# Lecture Series

## Joint Princeton-Rutgers Seminar on Geometric PDE's

### Spring 2017 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Organizer on Princeton side: Alice Chang }

• April 18, 2017, 1:40pm, Hill 705, Rutgers University

Speaker: Philip Isett, MIT

Title: A Proof of Onsager's Conjecture for the Incompressible Euler Equations

Abstract: In an effort to explain how anomalous dissipation of energy occurs in hydrodynamic turbulence, Onsager conjectured in 1949 that weak solutions to the incompressible Euler equations may fail to exhibit conservation of energy if their spatial regularity is below 1/3-Holder. I will discuss a proof of this conjecture that shows that there are nonzero, (1/3-\epsilon)-Holder Euler flows in 3D that have compact support in time. The construction is based on a method known as "convex integration," which has its origins in the work of Nash on isometric embeddings with low codimension and low regularity. A version of this method was first developed for the incompressible Euler equations by De Lellis and Szekelyhidi to build Holder-continuous Euler flows that fail to conserve energy, and was later improved by Isett and by Buckmaster-De Lellis-Szekelyhidi to obtain further partial results towards Onsager's conjecture. The proof of the full conjecture combines convex integration using the "Mikado flows" introduced by Daneri-Szekelyhidi with a new "gluing approximation" technique. The latter technique exploits a special structure in the linearization of the incompressible Euler equations.

• April 18, 2017, 3:00pm, Hill 705, Rutgers University

Speaker: Matthew J. Gursky, University of Notre Dame

Title: Some existence and non-existence results for Poincare-Einstein metrics

Abstract: I will begin with a brief overview of the existence question for conformally compact Einstein manifolds with prescribed conformal infinity. After stating the seminal result of Graham-Lee, I will discuss a non-existence result (joint with Qing Han) for certain conformal classes on the 7-dimensional sphere. I will also mention some ongoing work (with Gabor Szekelyhidi) on a version of "local existence" of Poincare-Einstein metrics.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Fall 2016 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Organizer on Princeton side: Alice Chang }

• October 21, 2016, 2:00pm, Princeton - Fine Hall 224

Speaker: Yanyan Li, Rutgers University

Title: Blow up analysis of solutions of conformally invariant fully nonlinear elliptic equations

Abstract: We establish blow-up profiles for any blowing-up sequence of solutions of genera l 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 stand ard 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 applic ation of this result, we establish a quantitative Liouville theorem. This is a joint work with Luc Nguyen.

• October 21, 2016, 3:30pm, Princeton - Fine Hall 224

Speaker: Yi Wang, Johns Hopkins University

Title: A fully nonlinear Sobolev trace inequality

Abstract: The $$k$$-Hessian operator $$sigma_k$$ is the $$k$$-th elementary symmetric function of the eigenvalues of the Hessian. It is known that the $$k$$-Hessian equation $$sigma_k(D^2 u)=f$$ with Dirichlet boundary condition $$u=0$$ is variational; indeed, this problem can be studied by means of the $$k$$-Hessian energy $$int -u sigma_k(D^2 u)$$. We construct a natural boundary functional which, when added to the $$k$$-Hessian energy, yields as its critical points solutions of $$k$$-Hessian equations with general non-vanishing boundary data. As a consequence, we prove a sharp Sobolev trace inequality for $$k$$-admissible functions $$u$$ which estimates the $$k$$-Hessian energy in terms of the boundary values of $$u$$. This is joint work with Jeffrey Case.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Spring 2016 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Organizer on Princeton side: Alice Chang }

• April 5, 2016, 4:00pm Hill 705

Speaker: Changfeng Gui, University of Connecticut

Title: Moser-Trudinger type inequalities, mean field equations and Onsager vortices

Abstract: In this talk, I will present a recent work confirming the best constant of a Moser-Trudinger type inequality conjectured by A. Chang and P. Yang. The proof is based on a new and powerful lower bound of total mass for mean field equations. Other applications of the lower bound include the classification of certain Onsager vortices on the sphere, the radially symmetry of solutions to Gaussian curvature equation on the plane, classification of solutions for mean field equations on tori and the sphere, etc. The resolution of several interesting problems in these areas will be presented. The work is jointly done with Amir Moradifam from UC Riverside.

