# Seminars & Events for Joint Princeton Rutgers Geometric PDEs Seminar

##### Ancient solutions to geometric flows

We will discuss ancient or eternal solutions to geometric parabolic partial differential equations. These are special solutions that appear as blow up limits near a singularity. They often represent models of ingularities. We will address the classification of ancient solutions to geometric flows such as the Mean Curvature flow, the Ricci flow and the Yamabe flow, as well as methods of constructing new ancient solutions from the gluing of two or more solitons. We will also include future research directions.

##### Geometry of asymptotically flat graphical hypersurfaces in Euclidean space

We consider a special class of asymptotically flat manifolds of nonnegative scalar curvature that can be isometrically embedded in Euclidean space as graphical hypersurfaces. In this setting, the scalar curvature equation becomes a fully nonlinear equation with a divergence structure, and we prove that the graph must be weakly mean convex. The arguments use some intriguing relation between the scalar curvature and mean curvature of the graph and the mean curvature of its level sets. Those observations enable one to give a direct proof of the positive mass theorem in this setting in all dimensions, as well as the stability statement that if the ADM masses of a sequence of such graphs approach zero, then the sequence converges to a flat plane in both Federer-Flemings flat topology and Sormani-Wenger's intrinsic flat topology.

##### Mean Curvature Flow of Cones

For smooth initial hypersurfaces one has short time existence and uniqueness of solutions to Mean Curvature Flow. For general initial data Brakke showed that varifold solutions exist, but that they need not be unique if the initial data are non smooth. In this talk I will discuss the multitude of solutions to MCF that exist if the initial hypersurface is a cone that is smooth except at the origin. Some of the examples go back to older work with Chopp, Ilmanen, and Velazquez, other examples are recent.

##### Geometry of the space of probability measures

The space of probability measures, on a compact Riemannian manifold, carries the Wasserstein metric coming from optimal transport. Otto found a remarkable formal Riemannian metric on this infinite-dimensional space. It is a challenge to make rigorous sense of the ensuing formal calculations, within the framework of metric geometry. I will describe what is known about geodesics, curvature, tangent spaces (cones) and parallel transport.

##### Uniqueness of blowups and Lojasiewicz inequalities

The mean curvature flow (MCF) of any closed hypersurface becomes singular in finite time. Once one knows that singularities occur, one naturally wonders what the singularities are like. For minimal varieties the first answer, by Federer-Fleming in 1959, is that they weakly resemble cones. For MCF, by the combined work of Huisken, Ilmanen, and White, singularities weakly resemble shrinkers. Unfortunately, the simple proofs leave open the possibility that a minimal variety or a MCF looked at under a microscope will resemble one blowup, but under higher magnification, it might (as far as anyone knows) resemble a completely different blowup. Whether this ever happens is perhaps the most fundamental question about singularities.

##### $C^{1,\alpha}$ regularity for the parabolic homogeneous p-Laplacian equation

It is well known that p-harmonic functions are $C^{1,\alpha}$ regular, for some $\alpha>0$. The classical proofs of this fact uses variational methods. In a recent work, Peres and Sheffield construct p-Harmonic functions from the value of a stochastic game. This construction also leads to a parabolic versions of the problem. However, the parabolic equation derived from the stochastic game is not the classical parabolic p-Laplace equation, but a homogeneous version. This equation is not in divergence form and variational methods are inapplicable. We prove that solutions to this equation are also $C^{1,\alpha}$ regular in space. This is joint work with Tianling Jin.

##### Ancient solutions to Navier-Stokes equations

In the talk, I shall try to explain the relationship between the so-called ancient (backwards) solutions to the Navier-Stokes equations in the space or in a half space and the global well-posedness of initial boundary value problems for these equations. Ancient solutions itself are an interesting part of the the theory of PDE's. Among important questions to ask are classification, smoothness, existence of non-trivial solutions, etc. The latter problem is in fact a Liouville type theory for non-stationary Navier-Stokes equations. The essential part of the talk will be addressed the so-called mild bounded ancient solutions. The Conjecture is that {\it any mild bounded ancient solution is a constant}, which should be identically zero in the case of the half space.

##### Multiparameter sweepouts and the existence of minimal hypersurfaces

It follows from the work of Almgren in the 1960s that the space of unoriented closed hypersurfaces, in a compact Riemannian manifold M, endowed with the flat topology, is weakly homotopically equivalent to the infinite dimensional real projective space. Together with Andre Neves, we have used this nontrivial structure, and previous work of Gromov and Guth on the associated multiparameter sweepouts, to prove the existence of infinitely many smooth embedded closed minimal hypersurfaces in manifolds with positive Ricci curvature and dimension at most 7. This is motivated by a conjecture of Yau (1982). We will discuss this result, the higher dimensional case and current work in progress on the problem of the Morse index.