# PDE Geometric Analysis seminar

All talks will be *in person* unless specified otherwise.

(Please make sure to check the seminar webpage regularly so you will be constantly correctly informed on the format and time of the seminar.)

To subscribe to the PDEGA seminar mailing list: email mathpdega+subscribe@g-groups.wisc.edu

### Previous PDE/GA seminars

# Fall 2024

The first day of class is Wednesday, September 4, 2024.

**September 9, 2024**

Giulia Mescolini (EPFL). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Vanishing viscosity non-unique solutions to the forced 2D Euler equations

Abstract: In the last decades, different techniques were developed to prove results around the topic of (non-)uniqueness of fluid dynamical PDEs. It is then an important question to understand if there is a selection principle for these equations, namely if such non-unique solutions can also be obtained in the limit of regularised problems (in which, for instance, a dissipative term is introduced: the vanishing viscosity limit). Remarkably, in the context of conservation laws, selection happens.

We investigate this problem in the context of the forced 2D Euler equations, for which Vishik (2018) recently constructed non-unique solutions. We prove that the unique Leray solutions to the 2D Navier-Stokes system, forced with Vishik's force and starting from a perturbation of Vishik's initial datum, converge to a 1-parameter family of solutions when we take the double limit of vanishing viscosity and perturbation size.

**September 16, 2024**

Cole Graham (UW-Madison)

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Shock formation in weakly viscous conservation laws

Abstract: The compressible Euler equations readily form shocks, but in 1D the inclusion of viscosity prevents such singularities. In this talk, we will quantitatively examine the interaction between generic shock formation and viscous effects as the viscosity tends to zero. We thereby obtain sharp rates for the vanishing-viscosity limit in Hölder norms, and uncover universal viscous structure near shock formation. The results hold for large classes of viscous hyperbolic conservation laws, including compressible Navier–Stokes with *physical* rather than artificial viscosity. This is joint work with John Anderson and Sanchit Chaturvedi.

**September 23, 2024**

Gong Chen (Georgia Tech). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Dynamics of solutions to Klein-Gordon equations around multi-solitons

Abstract: I will discuss the dynamics of solutions to Klein-Gordon equations around multi-solitons including asymptotic stability and classification of multisoliton, and to classify the initial data for the global behavior in an open neighborhood of multi-solitons. This talk is based on joint works with Jacek Jendrej and Kenji Nakanishi.

**September 30, 2024**

Thu Le (UW-Madison).

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Sampling methods for solving electromagnetic inverse scattering problems associated with Maxwell’s equations

Abstract: We consider the electromagnetic inverse scattering problem, which aims to reconstruct the location and shape of an unknown object from boundary measurements of the electric field scattered by that object at a fixed frequency. This problem has applications in radar and nondestructive testing. We study a modification of the Orthogonality Sampling Method (OSM) for time-harmonic Maxwell’s equations in anisotropic media and develop both OSM and the modified OSM for bi-anisotropic media. This modification allows the method to work with more types of polarization vectors associated with the scattering data. We provide theoretical justification for our methods using factorization analysis of the far-field operator and the Funk–Hecke formula. 3D numerical results will be presented using simulated data alongside experimental data provided by the Fresnel Institute. We compare the performance of the OSM, the modified OSM, and a classical factorization method. The results show that the modified OSM performs better than the original version and the factorization method on this specific sparse and limited-aperture real dataset in a fast and simple manner. This is a joint work with Dinh-Liem Nguyen, Hayden Schmidt, and Trung Truong.

**October 7, 2024**

Gi-Chan Bae (Seoul Nat. University). Host: Chanwoo Kim

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: The relativistic quantum Boltzmann equation near equilibrium

Abstract: The relativistic quantum Boltzmann equation (or the relativistic Uehling-Uhlenbeck equation) describes the dynamics of single-species fast-moving quantum particles. With the recent development of relativistic quantum mechanics, the relativistic quantum Boltzmann equation has been widely used in physics and engineering such as in quantum collision experiments and the simulations of electrons in graphene. Despite such importance, there has been no mathematical theory on the existence of solutions for the relativistic quantum Boltzmann equation to the best of authors’ knowledge. In this talk, we consider the global existence of a unique classical solution to the relativistic Boltzmann equation for both bosons and fermions when the initial distribution is nearby a global equilibrium. This is joint work with J. W. Jang and S. B. Yun.

**October 11, 2024 (Colloquium)**

Mikaela Iacobelli (ETH Zurich). Host: Qin Li

Format: in-person. Time: 4:00-5:00PM, **VV 911**

**October 14, 2024**

Myoungjean Bae (KAIST). Host: Mikhail Feldman

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Multi-dimensional solutions to steady Euler-Poisson system, part 1

Abstract: In the first part of my talk, I will introduce recent results on the existence of (i) subsonic solution, (ii) supersonic solution, (iii) continuous transonic solution to steady Euler-Poisson system. And, outlined proofs for (i) and (ii) will be presented. In addition, I will explain how this result can be related to de Laval nozzle flow problem, which is one of long standing open problems in the study of multi-dimensional compressible flows.

**October 21, 2024**

Zachary Bradshaw (U of Arkansas). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Properties of hypothetical non-unique Navier-Stokes flows

Abstract: Recent analytical and computational results suggest that some solutions to the Navier-Stokes equations in physically motivated classes are non-unique. In this talk we explore what can and cannot be said about general non-uniqueness scenarios in these classes. In particular, we explore how fast hypothetical non-unique solutions can separate. We additionally identify uniqueness criteria in terms of the difference of two hypothetical non-unique solutions, a.k.a. the error. The advantage of this type of result compared to traditional uniqueness results which only involve the Navier-Stokes flows themselves is that severity of non-uniqueness, as understood from the viewpoint of predictability, is encoded in the error. This talk is based in part on joint work with Patrick Phelps.

**October 25, 2024 (Colloquium)**

Connor Mooney (UC Irvine). Host: Dallas Albritton

Format: in-person. Time: 4:00-5:00PM, VV B239

**October 28, 2024**

Sam Punshon-Smith (Tulane University). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30 pm, VV901

Title: Finite Lyapunov Exponents for Stochastic Fluid Equations

Abstract: Lyapunov exponents have long been a useful tool for analyzing the long-term behavior of dynamical systems, but in infinite dimensions, their use is limited, as they may all collapse to $-\infty$. This talk presents recent results (with Hairer, Rosati, and Yi) demonstrating the finiteness of the top Lyapunov exponent for the 2d stochastic Navier-Stokes equations with non-degenerate noise, and for the advection-diffusion equation advected with this velocity field. We do this by proving a "high frequency stochastic instability" mechanism that causes Fourier mass to drift to low frequencies with high probability. As a corollary, we obtain an almost sure exponential lower bound for the advection-diffusion equation in this setting. Additionally, we provide estimates on decay rate in terms of the diffusivity parameter.

**November 1-3, 2024**

A workshop (information will be announced). Host: Mikhail Feldman

**November 4, 2024**

Daniel Tataru (UC Berkeley). Host: Mihaela Ifrim

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: The small data global well-posedness conjectures for dispersive flows

Abstract: The key property of linear dispersive flows is that waves with different frequencies travel with different group velocities, which leads to the phenomena of dispersive decay. Nonlinear dispersive flows also allow for interactions of linear waves, and their long time behavior is determined by the balance of linear dispersion on one hand, and nonlinear effects on the other hand. The first goal of this talk will be to present a new set of conjectures which aim to describe the global well-posedness and the dispersive properties of solutions in the most difficult case when the nonlinear effects are dominant, assuming only small initial data. This covers many interesting physical models, yet, as recently as a few years ago, there was no clue even as to what one might reasonably expect. The second objective of the talk will be to describe some very recent results in this direction. This is joint work with Mihaela Ifrim.

**November 11, 2024**

Leonid Berlyand (Penn State). Host: Mikhail Feldman

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Stability and Bifurcations in a Free Boundary PDE Models of Cell Motility

Abstract: We begin with a brief overview of the rapidly developing research area of active matter (a.k.a. active materials). These materials are intrinsically out of equilibrium resulting in novel physical properties whose modeling requires the development of new mathematical tools. We focus on studying the onset of motion of a living cell (e.g., a keratocyte) driven by myosin contraction. We introduce a minimal two-dimensional free-boundary PDE (FBP) model that captures the evolution of the cell shape and nonlinear diffusion of myosin.

We first consider a linear diffusion model with two sources of nonlinearity: Keller-Segel cross-diffusion term and the free-boundary that models moving/deformable cell membranes. Here we establish asymptotic linear stability and derive the explicit formula for the stability-determining eigenvalue.

Next, we consider the effect of nonlinear myosin diffusion, which results in the change of the bifurcation type from super- to subcritical, and obtain an asymptotic representation of the bifurcation curve (for small velocities). This allows us to derive an explicit formula for the curvature at the bifurcation point that controls the bifurcation type. In the most *recent* work in progress with the Heidelberg biophysics group, we study the relation between various types of nonlinear diffusion and *bistability*.

