#### Seminar

This is a Weekly Online Seminar in the areas of geometric and functional inequalities and closely related areas of partial differential equations, geometric analysis, etc. It is usually held between 9:00-11:00am US Eastern Time on Mondays. The researchseminars.org page may be found here. This seminar has been regularly running since July 2020.

A unique Zoom link for each talk is sent out in a mailing list each week. Please subscribe below to join the mailing list or email geometricinequalitiesandpdes@gmail.com. If you wish to attend a talk without subscribing to the mailing list, you may email geometricinequalitiesandpdes@gmail.com to request the Zoom link for a given talk.

If you have any other questions or comments, please email one of the organizers.

**Organizers**

Joshua Flynn

CRM-ISM, McGill University

(joshua.flynn@mcgill.ca)

Nguyen Lam

Memorial University of Newfoundland

(nlam@grenfell.mun.ca)

Jungang Li

University of Science and Technology of China

(jungangli@ustc.edu.cn)

Guozhen Lu

University of Connecticut

(guozhen.lu@uconn.edu)

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#### Next Talk

*Alessio Falocchi**(*Politecnico di Milano*)*

Nov 21, 2022

10:00 ET

**Some results on the 3D Stokes eigenvalue problem under Navier boundary conditions.**

*We study the Stokes eigenvalue problem under Navier boundary conditions in $C^{1,1}$-domains $\Omega\subset \mathbb{R}^3$. Differently from the Dirichlet boundary conditions, zero may be the least eigenvalue. We fully characterize the domains where this happens, showing the related validity/failure of a suitable Poincar\'{e}-type inequality.
As application we prove regularity results for the solution of the evolution Navier-Stokes equations under Navier boundary conditions in a class of merely {\em Lipschitz domains} of physical interest, that we call {\em sectors}.
This is a joint work with Filippo Gazzola, Politecnico di Milano.*

#### Scheduled Talks

*Deping Ye**(*Memorial University of Newfoundland*)*

Jan 23, 2023

10:00 ET

**TBA.**

*TBA.*

*Leonard Gross**(Cornell University)*

Jan 30, 2023

10:00 ET

**TBA.**

*TBA.*

*Nguyen Lam**(*Memorial University of Newfoundland*)*

Feb 20, 2023

10:00 ET

**TBA.**

*TBA.*

*Benjamin Schlein**(*University of Zurich*)*

Mar 06, 2023

10:00 ET

**TBA.**

*TBA.*

Pierre-Damien Thizy

*(*University of Lyon 1 (Claude Bernard)

*)*

Apr 10, 2023

10:00 ET

**TBA.**

*TBA.*

#### Previous Talks

*Philipp Reiter**(*University of Technology at Chemnitz*)*

Nov 14, 2022

10:00 EDT

**Elasticity models with self-contact.**

*Maintaining the topology of objects undergoing deformations is a crucial
aspect of elasticity models. In this talk we consider two different
settings in which impermeability is implemented via regularization by a
suitable nonlocal functional.
The behavior of long slender objects may be characterized by the classic
Kirchhoff model of elastic rods. Phenomena like supercoiling which play an
essential role in molecular biology can only be observed if
self-penetrations are precluded. This can be achieved by adding a
self-repulsive functional such as the tangent-point energy. We discuss the
discretization of this approach and present some numerical simulations.
In case of elastic solids whose shape is described by the image of a
reference domain under a deformation map, self-interpenetrations can be
ruled out by claiming global invertibility. Given a suitable stored energy
density, the latter is ensured by the Ciarlet–Nečas condition which,
however, is difficult to handle numerically in an efficient way. This
motivates approximating the latter by adding a self-repulsive functional
which formally corresponds to a suitable Sobolev–Slobodeckiĭ seminorm of
the inverse deformation.
This is joint work with Sören Bartels (Freiburg) and Stefan Krömer
(Prague).*

*Xiaolong Han**(*California State University*)*

Nov 07, 2022

10:00 EDT

**Fractal uncertainty principle for discrete Fourier transform and random Cantor sets.**

*The Fourier uncertainty principle describes a fundamental phenomenon that a function and its Fourier transform cannot simultaneously localize. Dyatlov and his collaborators (Zahl, Bourgain, Jin, Nonnenmacher) recently introduced a concept of Fractal Uncertainty Principle (FUP). It is a mathematical formulation concerning the limit of localization of a function and its Fourier transform on sets with certain fractal structure.
The FUP has quickly become an emerging topic in Fourier analysis and also has important applications to other fields such as wave decay in obstacle scattering. In this talk, we consider the discrete Fourier transform and the fractal sets are given by discrete Cantor sets. We present the FUP in this discrete setting with a much more favorable estimate than the one known before, when the Cantor sets are constructed by a random procedure. This is a joint work with Suresh Eswarathasan.*

*Alexandru Kristaly**(*Babes-Bolyai University*)*

Oct 31, 2022

10:00 EDT

**Lord Rayleigh’s conjecture for clamped plates in curved spaces.**

*This talk is focused on the vibrating clamped plate problem, initially formulated by Lord Rayleigh in 1877, and solved by M. Ashbaugh & R. Benguria (Duke Math. J., 1995) and N. Nadirashvili (ARMA, 1995) in 2 and 3 dimensional euclidean spaces. We consider the same problem on both negatively and positively curved spaces, and provide various answers depending on the curvature, dimension and the width/size of the clamped plate.*

*Joshua Flynn**(*CRM-ISM, McGill University*)*

Oct 24, 2022

10:00 EDT

**Sharp Uncertainty Principles for Physical Vector Fields and Second Order Derivatives.**

*The Heisenberg uncertainty principle is a fundamental result in quantum mechanics. Related inequalities are the hydrogen and Hardy uncertainty principles and all three belong to the family of geometric inequalities known as the Caffarelli-Kohn-Nirenberg inequalities. In this talk, we present our recent results pertaining to uncertainty principles and CKN inequalities with a particular focus on higher order derivatives and vector-valued cases. Presented works were done jointly with G. Lu, N. Lam and C. Cazacu.*

*Jianxiong Wang**(University of Connecticut)*

Oct 17, 2022

10:00 EDT

**Symmetry of solutions to higher and fractional order semilinear equations on hyperbolic spaces.**

*We show that nontrivial solutions to higher and fractional order equations with certain nonlinearity are radially symmetric and nonincreasing on geodesic balls in the hyperbolic space as well as on the entire space . Applying Helgason-Fourier analysis techniques on , we developed a moving plane approach for integral equations on . We also established the symmetry to solutions of certain equations with singular terms on Euclidean spaces. Moreover, we obtained symmetry to solutions of some semilinear equations involving fractional order derivatives.*

*Tobias König**(Goethe-Universität Frankfurt)*

Oct 10, 2022

10:00 EDT

**Multibubble blow-up analysis for the Brezis-Nirenberg problem in three dimensions.**

*In this talk, I will present a recent result about blow-up asymptotics in the three-dimensional Brezis-Nirenberg problem. More precisely, for a smooth bounded domain and smooth functions and , consider a sequence of positive solutions to on with zero Dirichlet boundary conditions, which blows up as . We derive the sharp blow-up rate and characterize the location of concentration points in the general case of multiple blow-up, thereby obtaining a complete picture of blow-up phenomena in the framework of the Brezis-Peletier conjecture in dimension . I will also indicate a forthcoming new result parallel to this one for dimension .
This is joint work with Paul Laurain (IMJ-PRG Paris and ENS Paris).*