• April 5, 2016, 5:15pm, Hill 705

Speaker: Jacob Bernstein, Johns Hopkins University

Title: Hypersurfaces of low entropy

Abstract: The entropy is a natural geometric quantity introduced by Colding and Minicozzi and may be thought of as a rough measure of the complexity of a hypersurface of Euclidean space. It is closely related to the mean curvature flow. On the one hand, the entropy controls what types of singularities the flow develops. On the other, the flow may be used to study the entropy. In this talk I will survey some recent results with Lu Wang that show that hypersurfaces of low entropy can't be too complicated.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Fall 2015 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Organizer on Princeton side: Alice Chang }

• Oct. 16, 2015, Friday, 3:00pm, Fine Hall 314, Princeton University

Speaker: Andrea Malchiodi, Scuola Normale Superiore

Title: Embedded Willmore tori in three-manifolds with small area constraint

Abstract: While there are lots of contributions on Willmore surfaces in the three-dimensional Euclidean space, the literature on curved manifolds is still relatively limited. One of the main aspects of the Willmore problem is the loss of compactness under conformal transformations. We construct embedded Willmore tori in manifolds with a small area constraint by analyzing how the Willmore energy under the action of the Mobius group is affected by the curvature of the ambient manifold. The loss of compactness is then taken care of using minimization arguments or Morse theory.

• Oct. 16, 2015, Friday, 4:15pm, Fine Hall 314, Princeton University

Speaker: Daniela De Silva, Columbia University

Title: The two membranes problem

Abstract: We will consider the two membranes obstacle problem for two different operators, possibly non-local. In the case when the two operators have different orders, we discuss how to obtain $$C^{1,\gamma}$$ regularity of the solutions. In particular, for two fractional Laplacians of different orders, one obtains optimal regularity and a characterization of the boundary of the coincidence set. This is a joint work with L. Caffarellii and O. Savin.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Spring 2015 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Antonio Ache}

• May 5, 2015, Tuesday, 4:00pm, Hill 705, Rutgers University

Speaker: Gregory Seregin, The University of Oxford

Title: Ancient solutions to Navier-Stokes equations

Abstract: 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. The validity of the Conjecture would rule out Type I blowups that have the same kind of singularity as possible self-similar solutions. I am going to list known cases for which the Conjecture has been proven: the Stokes system, the 2D Navier-Stokes system, axially symmetric solutions in the whole space. Very little is known in the case of the half space. Other type of ancients solutions to the Navier-Stokes equations will be mentioned as well.

• May 5, 2015, Tuesday, 5:15pm, Hill 705, Rutgers University

Speaker: Fernando Marques, Princeton University

Title: Multiparameter sweepouts and the existence of minimal hypersurfaces

Abstract: 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.

• March 27, 2015, Friday, 3:00pm, Room 314, Fine Hall, Princeton University

Speaker: William Minicozzi, MIT

Title: Uniqueness of blowups and Lojasiewicz inequalities

Abstract: 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. We will discuss the proof of this long standing open question for MCF at all generic singularities and for mean convex MCF at all singularities. This is joint work with Toby Colding.

• March 27, 2015, Friday, 4:15pm, Room 314, Fine Hall, Princeton University

Speaker: Luis Silvestre, University of Chicago

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

Abstract: 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.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Fall 2014 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Antonio Ache}

• Dec. 4, 2014, Thursday, 4:30pm, Fine 214, Princeton University

Speaker: Sigurd Angenent, University of Wisconsin, Madison

Title: Mean Curvature Flow of Cones

Abstract: 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.

• Dec. 4, 2014, Thursday, 5:30pm, Fine 214, Princeton University

Speaker: John Lott, University of California, Berkeley,

Title: Geometry of the space of probability measures

Abstract: 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.

• Oct. 8, 2014, Weds., 4:45pm, Hill 705

Title: Ancient solutions to geometric flows

Abstract: 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 singularities. 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.

• Oct. 8, 2014, Weds., 5:45pm, Hill 705

Speaker: Lan-Hsuan Huang, University of Connecticut

Title: Geometry of asymptotically flat graphical hypersurfaces in Euclidean space

Abstract: 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.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Spring 2014 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Antonio Ache}

• April 30, 2014, Weds., 2:00pm, Jadwin A06, Princeton University

Speaker: Fanghua Lin, Courant Institute

Title: Large N asymptotics of Optimal partitions of Dirichlet eigenvalues

Abstract: In this talk, we will discuss the following problem: Given a bounded domain . i n R^n, and a positive energy N, one divides . into N subdomains, .j,j=1,2,...,N. We consider the so-called optimal partitions that give the least possible value for the sum of the first Dirichelet eigenvalues on these sumdomains among all a dmissible partitions of $$\Omega$$.