Finally, we discuss novel mathematical features of this FBP model with a focus on *non-self-adjointness* that plays a key role in the spectral stability analysis. Our mathematics reveals the physical origins of the non-self-adjointess of the operators in this FBP model.

Joint works with A. Safsten & V. Rybalko (Transactions of AMS 2023, and Phys. Rev. E 2022), with O. Krupchytskyi &T. Laux (Preprint 2024), and with A. Safsten & L. Truskinovsky (Preprint 2024). This work has been supported by NSF grants DMS-2404546, DMS-2005262, and DMS-2404546.

**November 18, 2024**

Yulun Xu (Stony Brook). Host: Mikhail Feldman

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: The existence and uniqueness of viscosity solutions of the nonlinear complex Hessian equations on Hermitian manifolds.

Abstract: We proved the existence of viscosity solutions to complex Hessian equations that satisfy a determinant domination condition. This viscosity solution was shown to be unique when the right hand side is strictly monotone increasing in terms of the solution. When the right hand side does not depend on the solution, we reduced it to the strict monotonicity of the solvability constant. This is a joint work with Jingrui Cheng.

**November 25, 2024** **Thanksgiving Week**

**December 2, 2024**

Sanchit Chaturvedi (NYU–Courant). Host: Cole Graham

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title:

Abstract:

**December 9, 2024 (No seminar)**

The last day of class is Wednesday, December 11.

# Spring 2025

The first day of class is Tuesday, January 21, 2025.

**Feb 17, 2025**

Moon-Jin Kang (KAIST & UT-Austin). Host: Chanwoo Kim

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title:

Abstract:

**March 24, 2025 Spring Break**

**March 28-30, 2025**

Workshop on Kinetic Theory and Fluids, Host: Dallas Albritton, Chanwoo Kim

Format: in-person. VV 901.

Information: https://sites.google.com/view/madisonpde

**March 31, 2024**

Jin Woo Jang (POSTECH). Host: Chanwoo Kim

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title:

Abstract:

The last day of class is Friday, May 2, 2025.

## PDE GA Seminar Schedule Fall 2023-Spring 2024

# Spring 2024

The first day of class is Tuesday, January 23, 2023.

**January 23, 2024 (special date/time)**

Donghyun Lee (POSTECH). Host: Chanwoo Kim

Format: in-person. Time: 3:00-4:00PM, Location: 901 Van Vleck Hall

Title: DYNAMICAL BILLIARD AND A LONG-TIME BEHAVIOR OF THE BOLTZMANN EQUATION IN GENERAL 3D TOROIDAL DOMAINS

Abstract: Establishing global well-posedness and convergence toward equilibrium of the Boltzmann equation with specular reflection boundary condition has been one of the central questions in the subject of kinetic theory. Despite recent significant progress in this question when domains are strictly convex, the same question without the strict convexity of domains is still totally open in 3D. The major difficulty arises when a billiard map has an infinite number of bounces in a finite time interval or when the map fails to be Lipschitz continuous, both of which happen generically when the domain is non-convex. In this paper, we develop a new method to control a billiard map on a surface of revolution generated by revolving any planar analytic convex closed curve (e.g., typical shape of tokamak reactors’ chamber). In particular, we classify and measure the size (to be small) of a pullback set (along the billiard trajectory) of the infinite-bouncing and singular- bouncing cases. As a consequence, we solve the open question affirmatively in such domains. To the best of our knowledge, this work is the first construction of global solutions to the hard-sphere Boltzmann equation in generic non-convex 3-dimensional domains. This is joint work with Gyounghun Ko and Chanwoo Kim.

**January 29, 2024**

Minh-Binh Tran (TAMU). Host: Hung Tran

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Some Results On the Kinetic Theory for Classical and Quantum Waves

Abstract: Kinetic equations can be used to describe the dynamics of nonlinear classical and quantum waves out of thermal equilibrium, as well as the propagation of waves in a random medium. In this talk, I will present some of our recent results on the kinetic theory of waves. I will discuss the analysis of those kinetic equations for waves. Next, I will focus on the numerical schemes we have been developing to resolve those equations. I will also address some control problems concerning kinetic equations for waves. The last part is devoted to some physical applications of wave kinetic theory for Bose-Einstein Condensates.

**January 30, 2024 (special date/time)**

Jin Woo Jang (POSTECH). Host: Chanwoo Kim

Format: in-person. Time: 3:00-4:00PM, Location: 901 Van Vleck Hall

Title: On the Relativistic Boltzmann Equation with Long Range Interactions

Abstract: In this talk, I will discuss three recent, interrelated results concerning the special relativistic Boltzmann equation without angular cutoff. In the non-relativistic situation without angular cutoff, the change of variables from $v \to v'$ is a crux of the widely used "cancellation lemma". Firstly, in collaboration with James Chapman and Robert M. Strain, we calculate this very complex ten variable Jacobian determinant in the special relativisticsituation and illustrate some numerical results which show that it has a large number of distinct points where it is machine zero. Secondly, with Strain, we prove the sharp pointwise asymptotics for the frequency multiplier of the linearized relativistic Boltzmann collision operator that has not been previously established. As a consequence of these calculations, we further explain why the well known change of variables p \to p' is not well defined in the special relativisticcontext. Finally, also with Strain, we will present our recent proof of global-in-time existence and uniqueness of the solutions nearby the relativistic Maxwellian to the special relativistic Boltzmann equation without any angular cutoff and its asymptotic stability. We work in the case of a spatially periodic box. We assume the generic hard-interaction and mildly-soft-interaction conditions on the collision kernel that were derived by Dudyński and Ekiel-Jeżewska (in 1985). In this physical situation, the angular function in the collision kernel is not locally integrable, and the collision operator behaves like a fractional diffusion operator.

**February 5, 2024**

Thierry Laurens (UW-Madison)

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Continuum Calogero--Moser models

Abstract: The focusing CCM model is a dispersive equation that describes a continuum limit of a particle gas interacting pairwise through an inverse square potential. Recently, Gérard and Lenzmann discovered solutions to this equation that exhibit frequency cascades.

In this talk, we will discuss a scaling-critical well-posedness result for the focusing and defocusing CCM models on the line. In the focusing case, this requires solutions to have mass less than that of the soliton. This is joint work with Rowan Killip and Monica Visan.

**February 12, 2024**

Laurel Ohm (UW-Madison)

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Free boundary dynamics of an elastic filament in 3D Stokes flow

Abstract: We consider a free boundary problem for a thin elastic filament immersed in 3D Stokes flow. The 3D fluid is coupled to the quasi-1D filament dynamics via a novel type of angle-averaged Neumann-to-Dirichlet operator. Much of the difficulty in the analysis lies in understanding this operator. We show that the principal part of this NtD map is the corresponding operator about a straight, periodic filament, for which we derive an explicit symbol. It is then possible to establish local well-posedness for an immersed filament evolving via a simple elasticity law. This establishes a mathematical foundation for the myriad computational results based on slender body approximations for thin immersed elastic structures.

**February 19, 2024**

No seminar

**February 26, 2024**

Jiwoong Jang (UW-Madison)

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Periodic homogenization of geometric equations without perturbed correctors.

Abstract: Proving homogenization has been a subtle issue for geometric equations due to the discontinuity when the gradient vanishes. To overcome the difficulty and conclude homogenization, the work of Caffarelli-Monneau suggests a sufficient condition using perturbed correctors. However, some noncoercive equations do not satisfy this condition. In this talk, we discuss homogenization of geometric equations without using perturbed correctors, and we conclude homogenization for the noncoercive equations. Also, we derive a rate of periodic homogenization of coercive geometric equations by utilizing the fact that they remain coercive under perturbation. We also present an example that homogenizes with a rate Ω(ε| log ε|).

**March 4, 2024**

Yuming Paul Zhang (Auburn). Host: Hung Tran.

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Convergence of Policy Iteration for Deterministic Control

Abstract: We study the convergence of policy iteration for (deterministic) optimal control problems. To overcome the problem of ill-posedness due to lack of regularity, we consider both discrete and semi-discrete schemes by adding a viscosity term via finite differences in space. We prove that PI for the schemes converges exponentially fast, and provide a bound on the error induced by the schemes. If time permits, I will also discuss the convergence of exploratory Hamilton--Jacobi--Bellman (HJB) equations arising from the entropy-regularized exploratory control problem. These are joint works with Wenpin Tang, Hung Tran and Xunyu Zhou.

**March 5, 2024 (special date/time)**

Wei Xiang (City U. of Hong Kong). Host: Mikhail Feldman

Format: in-person. Time: 4:00-5:00PM, Location: VV 901

Title: Convexity, uniqueness, and stability of the regular shock reflection-diffraction problem.