*Mimi Dai**(University of Illinois at Chicago)*

Oct 03, 2022

10:00 EDT

**Navier-Stokes equation: determining wavenumber, Kolmogorov’s dissipation number, and Kraichnan’s number.**

*We show the existence of determining wavenumber for the Naiver-Stokes equation in both 3D and 2D. Estimates on the determining wavenumber are established in term of the phenomenological Kolmogorov’s dissipation number (3D) and
Kraichnan’s number (2D). The results rigorously justify the criticality of Kolmogorov’s dissipation number and Kraichnan’s number.*

*Bianca Stroffolini**(Universit`a degli Studi di NAPOLI ”Federico II”)*

Sep 19, 2022

10:00 EDT

**Taylor formula and regularity properties for degenerate Kolmogorov equations with Dini continuous coefficients.**

*We study the regularity properties of the second order linear operator in :
where , are real valued matrices with constant coefficients, with symmetric and strictly positive. We prove that, if the operator satisfies Hörmander’s hypoellipticity condition, and is a Dini continuous function, then the second order derivatives of the solution to the equation are Dini continuous functions as well. We also consider the case of Dini continuous coefficients ‘s. A key step in our proof is a Taylor formula for classical solutions to that we establish under minimal regularity assumptions on .*

*Galia Dafni**(Concordia University)*

May 23, 2022

09:00 EDT

**Locally uniform domains and extension of nonhomogeneous BMO spaces.**

*n joint work with Almaz Butaev (Cincinnati), we study local versions of uniform domains, which can be identified with the epsilon-delta domains used by Jones to extend Sobolev spaces. We show that a domain is locally uniform if and only if it is an extension domain for the nonhomogeneous (also known as “local”) space of functions of bounded mean oscillation introduced by Goldberg, and denoted by bmo. We also prove analogous results for functions of vanishing mean oscillation.*

*Fritz Gesztesy**(Baylor University)*

May 16, 2022

10:00 EDT

**Continuity properties of the spectral shift function for massless Dirac operators and an application to the Witten index
.**

*We report on recent results regarding the limiting absorption principle for multi-dimensional, massless Dirac-type operators (implying absence of singularly continuous spectrum) and continuity properties of the associated spectral shift function.
We will motivate our interest in this circle of ideas by briefly describing the connection to the notion of the Witten index for a certain class of non-Fredholm operators.
This is based on various joint work with A. Carey, J. Kaad, G. Levitina, R. Nichols, D. Potapov, F. Sukochev, and D. Zanin.*

*Andrea Mondino**(University of Oxford)*

May 09, 2022

09:00 EDT

**Optimal transport and quantitative geometric inequalities.**

*The goal of the talk is to discuss a quantitative version of the Levy-Gromov isoperimetric inequality (joint with Cavalletti and Maggi) as well as a quantitative form of Obata’s rigidity theorem (joint with Cavalletti and Semola). Given a closed Riemannian manifold with strictly positive Ricci tensor, one estimates the measure of the symmetric difference of a set with a metric ball with the deficit in the Levy-Gromov inequality. The results are obtained via a quantitative analysis based on the localisation method via L1-optimal transport. For simplicity of presentation, the talk will present the results in case of smooth Riemannian manifolds with Ricci Curvature bounded below; moreover it will not require previous knowledge of optimal transport theory.*

**Pei-Yong Wang***(Wayne State University)*

May 02, 2022

10:00 EDT

**A Bifurcation Phenomenon Of The Perturbed Two-Phase Transition Problem.**

*This talk presents a joint work with F. Charro, A. Haj Ali, M. Raihen, and M. Torres on a bifurcation phenomenon in a two-phase, singularly perturbed, free boundary problem of phase transition. We show that the uniqueness of the solution for the two-phase problem breaks down as the boundary data decreases through a threshold value. For boundary values below the threshold, there are at least three solutions, namely, the harmonic solution which is treated as a trivial solution in the absence of a free boundary, a nontrivial minimizer of the functional under consideration, and a third solution of the mountain-pass type. We classify these solutions according to the stability through evolution. The evolution with initial data near a stable solution, such as the trivial harmonic solution or a minimizer of the functional, converges to the stable solution. On the other hand, the evolution deviates away from a non-minimal solution of the free boundary problem.*

**Alina Stancu***(Concordia University)*

Apr 25, 2022

09:00 EDT

**On the fundamental gap of convex sets in hyperbolic space.**

*The difference between the first two eigenvalues of the Dirichlet Laplacian on convex sets of R^n and, respectively S^n, satisfies the same strictly positive lower bound depending on the diameter of the domain. In work with collaborators, we have found that the gap of the hyperbolic space on convex sets behaves strikingly different even if a stronger notion of convexity is employed. This is very interesting as many other features of first two eigenvalues behave in the same way on all three spaces of constant sectional curvature. *

**Jiaping Wang***(University of Minnesota)*

Apr 18, 2022

10:00 EDT

**Spectrum of complete manifolds.**

*Spectrum of Laplacian is an important set of geometric invariants. The talk, largely based on joint work with Peter Li and Ovidiu Munteanu, concerns its structure and size on complete manifolds under various curvature conditions. The focus is on sharp estimates of the bottom spectrum in terms of either Ricci or scalar curvature lower bound.*

**Eric Carlen***(Rutgers University)*

Apr 11, 2022

09:00 EDT

**Some trace inequalities related to quantum entropy.**

*Many inequalities for trace functional are formulated as concavity/convexity theorems. These generally have an equivalent monotonicity version asserting monotonicity of the functional under some class of completely positive maps. The monotonicty formulation has advantages: (1) Often this has a direct physical interpretation. (2) Often a direct proof of the monotonicity version is simpler than a direct proof of the concavity/convexity version, and the later is always recovered using a simple partial trace argument. (3) Often the monotonicty theorem holds for a broader class of maps, not, necessarily completely positive, and is thus a strictly stronger result. We discus significant examples, some coming from recent joint work with Alexander Mueller-Hermes.*

**Michael Loss***(Georgia Institute of Technology)*

Apr 04, 2022

09:00 EDT

**Which magnetic fields support a zero mode?**

*I present some results concerning the size of magnetic fields that support zero modes for the three dimensional Dirac equation and related problems for spinor equations. The critical quantity, is the norm of the magnetic field . The point is that the spinor structure enters the analysis in a crucial way. This is joint work with Rupert Frank at LMU Munich.*