• April 30, 2014, Weds., 3:15pm, Jadwin A06, Princeton University

Speaker: Bruce Kleiner, Courant Institute

Title: Ricci flow through singularities

Abstract: It has been a long-standing problem in geometric analysis to find a good definition of generalized solutions to the Ricci flow equation that would formalize the heuristic idea of flowing through singularities. I will discuss a notion in the 3-d case that has good analytical properties, enabling one to prove existence and compactness of solutions, as well as a number of structural results. It may also be used to partly address a question of Perelman concerning the convergence of Ricci flow with surgery to a canonical flow through singularities. This is joint work with John Lott.

• March 11, 2014, Tuesday, 4:45pm, Hill 525, Rutgers University

Speaker: Peter Constantin, Princeton University

Title: Long time behavior of forced 2D SQG equations

Abstract: We prove the absence of anomalous dissipation of energy for the forced critical surface quasi-geostrophic equation (SQG) in {\mathbb {R}}^2 and the existence of a compact finite dimensional golbal attractor in {\mathbb T}^2. The absence of anomalous dissipation can be proved for rather rough forces, and employs methods that are suitable for situations when uniform bounds for the dissipation are not available. For the finite dimensionality of the attractor in the space-periodic case, the global regularity of the forced critical SQG equation needs to be revisited, with a new and final proof. We show that the system looses infinite dimensional information, by obtaining strong long time bounds that are independent of initial data. This is joint work with A. Tarfulea and V. Vicol.

• March 11, 2014, Tuesday, 5:45pm, Hill 525, Rutgers University

Speaker: Mihalis Dafermos, Princeton University

Title: The linear stability of the Schwarzschild solution under gravitational perturbations in general relativity

Abstract: I will discuss joint work with G. Holzegel and I. Rodnianski showing the linear stability of the celebrated Schwarzschild black hole solution in general relativity.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Fall 2013 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Luc Nguyen)

• Tuesday, Dec. 3, 4:30pm, Hill 705, Rutgers University

Speaker: Gaoyong Zhang, Polytechnic Institute of New York University

Title: The logarithmic Minkowski problem

Abstract: The logarithmic Minkowski problem asks for necessary and sufficient conditions in order that a nonnegative finite Borel measure in (n-1)-dimensional projective space be the cone-volume measure of the unit ball of an n-dimensional Banach spa ce. The solution to this problem is presented. Its relation to conjectured geometric inequalities that are stronger than the classical Brunn-Minkowski inequality will be explained.

• Tuesday, Dec. 3, 5:30pm, Hill 705, Rutgers University

Speaker: Sergiu Klainerman, Princeton University

Title: On the Reality of Black Holes

• Friday, Oct. 4, 4:15pm, Fine Hall 110, Princeton University

Speaker: Natasa Sesum, Rutgers University

Title: Yamabe flow, its singularity profiles and ancient solutions

Abstract: We will discuss conformally flat complete Yamabe flow and show that in some case s we can give the precise description of singularity profiles close to the extin ction time of the solution. We will also talk about a construction of new compac t ancient solutions to the Yamabe flow. This is a joint work with Daskalopoulos, King and Manuel del Pino

• Friday, Oct. 4, 3:00pm, Fine Hall 110, Princeton University

Speaker: Jeff Viaclovsky, University of Wisconsin-Madison

Title: Critical metrics on connected sums of Einstein four-manifolds

Abstract: I will discuss a gluing procedure designed to obtain canonical metrics on connected sums of Einstein four-manifolds. The main application is an existence result, using two well-known Einstein manifolds as building blocks: the Fubini-Study metric on CP^2, and the product metric on S^2 x S^2. Using these metrics in various gluing configurations, critical metrics are found on connected sums for a specific Riemannian functional, which depends on the global geometry of the factors. This is joint work with Matt Gursky.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Spring 2013 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Luc Nguyen)

• Tuesday, April 23, 5:30pm, Hill 425, Rutgers University

Speaker: Camillo De Lellis, Zurich

Title: Quantitative rigidity estimates

Abstract: For many classical rigidity questions in differential geometry it is natural to ask to which extent they are stable. I will review several recent results in the literature. A typical example is the following: there is a constant $$C$$ such th at, if $$Sigma$$ is a $$2$$-dimensional embedded closed surface in $$R^3$$, then $$min_ lambda |A- lambda g|_{L^2} leq C |A - {rm tr A} g/2|_{L^2}$$, where $$A$$ is the se cond fundamental form of the surface and $$g$$ the Riemannian metric as a submanif old of $$R^3$$.