Abstract: We will talk about our recent results on the convexity, uniqueness, and stability of regular reflection solutions for the potential flow equation in a natural class of self-similar solutions. The approach is based on a nonlinear version of the method of continuity.

**March 11, 2024**

Rajendra Beekie (Duke). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Uniform vorticity depletion and inviscid damping in the weakly viscous regime.

Abstract: In recent years there has been significant progress made in studying the long-time behavior of 2D Euler equations linearized around monotone shear flows. The key stabilizing mechanism is known as inviscid damping and is a consequence of the continuous spectrum of the linearized operator. When the shear flow has non-degenerate critical points, an important new effect, known as vorticity depletion, occurs. Relative to inviscid damping, less is known about vorticity depletion. Since these are both inviscid effects, a natural question is whether they persist in the presence of viscosity. In this talk, I will discuss how both inviscid damping and vorticity depletion can be shown to hold uniformly in the presence of sufficiently weak viscosity. The proof is based on a detailed resolvent analysis of the linearized 2D Navier-Stokes equations. Based on work in progress with Shan Chen (UMN) and Hao Jia (UMN).

**March 18, 2024**

No seminar, but Cole Graham (Brown) will give the Colloquium at 4 pm in VV B239, and it will concern PDEs.

**March 25, 2024**

Spring Break. No seminar.

**April 1, 2024**

Dominic Wynter (Cambridge). Host: Chanwoo Kim

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Shock Profiles for the Long-Range Boltzmann Equation

Abstract: The Boltzmann equation models gas dynamics in the low density or high Mach number regime, using a statistical description of molecular interactions. Shock wave solutions have been constructed for this equation for hard-sphere particle interactions and have recently been constructed for a related kinetic model of plasmas by Albritton, Bedrossian, and Novack. Along similar lines as these works, we construct traveling shock solutions for the Boltzmann equation when molecular interactions are long-range. We prove existence and uniqueness up to translation, near compressible Navier-Stokes traveling waves, using stability estimates for the Boltzmann equation and the stability theory of viscous shocks. Translation invariance gives a linearized operator with a one-dimensional kernel – we overcome this by stable/unstable dimensionality arguments on a discretized model, and thus construct our solution.

**April 8, 2024**

Fizay-Noah Lee (Vanderbilt). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title: Equilibrium Dynamics in Electroconvection

Abstract: In this talk I will give an overview of what is known about the long time dynamics of the Nernst-Planck system, which models electrodiffusion of charged particles. Specifically, I will talk about regimes where it is known that solutions dissipate to a unique steady state solution, and a special emphasis will be placed on the rate of convergence. In fact, in many physically relevant cases, there is either no rigorously established rate or there are very crude lower bounds on the rate. I will talk about recent work with Elie Abdo in which we make use of a logarithmic Sobolev inequality on bounded domains to establish "sharp" exponential rates of convergence for solutions of the Nernst-Planck system.

**April 11, 2024 (Thursday, 2:30 PM in VV901)**

Bjoern Bringman (Princeton) will give a Probability Seminar at 2:30 pm in VV 901, and it will concern PDEs.

**April 15, 2024**

Cancelled/Available

Format: in-person. Time: 3:30-4:30PM, VV 901.

Title:

**April 22, 2024**

Sarah Strikwerda (Penn). Host: Hung Tran.

Format: in-person. Time: 3:30-4:30PM, VV 901

Title: Multiscale coupling of Biot's equations and blood flow ODE

Abstract: Glaucoma is the leading cause of blindness worldwide and is treated by lowering intraocular pressure. However, some people who have high intraocular pressure do not develop glaucoma and others with average intraocular pressure develop glaucoma. We would like to understand the dynamics related to the development of glaucoma better through modeling. We consider an elliptic-parabolic coupled partial differential equation connected to a nonlinear ODE through an interface. The model describes fluid flowing through biological tissues with the ODE accounting for global features of blood flow that affect the tissue. We discuss a modeling choice on the interface coming from the fact that the ODE is 0D and the PDE is 3D and show existence of a solution to this problem using a fixed point method.

**April 29, 2024**

Joonhyun La (Princeton). Host: Dallas Albritton

Format: in-person. Time: 3:30-4:30PM, VV 901

Title: Local well-posedness and smoothing of MMT kinetic wave equation

Abstract: In this talk, I will prove local well-posedness of kinetic wave equation arising from MMT equation, which is introduced by Majda, Mclaughlin, and Tabak and is one of the standard toy models to study wave turbulence. Surprisingly, our result reveals a regularization effect of the collision operator, which resembles the situation of non-cutoff Boltzmann. This talk is based on a joint work with Pierre Germain (Imperial College London) and Katherine Zhiyuan Zhang (Northeastern).

*The last day of class is Friday, May 3, 2023.*

**May 17, 2024**

Jiaxin Jin (OSU). Host: Chanwoo Kim

Format: in-person. Time: 11AM-noon, Location: Van Vleck B223

Title: Nonlinear Asymptotic Stability of 3D Relativistic Vlasov-Poisson systems

Abstract: Motivated by solar wind models in the low altitude, we explore a boundary problem of the nonlinear relativistic Vlasov-Poisson systems in the 3D half space in the presence of a constant vertical magnetic field and strong background gravity with the inflow boundary condition. As the main result, we construct stationary solutions and establish their nonlinear dynamical asymptotic stability.

## PDE GA Seminar Schedule Fall 2022-Spring 2023

# Spring 2023

**January 30, 2023 **

Jingwen Chen (U Chicago)

Time: 3:30 PM -4:30 PM, in person in VV901

Title: Mean curvature flows in the sphere via phase transitions.

Abstract: In this talk, we will discuss some solutions of the mean curvature flow (MCF) of surfaces in the 3-sphere. We will recall a generalized notion of MCF introduced by Brakke in the 70s, as well as its regularization by a parabolic partial differential equation arising in the theory of phase transitions. We will talk about some existence problems for this parabolic equation, and use them to construct MCFs that join minimal surfaces of low area in the 3-sphere, and some recent progress on the spaces of MCFs using Morse-Bott theory.

This is joint work with Pedro Gaspar (Pontificia Universidad Católica de Chile).

**February 6, 2023**

**February 13, 2023**

Trinh Tien Nguyen (UW Madison)

Format: In person, Time: 3:30-4:30PM.

**Title:** The inviscid limit of Navier-Stokes for domains with curved boundaries

**Abstract:** Understanding fluids with small viscosity is one of the most fundamental problems in mathematical fluid dynamics. The problem remains open in general for domains with curved boundaries, due to boundary layers near the boundary and large vorticity in the inviscid limit. We introduce the framework that captures precisely the pointwise behavior of the vorticity for the Navier-Stokes equations on domains with boundaries, under the no-slip boundary conditions. With a deep understanding of the linear problem with a nonlocal boundary condition of vorticity on the half-space, we show that the inviscid limit holds for the fully nonlinear Navier-Stokes equations if the initial data is locally analytic near the boundary, on a general bounded domain, or an exterior of a disk.

**February 20, 2023**

Ovidiu Avadanei UC Berkeley

Format: Format: In person in Room VV901, Time: 3:30-4:30PM.

**Title: ** WELL-POSEDNESS FOR THE SURFACEQUASI-GEOSTROPHIC FRONT EQUATION

**Abstract: ** We consider the well-posedness of the surface quasi-geostrophic (SQG) front equation on the real line. Hunter-Shu-Zhang established well-posedness under a small data conditionas well as a convergence condition on an expansion of the equa-tion’s nonlinearity. In the present article, we establish uncondi-tional large data local well-posedness of the SQG front equationin the non-periodic case, while also improving the low regularitythreshold for the initial data. In addition, we establish global well-posedness theory in the rough data regime by using the testing bywave packet approach of Ifrim-Tataru.This is joint work with Albert Ai.

**February 27, 2023**

Yuxi Han (UW Madison)

Format: In person, Time: 3:30-4:30PM.

**Title:** Rate of Convergence in Periodic Homogenization for Convex Hamilton-Jacobi Equations with multiscales

**Abstract:** In reality, both macroscale and microscale variables are involved in various problems. But very often, we are only interested in macroscale behavior. Homogenization is the process of averaging out microscale behavior. In this talk, I will mainly focus on the rate of convergence in homogenization for convex Hamilton-Jacobi equations with multiscales. In particular, we show that for Cauchy problem, the rate of convergence is O(\sqrt{\epsilon}) and the power of \epsilon is optimal. This is a joint work with Jiwoong Jang.

**March 6, 2023**

Jinwoo Jang (Postech), Host: Chanwoo Kim

Format: In-person, Time: 3:30-4:30PM.