**Carolyn Gordon***(Dartmouth College)*

Mar 28, 2022

09:00 EDT

**Inverse spectral problems on compact Riemannian orbifolds.**

*Orbifolds are a generalization of manifolds in which various types of singularities may occur. After reviewing the notion of Riemannian orbifolds and their Hodge Laplacians, we will address the question: Does the spectrum of the Hodge Laplacian on p-forms detect the presence of singularities? This question remains open in the case of the Laplace-Beltrami operator (i.e., the case p=0), although many partial results are known. We will show that the spectra of the Hodge Laplacians on functions and 1-forms together suffice to distinguish manifolds from orbifolds with sufficiently large singular set. In particular, these spectra always distinguish low-dimensional orbifolds (dimension at most 3) with singularities from smooth manifolds. We also obtain weaker affirmative results for the spectrum on 1-forms alone and show via counterexamples that some of these results are sharp.
(This is based on recent joint work with Katie Gittins, Magda Khalile, Ingrid Membrillo Solis, Mary Sandoval, and Elizabeth Stanhope and work in progress with the same co-authors along with Juan Pablo Rossetti.)
Time permitting, we will also make a few remarks concerning the Steklov spectrum on Riemannian orbifolds with boundary. The Steklov spectrum is the spectrum of the Dirichlet-to-Neumann operator, which maps Dirichlet boundary values of harmonic functions to their Neumann boundary values.*

**Jérôme Vétois***(McGill University)*

Mar 07, 2022

09:00 EST

**Stability and instability results for sign-changing solutions to second-order critical elliptic equations.**

*In this talk, we will consider a question of stability (i.e. compactness of solutions to perturbed equations) for sign-changing solutions to second-order critical elliptic equations on a closed Riemannian manifold. I will present a stability result obtained in the case of dimensions greater than or equal to 7. I will then discuss the optimality of this result by constructing counterexamples in every dimension. This is a joint work with Bruno Premoselli (Université Libre de Bruxelles, Belgium).*

**Enno Lenzmann***(University of Basel)*

Feb 28, 2022

09:00 EST

**Symmetry and symmetry-breaking for solutions of PDEs via Fourier methods.**

*In this talk, I will review recent results on symmetry and symmetry-breaking for optimizing solutions of a general class of nonlinear elliptic PDEs. On one hand, I will discuss a novel approach to prove symmetry by using the so-called Fourier rearrangements, which can be applied to PDEs of arbitrary order (where classical method such as the moving plane method or the Polya-Szegö principle fail short). On the other hand, I will discuss recent results on symmetry-breaking for optimizers by using Fourier methods and the Stein-Tomas inequality. This talk is based on joint work with Tobias Weth and Jeremy Sok.*

**Xiaodan Zhou***(Okinawa Institute of Science and Technology)*

Feb 21, 2022

09:00 EST

**Quasiconvex envelope in the Heisenberg group.**

*Various notions of convexity of sets and functions in the Heisenberg group have been studied in the past two decades. In this talk, we focus on the horizontally quasiconvex ($h$-quasiconvex) functions in the Heisenberg group. Inspired by the first-order characterization and construction of quasiconvex envelope by Barron, Goebel and Jensen in the Euclidean space, we obtain a PDE approach to construct the $h$-quasiconvex envelope for a given function $f$ in the Heisenberg group. In particular, we show the uniqueness and existence of viscosity solutions to a non-local Hamilton-Jacobi equation and iterate the equation to obtain the $h$-quasiconvex envelope. Relations between $h$-convex hull of a set and the $h$-quasiconvex envelopes are also investigated. This is joint work with Antoni Kijowski (OIST) and Qing Liu (Fukuoka University/OIST).*

*Jian Song**(Rutgers University)*

Feb 14, 2022

10:00 EST

** Positivity conditions for complex Hessian equations. **

*In this talk, we will discuss the relation between complex Hessian equation and positivity of algebraic numerical conditions. In particular, we will prove a Naki-Moishezon criterion for Donaldson’s J-equation. *

*Paul Yang**(Princeton University)*

Feb 07, 2022

10:00 EST

**Sturm comparison for Jacobi vector fields and applications.**

* For CR manifolds of real dimension three, we study the Jacobi field equation. Under the condition that the torsion be parallel, we obtain comparison results against a family of homogeneous CR structures. As application, we describe the singularities of contact forms on the the homogeneous structures with finite total Q-prime curvature. This is ongoing joint work with Sagun Chanillo.*

*Vitali Kapovitch**(University of Toronto)*

Jan 31, 2022

09:00 EST

**Mixed curvature almost flat manifolds.**

*A celebrated theorem of Gromov says that given there is an such that if a closed Riemannian manifold satisfies then is diffeomorphic to an infranilmanifold. I will show that the lower sectional curvature bound in Gromov’s theorem can be weakened to the lower Bakry-Emery Ricci curvature bound. I will also discuss the relation of this result to the study of manifolds with Ricci curvature bounded below.*

*Luis Vega**(Basque Center for Applied Mathematics)*

Jan 24, 2022

10:00 EST

**New Conservation Laws and Energy Cascade for 1d Cubic NLS.**

*I’ll present some recent results concerning the IVP of 1d cubic NLS at the critical level of regularity. I’ll also exhibit a cascade of energy for the 1D Schrödinger map which is related to NLS through the so called Hasimoto transformation. For higher regularity these two equations are completely integrable systems and therefore no cascade of energy is possible.*

*Robert McCann**(University of Toronto)*

Dec 20, 2021

10:00 EST

**Inscribed radius bounds for lower Ricci bounded metric measure spaces with mean convex boundary.**

*Consider an essentially nonbranching metric measure space with the measure contraction property of Ohta and Sturm. We prove a sharp upper bound on the inscribed radius of any subset whose boundary has a suitably signed lower bound on its generalized mean curvature. This provides a nonsmooth analog of results dating back to Kasue (1983) and subsequent authors. We prove a stability statement concerning such bounds and — in the Riemannian curvature-dimension (RCD) setting — characterize the cases of equality. This represents joint work with Annegret Burtscher, Christian Ketterer and Eric Woolgar.*

*Linhan Li**(University of Minnesota)*

Dec 13, 2021

09:00 EST

**Comparison between the Green function and smooth distances.**

*In the upper half-space, the distance function to the boundary is a positive solution to Laplace’s equation that vanishes on the boundary, which can be interpreted as the Green function with pole at infinity for the Laplacian. We are interested in understanding the exact relations between the behavior of the Green function, the structure of the underlying operator, and the geometry of the domain. In joint work with G. David and S. Mayboroda, we obtain a precise and quantitative control of the proximity of the Green function and the distance function on the upper half-space by the oscillation of the coefficients of the operator. The class of the operators that we consider is of the nature of the best possible for the Green function to behave like a distance function. More recently, together with J. Feneuil and S. Mayboroda, we obtain analogous results for domains with uniformly rectifiable boundaries.*

*Qing Han**(University of Notre Dame)*

Dec 6, 2021

10:00 EST

**A Concise Boundary Regularity 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. The asymptotic behaviors of the conformal factors near boundary are known in smooth and sufficiently smooth domains. In this talk, we introduce the logarithm of the distance to boundary as an additional independent self-variable and establish a concise boundary regularity.*

*Juan Manfredi**(University of Pittsburgh)*

Nov 29, 2021

09:00 EST

**NATURAL -MEANS FOR THE -LAPLACIAN IN EUCLIDEAN SPACE AND THE HEISENBERG GROUP**

*We consider semi-discrete approximations to -harmonic functions based on the natural
-means of Ishiwata, Magnanini, and Wadade in 2017 (CVPDE 2017), who proved their local convergence. In the Euclidean case we prove uniform convergence in bounded Lipschitz domains. We also consider adapted semi-discrete approximations in the Heisenberg group and prove uniform convergence in bounded -domains.
This talk is based in joint work with András Domokos and Diego Ricciotti (Sacramento)
and Bianca Stroffolini (Naples)*

