Analysis Seminar

The singular set in the fully nonlinear obstacle problem

Ovidiu Savin
Columbia University
November 18, 2019

For the Obstacle Problem involving a convex fully nonlinear elliptic operator, we show that the singular set of the free boundary stratifies. The top stratum is locally covered by a $C^{1,\alpha}$-manifold, and the lower strata are covered by $C^{1,\log^\eps}$-manifolds. This essentially recovers the regularity result obtained by Figalli-Serra when the operator is the Laplacian.

The Surface Quasigeostrophic equation on the sphere

Angel Martinez Martinez
Member, School of Mathematics
October 28, 2019

In this talk I will describe joint work with D. Alonso-Orán and A. Córdoba where we extend a result, proved independently by Kiselev-Nazarov-Volberg and Caffarelli-Vasseur, for the critical dissipative SQG equation on a two dimensional sphere. The proof relies on De Giorgi technique following Caffarelli-Vasseur intermingled with a nonlinear maximum principle that appeared later in the approach of Constantin-Vicol. The final result can be paraphrased as follows: if the data is sufficiently smooth initially then it is smooth for all times.

Strong ill-posedness of the logarithmically regularized 2D Euler equations in the borderline Sobolev space

Hyunju Kwon
Member, School of Mathematics
October 21, 2019

The well-posedness of the incompressible Euler equations in borderline spaces has attracted much attention in recent years. To understand the behavior of solutions in these spaces, the logarithmically regularized Euler equations were introduced. In borderline Sobolev spaces, local well-posedness was proved Chae-Wu when the regularization is sufficiently strong, while strong ill-posedness of the unregularized case was established by Bourgain-Li. In this talk, I will discuss the strong ill-posedness of the remaining intermediate regime of regularization. 

On the (in)stability of the identity map in optimal transportation

Yash Jhaveri
Member, School of Mathematics
October 14, 2019

In the optimal transport problem, it is well-known that the geometry of the target domain plays a crucial role in the regularity of the optimal transport. In the quadratic cost case, for instance, Caffarelli showed that having a convex target domain is essential in guaranteeing the optimal transport’s continuity. In this talk, we shall explore how, quantitatively, important convexity is in producing continuous optimal transports.

Weak solutions of the Navier-Stokes equations may be smooth for a.e. time

Maria Colombo
École Polytechnique Fédérale de Lausanne; von Neumann Fellow, School of Mathematics
October 7, 2019

In a recent result, Buckmaster and Vicol proved non-uniqueness of weak solutions to the Navier-Stokes equations which have bounded kinetic energy and integrable vorticity. 

 

We discuss the existence of such solutions, which in addition are regular outside a set of times of dimension less than 1. 

The inviscid limit for the Navier-Stokes equations with data analytic only near the boundary

Fei Wang
University of Maryland
May 30, 2019

We address the inviscid limit for the Navier-Stokes equations in a half space, with initial datum that is analytic only close to the boundary of the domain, and has finite Sobolev regularity in the complement. We prove that for such data the solution of the Navier-Stokes equations converges in the vanishing viscosity limit to the solution of the Euler equation, on a constant time interval.

Singularity formation for some incompressible Euler flows

Tarek Elgindi
University of California, San Diego
May 6, 2019

We describe a recent construction of self-similar blow-up solutions of the incompressible Euler equation. A consequence of the construction is that there exist finite-energy $C^{1,a}$ solutions to the Euler equation which develop a singularity in finite time for some range of $a>0$. The approach we follow is to isolate a simple non-linear equation which encodes the leading order dynamics of solutions to the Euler equation in some regimes and then prove that the simple equation has stable self-similar blow-up solutions.