**Title:** Magnetic confinement for the 2D axisymmetric relativistic Vlasov-Maxwell system in an annulus

**Abstract:** This talk deals with the mathematical analysis of the magnetic confinement of the plasma via kinetic equations. We prove the global wellposedness of the Vlasov-Maxwell system in a two-dimensional annulus when a huge (but finite-in-time) external magnetic potential is imposed near the boundary. We assume that the solution is axisymmetric. The external magnetic potential well that we impose remains finite within a finite time interval and from that, we prove that the plasma never touches the boundary. In addition, we provide a sufficient condition on the magnitude of the external magnetic potential to guarantee that the plasma is confined in an annulus of the desired thickness which is slightly larger than the initial support. Our method uses the cylindrical coordinate forms of the Vlasov-Maxwell system. This is a joint-work with Robert M. Strain and Tak Kwong Wong.

**March 13, 2023**

**March 20, 2023**

Format: , Time:

**Title:**

**Abstract:**

**March 27, 2023**

Matt Jacobs (Purdue). Host: Hung Tran.

Format: In person, Time: 3:30-4:30PM.

**Title:** Lagrangian solutions to the Porous Media Equation (and friends)

**Abstract:** Many works have been devoted to understanding and predicting the time evolution of a growing population of cells (bacterial colonies, tumors, etc...). At the macroscopic scale, cell growth is typically modeled through Porous Media type equations that describe the change in cell density. While these cell growth PDEs have been studied since the 70s, our understanding is far from complete, particularly in the case where there are several distinct cell populations.

An important open question is whether it is possible for two populations that were separated at initial time to become mixed during the flow. For instance, can tumor cells get mixed into healthy cell regions?

In this talk, I will show that it is possible to construct non-mixing solutions to these equations. The key is to construct the Lagrangian flow map along the pressure gradient generated by the Porous Media Equation. The main obstruction is the fact that the pressure gradient is not sufficiently regular to apply any generic theory for Lagrangian flows. To overcome this difficulty, we develop a new argument combining features of the Porous Media Equation with the quantitative Lagrangian flow theory of Crippa and De Lellis.

**April 3, 2023**

Zhihan Wang (Princeton). Host: Sigurd Angenent

Format: In person, Time: 3:30-4:30PM.

**Title:** *Translating mean curvature flow with simple end.*

**Abstract:** Translators are known as candidates of Type II blow-up model for mean curvature flows. Various examples of mean curvature flow translators have been constructed in the convex case and semi-graphical case, most of which have either infinite entropy or higher multiplicity asymptotics near infinity. In this talk, we shall present the construction of a new family of translators with prescribed end. This is based on joint work with Ao Sun.

**April 10, 2023**

Gi-Chan Bae (Seoul Nat. University), Host: Chanwoo Kim

Format: In person, Time: 3:30-4:30PM.

**Title:** *Large amplitude solution of BGK model*

**Abstract:** Bhatnagar–Gross–Krook (BGK) equation is a relaxation model of the Boltzmann equation which is widely used in place of the Boltzmann equation for the simulation of various kinetic flow problems. In this work, we study the asymptotic stability of the BGK model when the initial data is not necessarily close to global equilibrium pointwisely. The main difficulty of the BGK equation comes from the highly nonlinear structure of the relaxation operator. To overcomes this issue, we derive refined control of macroscopic fields to guarantee the system enters a quadratic nonlinear regime, in which the highly nonlinear perturbative term relaxes to essentially quadratic nonlinearity after a long but finite time.
This is joint work with G.-H. Ko, D.-H. Lee and S.-B. Yun.

**April 17, 2023**

Jingrui Cheng (Stony Brook). Host: Misha Feldman.

Format: In person, Time: 3:30-4:30PM.

**Title:** *Interior W^{2,p} estimates for complex Monge-Ampere equations*

**Abstract:** The classical estimate by Caffarelli shows that a strictly convex solution to the real Monge-Ampere equations has W^{2,p} regularity if the right hand side is close to a constant. We partially generalize this result to the complex version, when the underlying solution is close to a smooth strictly plurisubharmonic function. The additional assumption we impose is related to the lack of Pogorelov type estimate in the complex case. The talk is based on joint work with Yulun Xu.

**April 24, 2023**

Ben Pineau (Berkeley)

Format: In person, Time: 3:30-4:30PM.

**Title: ** Sharp Hadamard local well-posedness for the incompressible free boundary Euler equations, rough solutions, and continuation criterion

**Abstract:** We provide a complete, optimal local well-posedness theory for the free boundary incompressible Euler equations on a connected fluid domain, in $H^s$ based spaces. We establish the following results: (i) local well-posedness in the Hadamard sense, i.e., local existence, uniqueness, and the first proof of continuous dependence on the data in low regularity Sobolev spaces; (ii) Enhanced uniqueness: our uniqueness result holds at the level of essentially the Lipschitz norm of the velocity and $C^{1,\frac{1}{2}}$ regularity of the free surface; (iii) stability bounds: Coupled with our uniqueness result are more general stability bounds; namely, we construct a nonlinear functional which measures in a suitable sense, the distance between two solutions (even when defined on different domains) and we show that this distance is propagated by the flow; (iv) we establish sharp, essentially scale invariant energy estimates for solutions by making use of a newly constructed family of refined elliptic estimates; (v) The first proof of a sharp continuation criterion in pointwise norms, at the level of scaling, showing essentially that solutions can be continued as long as the velocity is in $L_T^1W^{1,\infty}$ and the free surface is in $L_T^1C^{1,\frac{1}{2}}$; (vi) A novel proof of construction of regular solutions. Our entire approach is in the Eulerian framework and can be adapted to work in relatively general fluid domains. This is joint work in progress with Mihaela Ifrim, Daniel Tataru and Mitchell Taylor.

**May 1, 2023**

Chris Henderson (Arizona). Host: Chanwoo Kim

Time: 3:30 PM -4:30 PM, in person in VV901

**Title:**The Boltzmann equation with large data

**Abstract:** The Boltzmann equation is a nonlocal, nonlinear equation arising in gas dynamics for which (large data) global well-posedness is an extremely difficult problem that is nearly completely open. In this talk I will discuss a recent program to understand a more tractable, related question: what is the largest space in which local well-posedness holds and what quantities prevent blow-up at finite times? This program covers several papers and intertwines with a recent push to understand the regularity theory of kinetic Fokker-Planck-type equations. I will give a broad outline of the proof, and then, instead of slogging through the whole thing, I will focus on a simple proof of a technically important and physically interesting lower bound -- how ``vacuum regions* fill in. This is a joint work with Snelson and Tarfulea.*

**May 8, 2023**

Lei Wu (Lehigh). Host: Chanwoo Kim

Time: 3:30 PM -4:30 PM, in person in **VV B223 (special room)**

**Title:**Ghost Effect from Boltzmann Theory

**Abstract:**It is a classical and fundamental problem to study the hydrodynamic limits of kinetic equations as the Knudsen number \varepsilon\rightarrow 0. In this talk, we focus on the stationary Boltzmann equation with diffuse-reflection boundary condition where the Mach number is O(\varepsilon) but the temperature variance is O(1). We rigorously derived the limiting fluid system, the so-called ghost-effect equations, in which an infinitesimal boundary variance will have macroscopic observable effects. This pure kinetic effect reveals the incompleteness of traditional fluid theory. Our proof relies on newly developed kernel estimates with conservation laws and novel BV estimates for the cutoff boundary layer.

# Fall 2022

**September 12, 2022**

**September 20, 2022 (Tuesday)** joint PDE and Analysis Seminar

Andrej Zlatos (UCSD). Host: Hung Tran.

Format: in-person. Time: 4-5PM, VV B139.

Title: Homogenization in front propagation models

Abstract: Homogenization is a general principle that the dynamics of physical processes occurring in periodic or random environments often become effectively homogeneous in the long-time-large-scale limit. I will presents results showing that homogenization occurs for reaction-diffusion equations with both time-periodic-spatially-random and space-time-random KPP reactions and coefficients. These results rely on two crucial new tools: virtual linearity of KPP reaction-diffusion dynamics and a non-autonomous versions of Kingman’s subadditive ergodic theorem.

**September 26, 2022 **

Haotian Wu (The University of Sydney, Australia). Host: Sigurd Angenent.

Format: in person, Time: 3:30pm-4:30pm VV 901

__Title:__ *Mean curvature flow of noncompact hypersurfaces with Type-II curvature blow-up*

__Abstract:__ The mean curvature flow (MCF) deforms a hypersurface in the direction of its mean curvature vectors. Singularities in mean curvature flow can form in either finite or infinite time. We present some results concerning the precise asymptotics of non-compact MCF solutions with either Type-IIa (in finite time) or Type-IIb (in infinite time) curvature blow-up. This is based on joint works with Jim Isenberg (Oregon) and Zhou Zhang (Sydney).

**October 3, 2022**

Format: , Time:

Title:

Abstract:

**October 10, 2022**

Alexander Kiselev (Duke). Host: Sergey Denisov.