*Jyotshana Prajapat**(University of Mumbai)*

Nov 22, 2021

09:00 EST

**Geodetically convex sets in Heisenberg group .**

*A classification of geodetically convex subsets of Heisenberg group of homogeneous dimension 4 was proved by Monti-Rickly. We extend their result to a higher dimension Heisenberg group. This is ongoing work with my PhD student Anoop Varghese.*

*Eric Chen**(University of California at Berkeley)*

Nov 08, 2021

10:00 EST

**Integral curvature pinching and sphere theorems via the Ricci flow.**

*I will discuss how uniform Sobolev inequalities obtained from the monotonicity of Perelman’s W-functional can be used to prove curvature pinching theorems on Riemannian manifolds. These are based on scale-invariant integral norms and generalize some earlier pointwise and supercritical integral pinching statements. This is joint work with Guofang Wei and Rugang Ye.*

*Guofang Wang**(University of Freiburg)*

Nov 01, 2021

10:00 EDT

**Geometric inequalities in the hyperbolic space and their applications.**

*We will talk about Alexandrov-Fenchel type inequalities in the hyperbolic space and their applications in a higher order mass of asymptotically hyperbolic manifolds. The talk is based on a series of work joint with Yuxin Ge, Jie Wu and Chao Xia*

*Stefan Steinerberger**(University of Washington)*

Oct 25, 2021

10:00 EDT

**Mean-Value Inequalities for Convex Domains.**

*The Mean Value Theorem implies that the average value of a subharmonic
function in a disk can be bounded from above by the average value on the boundary.
What happens if we replace the disk by another domain? Maybe surprisingly, the problem
has a relatively clean answer — we discuss a whole range of mean value inequalities for
convex domains in IR^n. The extremal domain remains a mystery for most of them.
The techniques are an amusing mixture of classical potential theory, complex analysis,
a little bit of elliptic PDEs and, surprisingly, the theory of solids from the 1850s.*

*Gabriele Grillo**(Politecnico di Milano)*

Oct 11, 2021

09:00 EDT

**Nonlinear characterizations of stochastic completeness.**

*A manifold is said to be stochastically complete if the free heat semigroup preserves probability. It is well-known that this property is equivalent to nonexistence of nonnegative, bounded solutions to certain (linear) elliptic problems, and to uniqueness of solutions to the heat equation corresponding to bounded initial data. We prove that stochastic completeness is also equivalent to similar properties for certain nonlinear elliptic and parabolic problems. This fact, and the known analytic-geometric characterizations of stochastic completeness, allow to give new explicit criteria for existence/nonexistence of solutions to certain nonlinear elliptic equations on manifolds, and for uniqueness/nonuniqueness of solutions to certain nonlinear diffusions on manifolds.*

*Sundaram Thangavelu**(INDIAN INSTITUTE OF SCIENCE)*

Oct 04, 2021

09:00 EDT

**On the extension problem for the sublaplacian on the Heisenberg group.**

*In this talk we plan to describe some results on the extension problem associated to the sublaplacian on the Heisenberg group The Dirichlet to Neumann map induced by this problem leads to conformally invariant fractional powers of We use the results to prove a version of Hardy’s inequality for such fractional powers. These results are based on my joint work with Luz Roncal.*

*Yoshikazu Giga**(University of Tokyo)*

Sep 27, 2021

09:00 EDT

**On the Helmholtz decomposition of BMO spaces of vector fields.**

*The Helmholtz decomposition of vector fields is a fundamental tool for analysis of vector fields especially to analyze the Navier-Stokes equations in a domain. It gives a unique decomposition of a (tangential) vector field defined in a domain of an Euclidean space (or a riemannian maniford) into a sum of a gradient field and a solenoidal field with supplemental condition like a boundary condition.It is well-known that such decomposition gives an orthogonal decomposition of the space of vector fields in an arbitrary domain and known as the Weyl decomposition. It is also well-studied that in various domains including the half space, smooth bounded and exterior domain, it gives a topological direct sum decomposition of the space of vector fields for . The extension to the case (or ) is impossible because otherwise it would imply the boundedness of the Riesz type operator in (or ) which is absurd. In this talk, we extend the Hemlholtz decomposition in a space of vector fields with bounded mean oscillations (BMO) when the domain of vector field is a smooth bounded domain in an Euclidean space. There are several possible definitions of a BMO space of vector fields. However, to have a topological direct sum decomposition, it turns out that components of normal and tangential to the boundary should be handled separately. This decomposition problem is equivalent to solve the Poisson equation with the divergence of the original vector field as a data with the Neumann data with the normal trace of . The desired gradient field is the gradient of the solution of this Poisson equation. To solve this problem we construct a kind of volume potential so that the problem is reduced to the Neumann problem for the Laplace equation. Unfortunately, taking usual Newton potential causes a problem to estimate necessary norm so we construct another volume potential based on normal coordinate. We need a trace theorem to control norm of the normal trace. This is of independent interest. Finally, we solve the Neumann problem with data in a necessary space. The Helmholtz decomposition for BMO vector fields is previously known only in the whole Euclidean space or the half space so this seems to be the first result for a domain with a curved boundary. This is a joint work with my student Z.Gu (University of Tokyo).*

*Jill Pipher**(Brown University)*

Sep 20, 2021

10:00 EDT

**Boundary value problems for -elliptic operators.**

*We give some background about the regularity of solutions to real and complex elliptic operators, motivating a new algebraic condition (-ellipticity). We introduce this condition in order to solve new boundary value problems for operators with complex coefficients. Results with M. Dindos, and with M. Dindos and J. Li, are discussed*

*Luis Silvestre**(University of Chicago)*

Sep 13, 2021

10:00 EDT

**Regularity estimates for the Boltzmann equation without cutoff.**

*We study the regularization effect of the inhomogeneous Boltzmann equation without cutoff. We obtain a priori estimates for all derivatives of the solution depending only on bounds of its hydrodynamic quantities: mass density, energy density and entropy density. As a consequence, a classical solution to the equation may fail to exist after a certain time T only if at least one of these hydrodynamic quantities blows up. Our analysis applies to the case of moderately soft and hard potentials. We use methods that originated in the study of nonlocal elliptic and parabolic equations: a weak Harnack inequality in the style of De Giorgi, and a Schauder-type estimate.*

*Lorenzo D’Ambrosio**(Universita di Bari)*

Jul 05, 2021

09:00 EDT

**Liouville theorems for semilinear biharmonic equations and inequalities.**

*We study nonexistence results for a coercive semilinear biharmonic equation on the whole . The analysis is made for general solutions without any assumption on their sign nor on their behaviour at infinity. A relevant role is played by some extensions of the Hardy-Rellich inequalities for general functions (not necessarily compactly supported).*

*Susanna Terracini**(Universitá di Torino)*

Jun 28, 2021

09:00 EDT

**Free boundaries in segregation problems.**

*We first consider classes of variational problems for densities that repel each other at distance. Examples are given by the minimizers of Dirichlet functional or the Rayleigh quotient
over the class of functions attaining some boundary conditions on , and subjected to the constraint
As second class of problems, we consider energy minimizers of Dirichlet energies with different metrics
with constraint
For these problems, we investigate the optimal regularity of the solutions, prove a free-boundary extremality condition, and derive some preliminary results characterising the emerging free boundary.*