• Tuesday, April 23, 4:30pm, Hill 425, Rutgers University

Speaker: Xiaochun Rong, Rutgers University

Title: Degenerations of Ricci Flat Kahler Metrics under extremal transitions and flops

Abstract: We will discuss degeneration of Ricci-flat Kahler metrics on Calabi-Yau manifold s under algebraic geometric surgeries: extremal transitions or flops. We will pr ove a version of Candelas and de la Ossa's conjecture: Ricci-flat Calabi-Yau man ifolds related via extremal transitions and flops can be connected by a path con sisting of continuous families of Ricci-flat Calabi-Yau manifolds and a compact metric space in the Gromov-Hausdorff topology. This is joint work with Yuguang Zhang

• Friday, March 1, 4:00pm, Fine Hall 110, Princeton University

Speaker: Jian Song, Rutgers University

Title: Analytic minimal model program with Ricci flow

Abstract: I will introduce the analytic minimal model program proposed by Tian and myself to study formation of singularities of the Kahler-Ricci flow. We also construct geometric and analytic surgeries of codimension one and higher codimensions equ ivalent to birational transformations in algebraic geometry by Ricci flow.

• Friday, March 1, 3:00pm, Fine Hall 110, Princeton University

Speaker: Antonio Ache, Princeton University

Title: On the uniqueness of asymptotic limits of the Ricci flow

Abstract: Given a compact Riemannian manifold we consider a solution of a normalization of the Ricci flow which exists for all time and such that both the full curvature tensor and the diameter of the manifold are uniformly bounded along the flow. It was proved by Natasa Sesum that any such solution of the normalized Ricci flow is sequentially convergent to a shrinking gradient Ricci soliton and moreover the limit is independent of the sequence if one assumes that one of the limiting solitons satisfies a certain integrability condition. We prove that this integrability condition can be removed using an idea of Sun and Wang for studying the stability of the Kaehler-Ricci flow near a Kaehler-Einstein metric. The method relies on the monotonicity of Perelman's W-functional along the Ricci flow and a Lojasiewicz-Simon inequality for the mu-functional. If time permits we will compare this result with recent Theorems on the stability of the Ricci flow.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Fall 2012 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Luc Nguyen)

• Thursday, September 13, 5:00pm, Hill 705, Rutgers University

Speaker: Jie Qing, University of California, Santa Cruz

Title: Hypersurfaces in hyperbolic space and conformal metrics on domains in sphere

Abstract: In this talk I will introduce a global correspondence between properly immersed horospherically convex hyper surfaces in hyperbolic space and complete conforma l metrics on subdomains in the boundary at infinity of hyperbolic space. I will discuss when a horospherically convex hypersurface is proper, when its hyperboli c Gauss map is injective, and when it is embedded. These are expected to be usef ul to the understandings of both elliptic problems of Weingarten hypersurfaces i n hyperbolic space and elliptic problems of complete conformal metrics on subdom ains in sphere.

• Thursday, September 13, 4:00pm, Hill 705, Rutgers University

Speaker: Alessio Figalli, University of Texas, Austin

Title: Regularity Results For Optimal Transport Maps

Abstract: Knowing whether optimal maps are smooth or not is an important step towards a qualitative understanding of them. In the 90's Caffarelli developed a regularity theory on R^n for the quadratic cost, which was then extended by Ma-Trudinger-Wang and Loeper to general cost functions which satisfy a suitable structural condition. Unfortunately, this condition is very restrictive, and when considered on Riemannian manifolds with the cost given by the squared distance, it is satisfied only in very particular cases. Hence the need to develop a partial regularity theory: is it true that optimal maps are always smooth outside a "small" singular set? The aim of this talk is to first review the "classical" regularity theory for optimal maps, and then describe some recent results about their partial regularity.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Spring 2012 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Luc Nguyen)

• Monday, April 30, 4:30-6:30pm, Fine Hall 110, Princeton University

Speaker: Andre Neves, Imperial College

Title: Min-max theory and the Willmore Conjecture

Abstract: In 1965, T. J. Willmore conjectured that the integral of the square of the mean curvature of any torus immersed in Euclidean three-space is at least 2 pi^2. I w ill talk about my recent joint work with Fernando Marques in which we prove this conjecture using the min-max theory of minimal surfaces.