Date/time/place: Monday, October 10, 3:30-4:30 pm, VV 901 (if more space will be needed, we have VV B119 to migrate to).

Speaker: Sasha Kiselev (Duke)

Title: The flow of polynomial roots under differentiation Abstract: The question of how polynomial roots move under differentiation is classical. Contributions to this subject have been made by Gauss, Lucas, Marcel Riesz, Polya and many others. In 2018, Stefan Steinerberger derived formally a PDE that should describe the dynamics of roots under differentiation in certain situations. The PDE in question is of hydrodynamic type and bears a striking resemblance to the models used in mathematical biology to describe collective behavior and flocking of various species- such as fish, birds or ants. I will discuss joint work with Changhui Tan in which we establish global regularity of Steinerberger's equation and make a rigorous connection between its solutions and evolution of roots under differentiation for a class of trigonometric polynomials.

**October 17, 2022**

Nicolas Garca Trillos (Stats, UW Madison). Host: Hung Tran.

Format: in person, Time: 3:30pm-4:30pm VV 901

**Title:** Analysis of adversarial robustness and of other problems in modern machine learning.

**Abstract:** Modern machine learning methods, in particular deep learning approaches, have enjoyed unparalleled success in a variety of challenging application fields like image recognition, medical image reconstruction, and natural language processing. While a vast majority of previous research in machine learning mainly focused on constructing and understanding models with high predictive power, consensus has emerged that other properties like stability and robustness of models are of equal importance and in many applications essential. This has motivated researchers to investigate the problem of adversarial training (or how to make models robust to adversarial attacks), but despite the development of several computational strategies for adversarial training and some theoretical development in the broader distributionally robust optimization literature, there are still several theoretical questions about it that remain relatively unexplored. In this talk, I will take an analytical perspective on the adversarial robustness problem and explore three questions: 1)What is the connection between adversarial robustness and inverse problems?, 2) Can we use analytical tools to find lower bounds for adversarial robustness problems?, 3) How do we use modern tools from analysis and geometry to solve adversarial robustness problems? At its heart, this talk is an invitation to view adversarial machine learning through the lens of mathematical analysis, showcasing a variety of rich connections with perimeter minimization problems, optimal transport, mean field PDEs of interacting particle systems, and min-max games in spaces of measures.

**October 24, 2022**

Format: , Time:

Title:

Abstract:

**October 31, 2022 **

Yuan Gao (Purdue). Host: Hung Tran.

Format: in person, Time: 3:30pm-4:30pm VV 901

Title: A selection principle for weak KAM solutions via Freidlin-Wentzell large deviation principle of invariant measures

Abstract: We will give a gentle introduction of weak KAM theory and then reinterpret Freidlin-Wentzell's variational construction of the rate function in the large deviation principle for invariant measures from the weak KAM perspective. We will use one-dimensional irreversible diffusion process on torus to illustrate some essential concepts in the weak KAM theory such as the Peierls barrier, the projected Mather/Aubry/Mane sets. We provide alternative proofs for Freidlin-Wentzell's variational formulas for both self-consistent boundary data at each local attractors and for the rate function are formulated as the global adjustment for the boundary data and the local trimming from the lifted Peierls barriers. Based on this, we proved the rate function is a weak KAM solution to the corresponding stationary Hamilton-Jacobi equation satisfying the selected boundary data on projected Aubry set, which is also the maximal Lipschitz continuous viscosity solution. The rate function is the selected unique weak KAM solution and also serves as the global energy landscape of the original stochastic process. A probability interpretation of the global energy landscape from the weak KAM perspective will also be discussed.

**November 7, 2022 **

Beomjun Choi (Postech)

Format: in person, Time: 3:30pm-4:30pm VV 901

Title: Liouville theorem for surfaces translating by powers of Gauss curvature

Abstract: We classify the entire solutions to degenerate Monge-Ampere equations $\det D^2u = (1+|Du|^2)^\beta$ on $\mathbb{R}^2$ for all $\beta<0$. The graphs of such solutions are the translating solitons to the flows by sub-affine-critical powers of Gauss curvature. In view of the Legendre transformation, this classifies the entire solutions to $\det D^2v = (1+|x|^2)^{-\beta}$ as well.

For the affine-critical-case $\det D^2u =1$, the celebrated result by Jorgens, Calabi and Pogorelov shows every solution must be a convex paraboloid and hence the level sets are homothetic ellipses. In our case, the level sets of given solution converge to a circle or a curve with $k$-fold symmetry for some $k>2$. These curves are closed shrinking curves to the curve shortening flows, classified by B. Andrews in 2003. Then we study the moduli space of solutions for each prescribed asymptotics. This is a joint work with K. Choi and S. Kim.

**November 14, 2022 **

Adrian Tudorascu (West Virginia University). Host: Misha Feldman, Hung Tran.

Format: in person, Time: 3:30pm-4:30pm VV 901

Title: Sticky Particles with Sticky Boundary

Abstract: We study the pressureless Euler system in an arbitrary closed subset of the real line. The reflective boundary condition renders an ill-posed problem. Instead, we show that the sticky boundary condition is natural and yields a well-posed problem which is treated by means of sticky particles solutions, Lagrangian solutions and an appropriate (and natural) reflection principle.

**November 21, 2022 **

Jason Murphy (Missouri S&T)

Format: online

Join Zoom Meeting https://uwmadison.zoom.us/j/94877483456?pwd=cG9Ec1dhb2ErcXhFVW1aN2hCYXRBUT09 Meeting ID: 948 7748 3456 Passcode: 303105

Time: 3:30 PM -4:30 PM

Title: Sharp scattering results for the 3d cubic NLS

Abstract: I will discuss several sharp scattering results for three-dimensional cubic nonlinear Schrödinger equations, including both the free NLS and the NLS with an external potential. After reviewing the proof of scattering below the mass/energy ground state threshold, I will discuss some work on scattering at the threshold for NLS with repulsive potentials. The talk will discuss joint works with B. Dodson; R. Killip, M. Visan, and J. Zheng; and C. Miao and J. Zheng.

**November 28, 2022 **

No Seminar- Thanksgiving

**December 5, 2022 **

James Rowan (UC Berkeley)

Time: 3:30 PM -4:30 PM

Title: Solitary waves for infinite depth gravity water waves with constant vorticity

Abstract: We show that solitary waves exist for pure gravity water waves in infinite depth in the presence of constant (nonzero) vorticity. The proof relies on the fact that this particular water-wave system is well-approximated by the Benjamin-Ono equation, which also allows a description of the profile of the solitary wave in terms of the Benjamin-Ono soliton. This is joint work with Lizhe Wan.

**December 12, 2022 **

Calum Rickard UC Davis

Format: in-person in room VV901

Time: 3:00 PM -4:00 PM

Title: An infinite class of shocks for compressible Euler

Abstract: We consider the two dimensional compressible Euler equations with azimuthal symmetry and construct an infinite class of shocks by establishing shock formation for a new Hölder family of so-called pre-shocks for all nonnegative integers. Moreover, a precise description of the dominant Riemann variable in the Hölder space is given in the form of a fractional series expansion. This is joint work with Sameer Iyer, Steve Shkoller and Vlad Vicol.

### Schedule for Fall 2021-Spring 2022

## PDE GA Seminar Schedule Fall 2021-Spring 2022

# Spring 2022

**January 31th, 2022.**

**February 7th, 2022.**

Jonah Duncan from John Hopkins University ; Format: online seminar via Zoom (see link above), Time: 3:30-4:30PM

Title: Estimates and regularity for the k-Yamabe equation

Abstract: The k-Yamabe problem is a fully nonlinear generalisation of the Yamabe problem, concerned with finding conformal metrics with constant k-curvature. In this talk, I will start by introducing the k-Yamabe problem, including a brief survey of established results and open problems. I will then discuss some recent work (joint with Luc Nguyen) on estimates and regularity for the k-Yamabe equation, addressing solutions in both the so-called positive and negative cones.

**February 14th, 2022.**

Sigurd Angenent; Format: online seminar via Zoom/ in person, Room:910, Time:3:30PM

Title: MCF after the Velázquez—Stolarski example.

Abstract: Velázquez (1995) constructed an example of a Mean Curvature Flow $M_t\subset\mathbb R^8$, $(-1<t<0)$ that blows up at the origin as $t\nearrow0$. Stolarski recently showed that in spite of the singularity the mean curvature on this solution is uniformly bounded. In joint work with Daskalopoulos and Sesum we constructed an extension of the Velázquez—Stolarski solution to positive times and show that it also has uniformly bounded mean curvature. In the talk I will describe the solutions and explain some of the ideas that show boundedness of the mean curvature.