*Roger Moser**(University of Bath)*

Jun 21, 2021

09:00 EDT

**The infinity-elastica problem.**

*The Euler elastica problem seeks to minimise the -norm of
the curvature of curves under certain boundary conditions. If we
replace the -norm with the -norm, then we obtain a
variational problem with quite different properties. Nevertheless, even
though the underlying functional is not differentiable, it turns out
that the solutions of the problem can still be described by
differential equations. An analysis of these equations then gives a
classification of the solutions.*

*Svitlana Mayboroda **(University of Minnesota)*

Jun 14, 2021

10:00 EDT

**Green Function vs. Geometry.**

*In this talk we will discuss connections between the geometric and PDE properties of sets. The emphasis is on quantifiable, global results which yield true equivalence between the geometric and PDE notions in very rough scenarios, including domains and equations with singularities and structural complexity. The main result establishes that in all dimensions , a -dimensional set in is regular (rectifiable) if and only if the Green function for elliptic operators is well approximated by affine functions (distance to the hyperplanes). To the best of our knowledge, this is the first free boundary result of this type for lower dimensional sets and the first free boundary result in the classical case without restrictions on the coefficients of the equation.*

*Sun-Yung Alice Chang **(Princeton University)*

Jun 07, 2021

10:00 EDT

**On bi-Lipschitz equivalence of a class of non-conformally flat spheres.**

*This is a report of some recent joint work with Eden Prywes and Paul Yang. The main result is a bi-Lipschitz equivalence of a class of metrics on 4-shpere under curvature constraints. The proof involves two steps: first a construction of quasiconformal maps between two conformally related metrics in a positive Yamabe class, followed by the step of applying the Ricci flow to establish the bi-Lipschitz equivalence from such a conformal class to the standard conformal class on 4-sphere.*

*Xiaojun Huang**(Rutgers University)*

May 17, 2021

10:00 EDT

**Revisit to a non-degeneracy property for extremal mappings.**

*I will discuss a generalization of my previous result on the localization of extremal maps near a strongly pseudo-convex point.*

*Michael Struwe*

(ETH Zürich)

May 10, 2021

10:00 EDT

**Normalized harmonic map flow.**

*Finding non-constant harmonic 3-spheres for a closed target manifold N is a prototype of a super-critical variational problem. In fact, the direct method fails, as the infimum of Dirichlet energy in any homotopy class of maps from the 3-sphere to any closed N is zero; moreover, the harmonic map heat flow may blow up in finite time, and even the identity map from the 3-sphere to itself is not stable under this flow.
To overcome these difficulties, we propose the normalized harmonic map heat flow as a new tool, and we show that for this flow the identity map from the 3-sphere to itself now, indeed, is stable; moreover, the flow converges to a harmonic 3-sphere also when we perturb the target geometry. While our results are strongest in the perturbative setting, we also outline a possible global theory.*

*Jungang Li **(Brown University)*

May 03, 2021

10:00 EDT

**Sharp critical and subcritical Moser-Trudinger inequalities on complete and noncompact Riemannian manifolds.**

*TBA.*

*Carlos Kenig*

(University of Chicago)

Apr 26, 2021

10:00 EDT

**Wave maps into the sphere.**

*We will introduce wave maps, an important geometric flow, and discuss, for the case when the target is the sphere, the asymptotic behavior near the ground state (without symmetry) and recent results in the general case (under co-rotational symmetry) in joint work with Duyckaerts, Martel and Merle.*

*Yanyan Li*

(Rutgers University)

Apr 19, 2021

09:00 EDT

**Regular solutions of the stationary Navier-Stokes equations on high dimensional Euclidean space.**

*We study the existence of regular solutions of the incompressible stationary Navier-Stokes equations in -dimensional Euclidean space with a given bounded external force of compact support. In dimensions , the existence of such solutions was known. In this paper, we extend it to dimensions . This is a joint work with Zhuolun Yang.*

*Jingzhi Tie*

(University of Georgia)

Apr 05, 2021

10:00 EDT

**CR analogue of Yau’s Conjecture on pseudo harmonic functions of polynomial growth.**

*Cheng and Yau derived the well-known gradient estimate for positive harmonic functions and obtained the classical Liouville theorem, which states that any bounded harmonic function is constant in complete noncompact Riemannian manifolds with nonnegative Ricci curvature. I will talk about the CR analogue of Yau’s conjecture. We need to derive the CR volume doubling property, CR Sobolev inequality, and mean value inequality. Then we can apply them to prove the CR analogue of Yau’s conjecture on the space consisting of all pseudoharmonic functions of polynomial growth of degree at most in a complete noncompact pseudohermitian -manifold. As a by-product, we obtain the CR analogue of volume growth estimate and Gromov precompactness theorem.*

*Brian Street*

(University of Wisconsin-Madison)

Mar 29, 2021

09:00 EDT

**Maximal Hypoellipticity**

* In 1974, Folland and Stein introduced a generalization of ellipticity known as maximal hypoellipticity. This talk will be an introduction to this concept and some of the ways it generalizes ellipticity.*

*Wenxiong Chen *

(Yeshiva University)

Mar 22, 2021

10:00 EDT

**Asymptotic radial symmetry, monotonicity, non-existence for solutions to fractional parabolic equations.**

*In this talk, we will consider nonlinear parabolic fractional equations
We develop a systematical approach in applying an asymptotic method of moving planes to investigate qualitative properties of positive solutions for fractional parabolic equations. To this end, we derive a series of needed key ingredients such as narrow region principles, and various asymptotic maximum and strong maximum principles for antisymmetric functions in both bounded and unbounded domains. Then we illustrate how these new methods can be employed to obtain asymptotic radial symmetry and monotonicity of positive solutions in a unit ball and on the whole space. Namely, we show that no matter what the initial data are, the solutions will eventually approach to radially symmetric functions. We will also consider the entire positive solutions on a half space, in the whole space, and with indefinite nonlinearity. Monotonicity and nonexistence of solutions are obtained. This is joint work with P. Wang, Y. Niu, Y. Hu and L. Wu.*

*Man Wah Wong *

(York University)

Mar 15, 2021

10:00 EDT

**Spectral Theory and Number Theory of the Twisted Bi-Laplacian.**

*We begin with the sub-Laplacian on the Heisenberg group and then the twisted Laplacian by taking its inverse Fourier transform with respect to the center of the group. The eigenvalues and the eigenfunctions of the twisted Laplacian are computed explicitly. Then we turn our attention to the product of the twisted Laplacian and its transpose, thus obtaining a fourth order partial differential operator dubbed the twisted bi-Laplacian. The connections between the spectral analysis of the twisted bi-Laplacian and Dirichlet divisors, the Riemann zeta function and the Dixmier trace are explained.*

*Enrique Zuazua*

(Friedrich-Alexander-Universität)

