contributions. In this talk we present some recent work, which brings a fresh perspective on this problem. Our new approach combines a Lagrangian analysis of the linearized problem with an Eulerian PDE framework in the nonlinear analysis, all the while respecting the symplectic structure. As a result, for the case of radial initial data, we see that solutions are global and in fact disperse to infinity via a modified scattering along trajectories of the linearized flow.
  This is joint work with Benoit Pausader (Brown University).
  
• Sharp well-posedness for the cubic NLS and mKdV on the line

  Speaker: Benjamin Harrop-Griffiths (University of California, Los Angeles) -
  

  When: Thu, December 3, 2020 - 3:30pm
  Where: Online
  

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  Abstract: The 1d cubic nonlinear Schrödinger equation (NLS) and the modified Korteweg-de Vries equation (mKdV) are two of the most intensively studied nonlinear dispersive equations. Not only are they important physical models, arising, for example, from the study of fluid dynamics and nonlinear optics, but they also have a rich mathematical structure: they are both members of the ZS-AKNS hierarchy of integrable equations. In this talk, we discuss an optimal well-posedness result for the cubic NLS and mKdV on the line. An essential ingredient in our arguments is the demonstration of a local smoothing effect for both equations, which in turn rests on the discovery of a one-parameter family of microscopic conservation laws. This is joint work with Rowan Killip and Monica Vișan.