**February 21th, 2022.**

Birgit Schoerkhuber; Format: online seminar via Zoom https://uwmadison.zoom.us/j/96354681353?pwd=SGlwUW1ockp6YklYYlppbDFZcW8zdz09

Time: 11:00 AM

Title: Nontrivial self-similar blowup in energy supercritical nonlinear wave equations

Abstract: Self-similar solutions play an important role in the dynamics of nonlinear wave equations as they provide explicit examples for finite-time blowup. This talk will be concerned with the focusing cubic and the quadratic wave equation, respectively, in dimensions where the models are energy supercritical. For both equations, we present new non-trivial self-similar solutions, which are completely explicit in all supercritical dimensions. Furthermore, we outline methods to analyse their stability. This involves a delicate spectral problem that we are able to solve rigorously in particular space dimensions. In these cases, we prove that the solutions are co-dimension one stable modulo translations. This is joint work with Irfan Glogić (Vienna) and Elek Csobo (Innsbruck).

**February 28th, 2021.**

Michael Hott; Format: online seminar via Zoom https://uwmadison.zoom.us/j/96354681353?pwd=SGlwUW1ockp6YklYYlppbDFZcW8zdz09

Time:3:30PM-4:30PM

Title: On quantum Boltzmann fluctuation dynamics at the presence of a BEC

Abstract: Boltzmann equations have served to describe transport properties in many instances. While usually heuristically justified by means of Markov processes, mathematically rigorous derivations from first principles started arising with the landmark works of Lanford and Cercignani, and there have been many important improvements ever since. To this day, it remains a very active mathematical and physical field. Starting with the Liouville-von Neumann equation for a weakly interacting highly condensed Bose gas in a finite periodic box, we will uncover a Boltzmann dynamics after identifying other dominating effects. Our work exhibits some parallels with a previous discussion by Zaremba-Niguni-Griffin. However, we will present an analytic dependence on physical parameters for the size of the individual terms in the expansion, for the size of errors, for the time of validity, as well as among physical parameters. We will also see how mathematical rigor uncovers important physical subtleties missed in the physical literature. Our work is the first rigorous derivation of a quantum Boltzmann equation from first principles. This is a joint work with Thomas Chen.

**March 7th, 2022.**

**March 14th, 2022.**

**March 21th, 2022.**

**March 28th, 2022.**

Monica Visan; Format: online seminar via Zoom, Time: 3:30PM-4:30PM

https://uwmadison.zoom.us/j/96354681353?pwd=SGlwUW1ockp6YklYYlppbDFZcW8zdz09

Meeting ID: 963 [1353|5468 1353] Passcode: 180680

Title: Determinants, commuting flows, and recent progress on completely integrable systems

Abstract: I will survey a number of recent developments in the theory of completely integrable nonlinear dispersive PDE. These include a priori bounds, the orbital stability of multisoliton solutions, well-posedness at optimal regularity, and the existence of dynamics for Gibbs distributed initial data. I will describe the basic objects that tie together these disparate results, as well as the diverse ideas required for each problem.

**April 4th, 2022.**

Marcelo Disconzi; Format: online seminar via Zoom/ in person, Room: 901, Time: 3:30PM

Title: General-relativistic viscous fluids.

Abstract: The discovery of the quark-gluon plasma that forms in heavy-ion collision experiments provides a unique opportunity to study the properties of matter under extreme conditions, as the quark-gluon plasma is the hottest, smallest, and densest fluid known to humanity. Studying the quark-gluon plasma also provides a window into the earliest moments of the universe, since microseconds after the Big Bang the universe was filled with matter in the form of the quark-gluon plasma. For more than two decades, the community has

intensely studied the quark-gluon plasma with the help of a rich interaction between experiments, theory, phenomenology, and numerical simulations. From these investigations, a coherent picture has emerged, indicating that the quark-gluon plasma behaves essentially like a relativistic liquid with viscosity. More recently, state-of-the-art numerical simulations strongly suggested that viscous and dissipative effects can also have non-negligible effects on gravitational waves produced by binary neutron star mergers. But despite the importance of viscous effects for the study of such systems, a robust and mathematically sound theory of relativistic fluids with viscosity is still lacking. This is due, in part, to difficulties to preserve causality upon the inclusion of viscous and dissipative effects into theories of relativistic fluids. In this talk, we will survey the history of the problem and report on a new approach to relativistic viscous fluids that addresses these issues.

**April 11th, 2022.**

Dallas Albitron; Format: online seminar via Zoom, Time: 3:30PM-4:30PM

https://uwmadison.zoom.us/j/96354681353?pwd=SGlwUW1ockp6YklYYlppbDFZcW8zdz09

Meeting ID: 963 [1353|5468 1353] Passcode: 180680

Title: Non-uniqueness of Leray solutions of the forced Navier-Stokes equations

Abstract: In a seminal work, Leray demonstrated the existence of global weak solutions to the Navier-Stokes equations in three dimensions. Are Leray's solutions unique? This is a fundamental question in mathematical hydrodynamics, which we answer in the negative, within the `forced' category, by exhibiting two distinct Leray solutions with zero initial velocity and identical body force. This is joint work with Elia Brué and Maria Colombo

**April 18th, 2022.**

Loc Nguyen (UNCC); Format: in-person seminar, Room: 901, Time: 3:30PM-4:30PM. Host: Hung Tran.

Title: The Carleman-based convexification approach for the 3D inverse scattering problem with experimental data.

Abstract: We study the inverse scattering problem for the three-dimensional Helmholtz equation with multi-frequency back scattering data. Our approach relies on a new derivation of a boundary value problem for a system of coupled quasi-linear elliptic partial differential equations. We solve this coupled system by developing the Carleman convexification method. Using the Carleman weight function, we construct a globally strictly convex cost functional and prove the global convergence to the exact solution of the gradient projection method. The Lipschitz stability estimate of the Carleman convexification method is proved also via a Carleman estimate. Finally, our theoretical finding is verified via several numerical tests with computationally simulated data and experimental data. These tests demonstrate that we can accurately recover all three important components of targets of interest: locations, shapes, and dielectric constants.

**April 25th, 2022.**

**May 2nd, 2022.**

Alexei Gazca; Format: online seminar via Zoom, Time:3:30PM -4:30PM

Title: Heat-conducting Incompressible Fluids and Weak-Strong Uniqueness

Abstract: In this talk, I will present some recent results obtained in collaboration with V. Patel (Oxford) in connection with a system describing a heat-conducting incompressible fluid. I will introduce the notion of a dissipative weak solution of the system and highlight the connections and differences to the existing approaches in the literature. One of the advantages of the proposed approach is that the solution satisfies a weak-strong uniqueness principle, which guarantees that the weak solution will coincide with the strong solution, as long as the latter exists; moreover, the solutions are constructed via a finite element approximation, leading (almost, not quite) to the first convergence result for the full system including viscous dissipation.

# Fall 2021

**September 20th, 2021.**

Simion Schulz (UW Madison); Format: in-person seminar, Room: 901, Time: 3:30PM-4:30PM

Title: Existence and regularity for a system of porous medium equations with small cross-diffusion and nonlocal drifts

Abstract: We prove existence and Sobolev regularity of solutions of a nonlinear system of degenerate parabolic PDEs with self- and cross-diffusion, transport/confinement and nonlocal interaction terms. The macroscopic system of PDEs is formally derived from a large particle system and models the evolution of an arbitrary number of (e.g. biological) species with quadratic porous medium interactions in a bounded domain of arbitrary dimension. The cross-interactions are scaled by a coefficient on which a necessary smallness condition is imposed. The strategy of our proof relies on a fixed point argument, followed by a vanishing viscosity scheme. This is joint work with Maria Bruna (Cambridge), Luca Alasio (Paris VI), and Simone Fagioli (Università degli Studi dell'Aquila).

**September 27th, 2021.**

Dohyun Kwon (UW Madison); Format: in-person seminar, Room: 901, Time: 3:30PM-4:30PM

Title: Volume-preserving crystalline and anisotropic mean curvature flow

Abstract: We consider the global existence of volume-preserving crystalline mean curvature flow in a non-convex setting. We show that a natural geometric property, associated with reflection symmetries of the Wulff shape, is preserved with the flow. Using this geometric property, we address the global existence and regularity of the flow for smooth anisotropies. For the non-smooth case, we establish global existence results for the types of anisotropies known to be globally well-posed. This is joint work with Inwon Kim (UCLA) and Norbert Požár (Kanazawa University).

**October 4th, 2021.**

Antoine Remind-Tiedrez (UW Madison); Format: in-person seminar, Room: 901, Time: 3:30PM-4:30PM

Title: Variational formulation, well-posedness, and iterative methods for moist potential vorticity inversion: a nonlinear PDE from atmospheric dynamics with free boundaries

Abstract: To describe the atmosphere on a synoptic scale (the scale at which high- and low-pressure systems are apparent on a weather map, for example) one may use the quasi-geostrophic equations, which are derived as a limit of the classical Boussinesq system under the assumptions of fast rotation and strong stratification. When incorporating the dynamics of water content in the atmosphere, a.k.a. moisture, one may then study the moist Boussinesq equations and its limit, the precipitating quasi-geostrophic equations.