Mar 08, 2021

09:00 EST

**UNILATERAL BOUNDS FOR NONLINEAR SEMIGROUPS AND TIME-INVERSION.**

*Abstract: Some classical nonlinear semigroups arising in mechanics induce unilateral bounds on solutions. Hamilton–Jacobi equations and 1-d scalar conservation laws are classical examples of such nonlinear effects: solutions spontaneously develop one-sided Lipschitz or semi-concavity conditions.
When this occurs the range of the semigroup is unilaterally bounded by a threshold.
On the other hand, in practical applications, one is led to consider the problem of time-inversion, so to identify the initial sources that have led to the observed dynamics at the final time.
In this lecture we shall discuss this problem answering to the following two questions: On one hand, to identify the range of the semigroup and, given a target, to characterize and reconstruct the ensemble of initial data leading to it.
Illustrative numerical simulations will be presented, and a complete geometric interpretation will also be provided.
We shall also present a number of open problems arising in this area and the possible link with reinforcement learning.*

*Manuel Del Pino*

(University of Bath)

Mar 01, 2021

09:00 EST

**Dynamics of concentrated vorticities in 2d and 3d Euler flows.**

*Abstract: A classical problem that traces back to Helmholtz and Kirchoff is the understanding of the dynamics of solutions to the 2d and 3d Euler equations of an inviscid incompressible fluid, when the vorticity of the solution is initially concentrated near isolated points in 2d or vortex lines in 3d. We discuss some recent result on existence and asymptotic behaviour of these solutions. We describe, with precise asymptotics, interacting vortices and travelling helices. We rigorously establish the law of of motion of of “leapfrogging vortex rings”, originally conjectured by Helmholtz in 1858. This is joint work with Juan Davila, Monica Musso and Juncheng Wei.*

**Saikat Mazumdar**

(Indian Institute of Technology Bombay)

Feb 22, 2021

09:00 EST

**EXISTENCE RESULTS FOR THE HIGHER-ORDER -CURVATURE EQUATION.**

*Abstract: In this talk, we will obtain some existence results for the -curvature equation
of arbitrary -th order, where is an integer, on a compact Riemannian
manifold of dimension . This amounts to solving a nonlinear elliptic
PDE involving the powers of Laplacian called the GJMS operator. The difficulty
in determining the explicit form of this GJMS operator together with a lack of
maximum principle complicates the issues of existence.
This is a joint work with Jérôme Vètois (McGill University).*

**Yehuda Pinchover**

(Technion -Israel Institute of Technology)

Feb 15, 2021

09:00 EST

**On families of optimal Hardy-weights for linear second-order elliptic operators.**

*Abstract: We construct families of optimal Hardy-weights for a subcritical linear second-order elliptic operator using a one-dimensional reduction. More precisely, we first characterize all optimal Hardy-weights with respect to one-dimensional subcritical Sturm-Liouville operators on , , and then apply this result to obtain families of optimal Hardy inequalities for general linear second-order elliptic operators in higher dimensions. This is a joint work with Idan Versano.*

**Martin Dindos**

(The University of Edinburgh)

Feb 08, 2021

09:00 EST

**On -ellipticity and connections to solvability of elliptic complex valued PDEs.**

*Abstract: The notion of an elliptic partial differential equation (PDE)
goes back at least to 1908, when it appeared in a paper J. Hadamard. In
this talk we present a recently discovered structural condition, called
-ellipticity, which generalizes classical ellipticity. It was
co-discovered independently by Carbonaro and Dragicevic on one hand, and
Pipher and myself on the other, and plays a fundamental role in many
seemingly mutually unrelated aspects of the theory of elliptic
complex-valued PDE. So far, -ellipticity has proven to be the key
condition for:
(i) convexity of power functions (Bellman functions)
(ii) dimension-free bilinear embeddings,
(iii) -contractivity and boundedness of semigroups
associated with elliptic operators,
(iv) holomorphic functional calculus,
(v) multilinear analysis,
(vi) regularity theory of elliptic PDE with complex coefficients.
During the talk, I will describe my contribution to this development, in
particular to (vi).*

**Yunfeng Zhang**

(University of Connecticut)

Feb 01, 2021

09:00 EST

**Schrödinger equations on compact globally symmetric spaces.**

*Abstract: Let be a compact manifold of dimension . Scale-invariant Strichartz estimates of the form
have only been proved for a few model cases of , most of which are compact globally symmetric spaces.
In this talk, we report that the above estimate holds true on an arbitrary compact globally symmetric space equipped with the canonical Killing metric, for all , where denotes the rank of . As an immediate application, we provide local well-posedness results for nonlinear Schrödinger equations of polynomial nonlinearities of degree on any compact globally symmetric space of large enough rank, in all subcritical spaces.
We also discuss bilinear Strichartz estimates on compact globally symmetric spaces, and critical and subcritical local well-posedness results for the cubic nonlinearity.*

**Zhen-Qing Chen**

(University of Washington)

Jan 25, 2021

10:00 EST

**Stability of Elliptic Harnack Inequality.**

*Abstract: Harnack inequality, if it holds, is a useful tool in analysis and probability theory. In this talk, I will discuss scale invariant elliptic Harnack inequality for general diffusions, or equivalently, for general differential operators on metric measure spaces, and show that it is stable under form-comparable perturbations for strongly local Dirichlet forms on complete locally compact separable metric spaces that satisfy metric doubling property. Based on Joint work with Martin Barlow and Mathav Murugan.*

**Matthew Gursky**

(University of Notre Dame)

Jan 18, 2021

09:00 EST

**Extremal Eigenvalues of the conformal laplacian.**

*Abstract: I will report on joint work with Samuel Perez-Ayala in which we consider the problem of extremizing eigenvalues of the conformal laplacian in a fixed conformal class. This generalizes the problem of extremizing the eigenvalues of the laplacian on a compact surface. I will explain the connection of this problem to the existence of harmonic maps, and to nodal solutions of the Yamabe problem (first noticed by Ammann-Humbert).*

**Gilles Carron**

(University of Nantes)

Jan 11, 2021

09:00 EST

**Euclidean heat kernel rigidity.**

*Abstract: This is joint work with David Tewodrose (Bruxelles). I will explain that a metric measure space with Euclidean heat kernel are Euclidean. An almost rigidity result comes then for free, and this can be used to give another proof of Colding’s almost rigidity for complete manifold with non negative Ricci curvature and almost Euclidean growth.*

**Betsy Stovall**

(University of Wisconsin-Madison)

Dec 14, 2020

10:00 EST

**Fourier restriction to degenerate hypersurfaces.**

*Abstract: In this talk, we will describe various open questions and recent progress on the Fourier restriction problem associated to hypersurfaces with varying or vanishing curvature.*

**Almut Burchard**

(University of Toronto)

Dec 14, 2020

09:00 EST

**Rearrangement inequalities on spaces of bounded mean oscillation.**

*Abstract: Spaces of bounded mean oscillation (BMO) are relatively
large function spaces that are often used in place
of L^\infinity to do basic Fourier analysis.
It is not well-understood how geometric properties
of the underlying point space enters into the functional
analysis of BMO. I will describe recent work with
Galia Dafni and Ryan Gibara, where we take some
steps towards geometric inequalities.
Specifically, we show that the symmetric decreasing
rearrangement in n-dimensions is bounded, but not
continuous in BMO. The question of sharp bounds
remains open.*

**Giovanna Citti**

(University of Bologna)