• Tuesday, March 6, 5:00pm, Hill 705, Rutgers University

Speaker: Paul Yang, Princeton University

Title: Compactness of conformally compact Einstein metrics

Abstract:

• Tuesday, March 6, 4:00pm, Hill 705, Rutgers University

Speaker: Ovidiu Savin, Columbia University

Title: The thin one-phase problem

Abstract: We discuss regularity properties of solutions and their free boundaries for minimizers of the thin Bernoulli problem. We show that Lipschitz free boundaries are classical and we obtain a bound on the Hausdorff dimension of the singular set of the free boundary of minimizers. This is a joint work with D. De Silva.

### Joint Princeton-Rutgers Seminar on Geometric PDE's ---- Fall 2011 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song; Coordinator on Princeton side: Luc Nguyen)

• Thursday, December 8, 5:00pm, Fine Hall 110, Princeton University

Speaker: Haim Brezis, Rutgers University

Title: Sobolev maps with values into the circle

Abstract: Real-valued Sobolev functions are well-understood and play an immense role. By c ontrast, the theory of Sobolev maps with values into the unit circle is not yet sufficiently developed. Such maps occur in a number of physical problems. The re ason one is interested in Sobolev maps, rather than smooth maps is to allow maps with point singularities, such as x/|x| in 2-d, or line singularities in 3-d wh ich appear in physical problems. It turns out that these classes of maps have a rich structure. Geometrical and topological effects are already conspicuous, eve n in this very simple framework. On the other hand, the fact that the target spa ce is the circle (as opposed to higher-dimensional manifolds) offers the option to study their lifting and raises some tough questions in Analysis.

• Thursday, December 8, 4:00pm, Fine Hall 110, Princeton University

Speaker: Gang Tian, Princeton University

Title: Bounding scalar curvature along Kahler-Ricci flow

Abstract:

• Thursday, Oct. 27, 5:00pm, Hill 552, Rutgers University

Speaker: Nassif Ghoussoub , University of British Columbia

Title: A self-dual polar decomposition for vector fields

Abstract: I shall explain how any non-degenerate vector field on a bounded domain of $$R^n$$ is monotone modulo a measure preserving involution $$S$$ (i.e., $$S2=Identity$$). This is to be compared to Brenier's polar decomposition which yields that any su ch vector field is the gradient of a convex function (i.e., cyclically monotone) modulo a measure preserving transformation. Connections to mass transport --whi ch is at the heart of Brenier's decomposition-- is elucidated. This is joint wor k with A. Momeni.

• Thursday, Oct. 27, 4:00pm, Hill 552, Rutgers University

Speaker: Aaron Naber, MIT

Title: Quantitative Stratification and regularity for Einstein manifolds, harmonic maps and minimal surfaces

Abstract: In this talk we discuss new techniques for taking ineffective local, e.g. tangent cone, understanding and deriving from this effective estimates on regularity. Our primary applications are to Einstein manifolds, harmonic maps between Riemannian manifolds, and minimal surfaces. For Einstein manifolds the results include, for all p<2, 'apriori' L^p estimates on the curvature |Rm| and the much stronger curvature scale r_{|Rm|}(x)=max{r>0:sup_{B_r(x)}|Rm|leq r^{-2}}. If we assume additionally that the curvature lies in some L^q we are able to prove that r^{-1}_{|Rm|} lies in weak L^2q. For minimizing harmonic maps f we prove W^{1,p}cap W^{2,p/2} estimates for p<3 for f and the stronger likewise defined regularity scale. These are the first gradient estimates for p>2 and the first L^p estimates on the hessian for any p. The estimates are sharp. For minimizing hypersurfaces we prove L^p estimates for p<7 for the second fundamental form and its regularity scale. The proofs include a new quantitative dimension reduction, that in the process stengthens hausdorff estimates on singular sets to minkowski estimates. This is joint work with Jeff Cheeger.

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Joint Princeton-Rutgers Seminar on Geometric PDE's Spring 2017 (Organizers on Rutgers side: Yanyan Li, Zheng-Chao Han, Natasa Sesum, Jian Song;

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