These models are important for atmospheric scientists in light of the role that the water cycle plays in atmospheric dynamics, notably through energy budgeting (such as for example when atmospheric circulations are driven by laten heat release in storms). Mathematically, these models present interesting challenges due to the presence of boundaries, whose locations are a priori unknown, between phases saturated and unsaturated in water (schematically: boundaries between clouds and their surroundings).

In particular, while the (dry) quasi-geostrophic equations rely on the inversion of a Laplacian, this becomes a much trickier adversary in the presence of free boundaries. In this talk we will discuss how this nonlinear equation underpinning the precipitating quasi-geostrophic equations can be characterized using a variational formulation and we will describe the many benefits one may derive from this formulation.

**October 11th, 2021.**

**October 18th, 2021.**

Wojciech Ozanski (USC); Format: online seminar via Zoom, Room:--, Time: 3:30PM-4:30PM

Title: Well-posedness of logarithmic spiral vortex sheets.

Abstract: We will discuss a family of 2D logarithmic spiral vortex sheets which include the celebrated spirals introduced by Prandtl (Vorträge aus dem Gebiete der Hydro- und Aerodynamik, 1922) and by Alexander (Phys. Fluids, 1971). We will discuss a recent result regarding a complete characterization of such spirals in terms of weak solutions of the 2D incompressible Euler equations. Namely, we will explain that a spiral gives rise to such solution if and only if two conditions hold across every spiral: a velocity matching condition and a pressure matching condition. Furthermore we show that these two conditions are equivalent to the imaginary part and the real part, respectively, of a single complex constraint on the coefficients of the spirals. This in particular provides a rigorous mathematical framework for logarithmic spirals, an issue that has remained open since their introduction by Prandtl in 1922, despite significant progress of the theory of vortex sheets and Birkhoff-Rott equations. We will also show well-posedness of the symmetric Alexander spiral with two branches, despite recent evidence for the contrary. Our main tools are new explicit formulas for the velocity field and for the pressure function, as well as a notion of a winding number of a spiral, which gives a robust way of localizing the spirals' arms with respect to a given point in the plane. This is joint work with P. Kokocki and T. Cieślak.

**October 25th, 2021.**

Maxwell Stolarski (ASU); Format: in person seminar, Room: 901, Time: 3:30pm-4:30pm

Title: Mean Curvature Flow Singularities with Bounded Mean Curvature

Abstract: Hypersurfaces moving by mean curvature flow often become singular in finite time. At this time, the flow may no longer be continued smoothly. The extension problem asks, "If M(t) is a solution to mean curvature flow defined up to time T, what conditions ensure that we may smoothly extend this solution to slightly later times?" For example, a result of Huisken says that if the 2nd fundamental forms of the evolving hypersurfaces remain uniformly bounded, then the mean curvature flow can be smoothly extended. One might then ask if a uniform bound on the mean curvature suffices to extend the flow. We'll discuss work that shows the answer is "no" in general, that is, there exist mean curvature flow solutions that become singular in finite time but have uniformly bounded mean curvature.

**November 1th, 2021.**

Lizhe Wan (UW Madison); Format: in-person seminar, Room: 901, Time: 3:30PM-4:30PM

Title: The Benjamin-Ono approximation for 2D gravity water waves with constant vorticity

Abstract: This article is concerned with infinite depth gravity water waves with constant vorticity in two space dimensions. We consider this system expressed in position-velocity potential holomorphic coordinates. We show that, for low-frequency solutions, the Benjamin-Ono equation gives a good and stable approximation to the system on the natural cubic time scale. The proof relies on refined cubic energy estimates and perturbative analysis.

**November 8th, 2021.**

Albert Ai (UW Madison);

Title: Well-posedness for the dispersion-generalized Benjamin-Ono equation

Abstract: In this talk we will consider the Cauchy problem for both low and high dispersive generalizations of the Benjamin-Ono equation. To address the nonlinear interactions, we use a pseudodifferential generalization of the gauge transform introduced by Tao for the original Benjamin-Ono equation. Further, we combine this with a paradifferential normal form. This approach allows for a much simpler functional setting, and improves the known low regularity well-posedness threshold across the range of the dispersive generalization. This is joint work with Grace Liu.

**November 15th, 2021.**

Sebastien Herr (Bielefeld University); Format: online seminar via Zoom, Time:10 AM

Please observe the time change!

Zoom Link: Register in advance for this meeting: https://uwmadison.zoom.us/meeting/register/tJcpcuqqqjMjE9VJ_-SaJ0gc6kS10CCTQTVP

Title: Global wellposedness for the energy-critical Zakharov system below the ground state

Abstract: The Zakharov system is a quadratically coupled system of a Schroedinger and a wave equation, which is related to the focussing cubic Schroedinger equation. We consider the associated Cauchy problem in the energy-critical dimension d=4 and prove that global well-posedness holds in the full (non-radial) energy space for any initial data with energy and wave mass below the ground state threshold. The result is based on a uniform Strichartz estimate for the Schr\"odinger equation with potentials solving the wave equation. A key ingredient in the non-radial setting is a bilinear Fourier extension estimate.

**November 22th, 2021.**

**November 29th, 2021.**

**December 6th, 2021.**

William Cooperman (University of Chicago); Format: in-person seminar, Room: 901, Time: 3:30PM-4:30PM. Host: Hung Tran.

Title: Quantitative homogenization of Hamilton-Jacobi equations

Abstract: We are interested in the rate at which solutions to a Hamilton-Jacobi equation converge, in the large-scale limit, to the solution of the effective problem. We'll describe prior work in various settings where homogenization occurs (periodic or random in space, coercive or only "coercive on average" in momentum as in the G equation). We'll also use a theorem of Alexander, originally proved in the context of first-passage percolation, to improve the rate of convergence when an optimal control formulation is available (for example, in the G equation or when the Hamiltonian is convex and coercive).

**December 13th, 2021.**

## PDE GA Seminar Schedule Fall 2020-Spring 2021

Welcome to the new mode of our PDEGA seminar this semester. Each week, we'll introduce to you two talks that are interesting and related to our interests. As the videos are already on Youtube or other platforms, you could choose to watch them whenever you want to; our goal here is merely to pick our favorite ones out of thousands of already available recorded talks.

**Week 1 (9/1/2020-9/5/2020)**

1. Paul Rabinowitz - The calculus of variations and phase transition problems. https://www.youtube.com/watch?v=vs3rd8RPosA

2. Frank Merle - On the implosion of a three dimensional compressible fluid. https://www.youtube.com/watch?v=5wSNBN0IRdA&feature=youtu.be

**Week 2 (9/6/2020-9/12/2020)**

1. Yoshikazu Giga - On large time behavior of growth by birth and spread. https://www.youtube.com/watch?v=4ndtUh38AU0

2. Tarek Elgindi - Singularity formation in incompressible fluids. https://youtu.be/29zUjm7xFlI

**Week 3 (9/13/2020-9/19/2020)**

1. Eugenia Malinnikova - Two questions of Landis and their applications. https://www.youtube.com/watch?v=lpTsW1noWTQ

2. Pierre Germain - On the derivation of the kinetic wave equation. https://youtu.be/ZbCjKwQ3KcE

**Week 4 (9/20/2020-9/26/2020)**

1. Robert M. Strain - Global mild solutions of the Landau and non-cutoff Boltzmann equation. https://www.youtube.com/watch?v=UWrCItk2euo&feature=youtu.be

2. Elena Kosygina - Stochastic homogenization of a class of nonconvex viscous HJ equations in one space dimension https://www.youtube.com/watch?v=tVZv0ftT3PM

**Week 5 (9/27/2020-10/03/2020)**

1. Isabelle Gallagher - From Newton to Boltzmann, fluctuations and large deviations. https://www.youtube.com/watch?v=BkrKkUVadDo

2. Connor Mooney - The Bernstein problem for elliptic functionals, https://www.youtube.com/watch?v=lSfnyfCL74c

**Week 6 (10/04/2020-10/10/2020)**

1. Felix Otto - The thresholding scheme for mean curvature flow and De Giorgi's ideas for gradient flows. https://www.youtube.com/watch?v=7FQsiZpQA7E

2. Inwon Kim - Evolution of star-shaped sets in Mean curvature flow with forcing http://www.birs.ca/events/2018/5-day-workshops/18w5033/videos/watch/201806190900-Kim.html

**Week 7 (10/11/2020-10/17/2020)**

1. Benoit Perthame - Multiphase models of living tissues and the Hele-Shaw limit. https://www.youtube.com/watch?v=UGVJnJCfw5s

2. Yifeng Yu - Properties of Effective Hamiltonians. https://www.youtube.com/watch?v=U06G4wjF-Hg

**Week 8 (10/18/2020-10/24/2020)**

1. Carlos Kenig - Asymptotic simplification for solutions of the energy critical nonlinear wave equation. https://youtu.be/jvzUqAxU8Xg

2. Kyeongsu Choi - Ancient mean curvature flows and singularity analysis. https://www.youtube.com/watch?v=Iu1iLjdFjKQ

Virtual Analysis and PDE Seminar (VAPS): https://sites.uci.edu/pdeonlineseminar/. First talk by Ovidiu Savin.