Dec 07, 2020

09:00 EST

**Degree preserving variational formulas for submanifolds.**

*Abstract: I present a joint work with M. Ritoré and G. Giovannardi related to an area functional for
submanifolds of fixed degree immersed in a graded manifold. The expression of this area functional
strictly depends on the degree of the manifold, so that, while computing the first variation,
we need to keep fixed its degree. We will show that there are isolated surfaces,
for which this type of degree preserving variations do not exist: they can be considered
higher dimensional extension of the subriemannian abnormal geodesics. *

**Annalisa Baldi**

(University of Bologna)

Nov 30, 2020

09:00 EST

**Poincaré and Sobolev inequalities for differential forms in Euclidean spaces and Heisenberg groups.**

*Abstract: In this talk I present some recent results obtained in collaboration with B. Franchi and P. Pansu about Poincaré and Sobolev inequalities for differential forms in Heisenberg groups (some results are new also for Euclidean spaces). For L^p, p>1, the estimates are consequence of singular integral estimates. In the limiting case L^1, the singular integral estimates are replaced with inequalities which go back to Bourgain-Brezis and Lanzani-Stein in Euclidean spaces, and to Chanillo-Van Schaftingen and Baldi-Franchi-Pansu in Heisenberg groups. Also the case p=Q (Q is the homogeneous dimension of the Heisenberg group ) is considered.*

**Emmanuel Hebey**

(Université de Cergy-Pontoise)

Nov 16, 2020

09:00 EST

**Schrödinger-Proca constructions in the closed setting**

*Abstract: We discuss Schrödinger-Proca constructions in the context of closed manifolds leading to the Bopp-Podolsky-Schrödinger-Proca and the Schrödinger-Poisson-Proca systems. The goal is to present an introduction to these equations (how we build them, what do they represent) and then to present the result we got on these systems about the convergence of (BPSP) to (SPP) as the Bopp-Podolsky parameter goes to zero.*

**Ling Xiao**

(University of Connecticut)

Nov 09, 2020

09:00 EST

**Entire spacelike constant curvature in Minkowski space.**

*Abstract: We prove that, in the Minkowski space, if a spacelike, -convex hypersurface with constant curvature has bounded principal curvatures, then is convex. Moreover, if is not strictly convex, after an rigid motion, splits as a product . We also construct nontrivial examples of strictly convex, spacelike hypersurface with constant curvature and bounded principal curvatures. This is a joint work with Changyu Ren and Zhizhang Wang.*

**Joshua Flynn**

(University of Connecticut)

Nov 02, 2020

10:00 EST

**Sharp Caffarelli-Kohn-Nirenberg Inequalities for Grushin Vector Fields and Iwasawa Groups.**

*Abstract: Caffarelli-Kohn-Nirenberg inequalities are established for the Grushin vector fields and for Iwasawa groups (i.e., the boundary group of a real rank one noncompact symmetric space). For all but one parameter case, this is done by introducing a generalized Kelvin transform which is shown to be an isometry of certain weighted Sobolev spaces. For the exceptional parameter case, the best constant is found for the Grushin vector fields by introducing Grushin cylindrical coordinates and studying the transformed Euler-Lagrange equation.*

**William Minicozzi**

(MIT)

Oct 26, 2020

10:00 EDT

**Mean curvature flow in high codimension.**

*Abstract: Mean curvature flow (MCF) is a geometric heat equation where a submanifold evolves to minimize its area. A central problem is to understand the singularities that form and what these imply for the flow. I will talk about joint work with Toby Colding on higher codimension MCF, where the flow becomes a complicated system of equations and much less is known.*

**Jiuyi Zhu**

(Louisiana State University)

Oct 19, 2020

10:00 EDT

**The bounds of nodal sets of eigenfunctions.**

*Abstract: Motivated by Yau’s conjecture, the study of the measure of nodal sets (Zero level sets) for eigenfunctions is interesting. We investigate the measure of nodal sets for Steklov, Dirichlet and Neumann eigenfunctions in the domain and on the boundary of the domain. For Dirichlet or Neumann eigenfunctions in the analytic domains, we show some sharp upper bounds of nodal sets which touch the boundary. We will also discuss some upper bounds of nodal sets for eigenfunctions of general eigenvalue problems. Furthermore, some sharp doubling inequalities and vanishing order are obtained. Part of the talk is based on joint work with Fanghua Lin.*

**Cristian Cazacu**

(University of Bucharest)

Oct 12, 2020

09:00 EDT

**Optimal constants in Hardy and Hardy-Rellich type inequalities.**

*Abstract: In this talk we discuss Hardy and Hardy-Rellich type inequalities, so important in establishing useful properties for differential operators with singular potentials and their PDEs. We recall some well-known and recent results and present some new extensions. We analyze singular potentials with one or various singularities. The tools of our proofs are mainly based on the method of supersolutions, proper transformations and spherical harmonics decomposition. We also focus on the best constants and the existence/nonexistence of minimizers in the energy space. This presentation is partially supported by CNCS-UEFISCDI Grant No. PN-III-P1-1.1-TE-2016-2233. *

**Laurent Saloff-Coste**

(Cornell University)

Oct 5, 2020

10:00 EDT

**Heat kernel on manifolds with finitely many ends.**

*Abstract: For over twenty years A. Grigor’yan and the speaker have studied heat kernel estimates on manifolds with a finite number of nice ends. Despite these efforts, question remains. In this talk, after giving an overview of what the problem is and what we know, the main difficulty will be explained and recent progresses involving joint work with Grigor’yan and Ishiwata will be explained. They provide results concerning Poincaré inequality in large central balls on such manifold.*

**Jungang Li**

(Brown University)

Sep 28, 2020

09:00 EDT

**Higher order Brezis-Nirenberg problems on hyperbolic spaces.**

*Abstract: The Brezis-Nirenberg problem considers elliptic equations whose nonlinearity is associated with critical Sobolev exponents. In this talk we will discuss a recent progress on higher order Brezis-Nirenberg problem on hyperbolic spaces. The existence of solutions relates closely to the study of higher order sharp Hardy-Sobolev-Maz’ya inequalities, which is due to G. Lu and Q. Yang. On the other hand, we obtain a nonexistence result on star-shaped domains. In addition, with the help of Green’s function estimates, we apply moving plane method to establish the symmetry of positive solutions. This is a joint work with Guozhen Lu and Qiaohua Yang..*

**Phan Thành Nam**

(LMU Munich)

Sep 21, 2020

09:00 EDT

**Lieb-Thirring inequality with optimal constant and gradient error term.**

*Abstract: In 1975, Lieb and Thirring conjectured that the kinetic energy of fermions is not smaller than its Thomas-Fermi (semiclassical) approximation, at least in three or higher dimensions. I will discuss a rigorous lower bound with the sharp semiclassical constant and a gradient error term which is normally of lower order in applications. The proof is based on a microlocal analysis and a variant of the Berezin-Li-Yau inequality. This approach can be extended to derive an improved Lieb-Thirring inequality for interacting systems, where the Gagliardo-Nirenberg constant appears in the strong coupling limit.*

**Jean Dolbeault**

(Université Paris-Dauphine)

Sep 14, 2020

09:00 EDT

**Stability in Gagliardo-Nirenberg inequalities.**

*Abstract: Optimal constants and optimal functions are known in some functional inequalities. The next question is the stability issue: is the difference of the two terms controlling a distance to the set of optimal functions ? A famous example is provided by Sobolev’s inequalities: in 1991, G. Bianchi and H. Egnell proved that the difference of the two terms is bounded from below by a distance to the manifold of the Aubin-Talenti functions. They argued by contradiction and gave a very elegant although not constructive proof. Since then, estimating the stability constant and giving a constructive proof has been a challenge.
This lecture will focus mostly on subcritical inequalities, for which explicit constants can be provided. The main tool is based on entropy methods and nonlinear flows. Proving stability amounts to establish, under some constraints, a version of the entropy – entropy production inequality with an improved constant. In simple cases, for instance on the sphere, rather explicit results have been obtained by the « carré du champ » method introduced by D. Bakry and M. Emery. In the Euclidean space, results based on constructive regularity estimates for the solutions of the nonlinear flow and corresponding to a joint research project with Matteo Bonforte, Bruno Nazaret, and Nikita Simonov will be presented.*

**Yi Wang**

(Johns Hopkins University)

Sep 07, 2020

09:00 EDT

**Rigidity of local minimizers of the functional.**

*Abstract: In this talk, I will present a result on the rigidity of local minimizers of the functional among all conformally flat metrics in the Euclidean (n + 1)-ball. We prove the metric is flat up to a conformal transformation in some (noncritical) dimensions. We also prove the analogous result in the critical dimension n + 1 = 4. The main method is Frank-Lieb’s rearrangement-free argument. If minimizers exist, this implies a fully nonlinear sharp Sobolev trace inequality. I will also discuss a nonsharp Sobolev trace inequality. This is joint work with Jeffrey Case.*

**Pengfei Guan**

(McGill University)

Aug 31, 2020

10:00 EDT

**A mean curvature type flow and isoperimetric problem in warped product spaces.**

*Abstract: We will discuss a mean curvature type flow with the goal to solve isoperimetric problem. The flow is induced from the variational properties associated to conformal Killing fields. Such flow was first introduced in space forms in a previous joint work with Junfang Li, where we provided a flow approach to the classical isoperimetric inequality in space form. Later, jointly with Junfang Li and Mu-Tao Wang, we considered the similar flow in warped product spaces with general base. Under some natural conditions, the flow preserves the volume of the bounded domain enclosed by a graphical hypersurface, and monotonically decreases the hypersurface area. Furthermore, the regularity and convergence of the flow can be established, thereby the isoperimetric problem in warped product spaces can be solved. The flow serves as an interesting way to achieve the optimal solution to the isoperimetric problem.*

**Rupert Frank**

(CalTech)

Aug 24, 2020

10:30 EDT

**REVERSE HARDY–LITTLEWOOD–SOBOLEV INEQUALITIES.**

*Abstract: We are interested in a new family of reverse Hardy–Littlewood–Sobolev inequalities which involve a power law kernel with positive exponent and a Lebesgue exponent <1. We characterize the range of parameters for which the inequality holds and present results about the existence of optimizers. A striking open question is the possibility of concentration of a minimizing sequence.
This talk is based on joint work with J. Carrillo, M. Delgadino, J. Dolbeault and F. Hoffmann.*

**Juncheng Wei**

University of British Columbia)

Aug 17, 2020

10:00 EDT

**Rigidity Results for Allen-Cahn Equation.**

*Abstract: I will discuss two recent rigidity results for Allen-Cahn: the first is Half Space Theorem which states that if the nodal set lies above a half space then it must be one-dimensional. The second result is the stability of Cabre-Terra saddle solutions in R^8, R^{10} and R^{12}.*

**Fengbo Hang**

(New York University)

Aug 10, 2020

10:00 EDT

**Concentration compactness principle in critical dimensions revisited.**

*Abstract: Concentration compactness principle for functions in on a n-dimensional domain was introduced by Lions in 1985 with the Moser-Trudinger inequality in mind. We will discuss some further refinements after Cerny-Cianchi-Hencl’s improvement in 2013. These refinements unifiy the approach for n=2 and n>2 cases and work for higher order or fractional order Sobolev spaces as well. They are motivated by and closely related to the recent derivation of Aubin’s Moser-Trudinger inequality for functions with vanishing higher order moments on the standard 2-sphere (one may see math.sjtu.edu.cn/conference/2020p&g/videos/20200707_FengboHang_M1.html for that part).*

**Dongmeng Xi **

(NYU)

Aug 03, 2020

10:00 EDT

**An isoperimetric type inequality via a modified Steiner symmetrization scheme.**

*Abstract: We establish an affine isoperimetric inequality using a symmetrization scheme that involves a total of 2n elaborately chosen Steiner symmetrizations at a time. The necessity of this scheme, as opposed to the usual Steiner symmetrization, will be demonstrated with an example. This is a joint work with Dr. Yiming Zhao.*

**Yiming Zhao **

(MIT)

Aug 03, 2020

09:00 EDT

**Reconstruction of convex bodies via Gauss map.**

*Abstract: In this talk, we will discuss the Gauss image problem, a problem that reconstructs the shape of a convex body using partial data regarding its Gauss map. In the smooth category, this problem reduces to a Monge-Ampere type equation on the sphere. But, we will use a variational argument that works with generic convex bodies. This is joint work with Károly Böröczky, Erwin Lutwak, Deane Yang, and Gaoyong Zhang.*

**Xavier Cabre **

(ICREA and UPC (Barcelona))

Jul 27, 2020

09:00 EDT

**Stable solutions to semilinear elliptic equations are smooth up to dimension 9.**

*Abstract: The regularity of stable solutions to semilinear elliptic PDEs has been studied since the 1970’s. In dimensions 10 and higher, there exist singular stable energy solutions. In this talk I will describe a recent work in collaboration with Figalli, Ros-Oton, and Serra, where we prove that stable solutions are smooth up to the optimal dimension 9. This answers to an open problem posed by Brezis in the mid-nineties concerning the regularity of extremal solutions to Gelfand-type problems.*

**Andrea Malchiodi**

(Scuola Normale Superiore)

Jul 20, 2020

10:00 EDT

**On the Sobolev quotient in sub-Riemannian geometry.**

*Abstract: We consider three-dimensional CR manifolds, which are modelled on the Heisenberg group. We introduce a natural concept of “mass” and prove its positivity under the condition that the scalar curvature is positive and in relation to their (holomorphic) embeddability properties. We apply this result to the CR Yamabe problem, and we discuss extremality of Sobolev-type quotients, giving some counterexamples for “Rossi spheres”. This is joint work with J.H.Cheng and P.Yang.*

**William Beckner**

(University of Texas at Austin)

Jul 20, 2020

09:00 EDT

**Symmetry in Fourier Analysis – Heisenberg to Stein-Weiss.**

*Abstract: Embedded symmetry within the Heisenberg group is used to couple geometric insight and analytic calculation to obtain a new sharp Stein-Weiss inequality with mixed homogeneity on the line of duality. SL(2,R) invariance and Riesz potentials define a natural bridge for encoded information that connects distinct geometric structures. Insight for Stein-Weiss integrals is gained from vortex dynamics, embedding on hyperbolic space, and conformal geometry. The intrinsic character of the Heisenberg group makes it the natural playing field on which to explore the laws of symmetry and the interplay between analysis and geometry on a manifold.*