**Week 9 (10/25/2020-10/31/2020)**

1. John Ball - Some energy minimization problems for liquid crystals. https://www.youtube.com/watch?v=-j0jc-y7JzE

2. Tristan Buckmaster - Stable shock wave formation for the isentropic compressible Euler equations. https://stanford.zoom.us/rec/play/DwuT8rE-K1uJC0LghYPtsoaNmPBk9-P5EK4ZeWh1mVNJELRHn-ay-gOVXHSTRz_0X3iUZDBoUVYq8zfd.Tuqy8urKY4jESivm?continueMode=true&_x_zm_rtaid=GiRX307iT7encyYgIEgh9Q.1603308889393.b4a9b3af5c64cc9ca735cffbe25d8b7b&_x_zm_rhtaid=764

**Week 10 (11/1/2020-11/7/2020)**

1. Sylvia Serfaty - Mean-Field limits for Coulomb-type dynamics. https://www.youtube.com/watch?v=f7iSTnAe808&feature=youtu.be

2. Luc Nguyen - Symmetry and multiple existence of critical points in 2D Landau-de Gennes Q-tensor theory http://www.birs.ca/events/2017/5-day-workshops/17w5110/videos/watch/201705041518-Nguyen.html

**Week 11 (11/8/2020-11/14/2020)**

1. Andrzej Święch - Finite dimensional approximations of Hamilton-Jacobi-Bellman equations in spaces of probability measures https://www.youtube.com/watch?v=KC514krtWAc

2. Alexandru Ionescu - On the nonlinear stability of shear flows and vortices, https://youtu.be/Zt_Izzi87V0

**Week 12 (11/15/2020-11/21/2020)**

1. Irene M. Gamba - Boltzmann type equations in a general framework: from the classical elastic flow, to gas mixtures, polyatomic gases, and more, https://youtu.be/fPlhAMGULtY

2. Andrej Zlatos - Euler Equations on General Planar Domains, https://www.youtube.com/watch?v=FdyyMZirRwk

**Week 13 (11/22/2020-11/28/2020)**

1. Camillo De Lellis - Flows of vector fields: classical and modern, https://www.youtube.com/watch?v=dVXSC3rtvok&feature=youtu.be

2. Wilfrid Gangbo - Analytical Aspect of Mean Field Games (Part 1/2), https://www.youtube.com/watch?v=KI5n6OYzzW8

**Week 14 (11/29/2020-12/5/2020)**

1. Juan Dávila - Leapfrogging vortex rings and other solutions with concentrated vorticity for the Euler equations, https://youtu.be/xfAKGc0IEUw

2. Yao Yao - Aggregation-diffusion equation: symmetry, uniqueness and non-uniqueness of steady states, https://www.youtube.com/watch?v=C_4qCimIMYc

**Week 15 (12/6/2020-12/12/2020)**

1. Pierre Gilles Lemarié-Rieusset - On weak solutions of the Navier-Stokes equations with infinite energy, https://www.youtube.com/watch?v=OeFJ6r-GLJc&feature=youtu.be

2. Albert Fathi - Weak KAM Theory: the connection between Aubry-Mather theory and viscosity solutions of the Hamilton-Jacobi equation, https://www.youtube.com/watch?v=0y8slhbQlTU

**Spring 2021**

**Week 1 (1/31/2021- 2/6/2021)**

1. Emmanuel Grenier - instability of viscous shear layers https://www.youtube.com/watch?v=0_EG4VWIYvU&feature=youtu.be

2. Robert Pego - Dynamics and oscillations in models of coagulation and fragmentation https://www.youtube.com/watch?v=3712lImYP84

**Week 2 ( 2/7/2021- 2/13/2021)**

1. Ryan Hynd, The Hamilton-Jacobi equation, past and present https://www.youtube.com/watch?v=jR6paJf7aek

2. Jacob Bedrossian - Chaotic mixing of the Lagrangian flow map and the power spectrum of passive scalar turbulence in the Batchelor regime https://youtu.be/3lNQNsdlGTE

Colloquium (2/12/2021): Bobby Wilson (University of Washington). More information can be found here http://www.math.wisc.edu/wiki/index.php/Colloquia.

**Week 3 ( 2/14/2021- 2/20/2021)**

1. Diogo A. Gomes - Monotone MFGs - theory and numerics https://www.youtube.com/watch?v=lj1L7AHHY3s

2. Hao Jia - nonlinear asymptotic stability in two dimensional incompressible Euler equations https://youtu.be/KMf7K2sTLXg

**Week 4 ( 2/21/2021- 2/27/2021)**

1. Anne-Laure Dalibard - Boundary layer methods in semilinear fluid equations https://www.msri.org/workshops/944/schedules/29309

2. Gui-Qiang G. Chen - On Nonlinear PDEs of Mixed Elliptic-Hyperbolic Type: Analysis and Connections https://www.youtube.com/watch?v=W3sa-8qtw68

**Week 5 ( 2/28/2021- 3/6/2021)**

1. Inwon Kim - A variational scheme for Navier-Stokes Equations https://www.msri.org/workshops/944/schedules/29317

2. Robert L. Jerrard - Solutions of the Ginzburg–Landau equatons with vorticity concentrating near a nondegenerate geodesic https://www.youtube.com/watch?v=M0NQh2PET_k

**Week 6 (3/7/2021-3/13/2021)**

1. Ondřej Kreml - Non-uniqueness of admissible weak solutions to the compressible Euler equations with smooth initial datas https://www.birs.ca/events/2020/5-day-workshops/20w5188/videos/watch/202011231027-Kreml.html

2. Rita Ferreira - Homogenization of a stationary mean-field game via two-scale convergence https://www.youtube.com/watch?v=EICMVmt5o9c

**Week 7 (3/14/2021-3/20/2021)**

1. Sergey Denisov - Small scale formation in 2D Euler dynamics https://www.youtube.com/watch?v=7ffUgTC34tM

2. Alexis Vasseur - Instability of finite time blow-ups for incompressible Euler https://www.birs.ca/events/2020/5-day-workshops/20w5188/videos/watch/202011231000-Vasseur.html

**Week 8 (3/21/2021- 3/27/2021)**

1. Peter Sternberg - Variational Models for Phase Transitions in Liquid Crystals Based Upon Disparate Values of the Elastic Constants https://www.youtube.com/watch?v=4rSPsDvkTYs

2. François Golse - Half-space problem for the Boltzmann equation with phase transition at the boundary https://mysnu-my.sharepoint.com/:v:/g/personal/bear0117_seoul_ac_kr/ETGjasFQ7ylHu04qUz4KomYB98uMHLd-q96DOJGwbbEB0A

**Week 9 (3/28/2021- 4/3/2021)**

1. Susan Friedlander - Kolmogorov, Onsager and a stochastic model for turbulence https://www.youtube.com/watch?v=xk3KZQ-anDM

2. Sergei Chernyshenko - Auxiliary functionals: a path to solving the problem of turbulence https://www.youtube.com/watch?v=NrF7n3MyCy4&list=PLf_ipOSbWC86n18q4JMn_1J04S90FpdeE&index=9

**Week 10 (4/4/2021- 4/10/2021)**

1. Camillo De Lellis - Transport equations and ODEs with nonsmooth coefficients https://www.msri.org/workshops/945/schedules/29235

2. Weinan E - PDE problems that arise from machine learning https://www.youtube.com/watch?v=5rb8DJkxfg8

**Week 11(4/11/2021- 4/17/2021)**

1. Marian Gidea - Topological methods and Hamiltonian instability https://youtu.be/aMN7zJZavDo

2. David Gerard-Varet - On the effective viscosity of suspensions http://www.birs.ca/events/2020/5-day-workshops/20w5188/videos/watch/202011230644-Gerard-Varet.html

**Week 12(4/18/2021- 4/24/2021)**

1. Takis Souganidis - Phase-field models for motion by mean curvature - 1 https://www.youtube.com/watch?v=fH8ygVAZm-0

2. Nader Masmoudi - Inviscid Limit and Prandtl System, https://youtu.be/tLg3HwVDlOo

**Week 13(4/25/2021- 5/1/2021)**

1. James Stone - Astrophysical fluid dynamics https://youtu.be/SlPSa37QMeI

2. Stefania Patrizi - Chaotic Orbits for systems of nonlocal equations http://www.birs.ca/events/2017/5-day-workshops/17w5116/videos/watch/201704050939-Patrizi.html

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## Abstracts

Title:

Abstract: