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Wed Sep 11

PDE Seminar

3:35pm - Vincent Hall 570
Parabolic problems with rough coefficients
Pierre Portal,, Australian National University

Using form methods, one can solve linear parabolic PDE in divergence form with $L^2$ data in appropriate energy spaces, even when the coefficients are merely bounded measurable in time and space, and no maximum principle is available. This goes back, at least, to the work of Lions and his school in the 1950s. When dealing with $L^p$ data, it is not so clear which $L^p$ like solution space one should use as a replacement of Lions’ energy space. Depending on the choice, one can solve, for instance, time dependent single equations (Aronson 1968), time independent systems for a range of values of $p$ (Auscher 2005), or stochastic problems with some spatial regularity (starting with Krylov 1994).
In this talk, I explain that, by choosing tent spaces as solution spaces (inspired by their role in elliptic boundary value problems on rough domains), one gets fairly general results, including both deterministic and stochastic problems, all values of $p$, systems as well as single equations, and rough coefficients in both time and space.

This summarises joint works with Pascal Auscher, Sylvie Monniaux, Jan van Neerven, and Mark Veraar.

Wed Sep 25

PDE Seminar

3:35pm - Vincent Hall 570
Random Tug of War games for the p-Laplacian
Marta Lewicka, University of Pittsburgh

We propose a new dynamic programming principle related to the Dirichlet problem for the homogeneous p-Laplace equation in connection with the Tug of War games with noise. We also discuss similar approximations in presence of the Robin boundary conditions. For the proofs, we use martingale techniques involving various couplings of random walks and yielding estimates on the involved probabilistic representations.

Wed Oct 09

PDE Seminar

3:35pm - Vincent Hall 570
Diffusion in the Mean for a Periodic Schrödinger Equation Perturbed by a Fluctuating Potential
Shiwen Zhang, UMN

We consider the solution to a tight-binding, periodic Schrödinger equation with a random potential evolving stochastically in time. If the potential evolves according to a stationary Markov process we obtain a positive, finite diffusion constant for the evolution of the solution. More generally, we show that the square amplitude of the wave packet, after diffusive rescaling, converges to a solution of the heat equation. (Joint work with J. Schenker and Z. Tilocco at the Michigan State University).

Wed Oct 16

PDE Seminar

3:35pm - Vincent Hall 570
Optimal local well-posedness for the derivative nonlinear Schrodinger's equation
Yu Deng, University of Southern California

In joint work with Andrea Nahmod and Haitian Yue, we prove local well-posedness for the derivative nonlinear Schrodinger's equation in Fourier-Lebesgue space which has the same scaling as H^s for any s>0. This closes the gap left open by the work of Grunrock-Herr where s>1/4. Here there is no trilinear estimate in any standard function space, instead we will construct the solution in a nonlinear submanifold (of a function space) by exploiting its structure. This is somehow inspired by the theory of para-controlled distributions that Gubinelli et al. developed for stochastic PDEs, but our arguments are purely deterministic.

Wed Nov 13

PDE Seminar

3:30pm - Vincent Hall 570
Serrin Lecture - Localization for the Anderson-Bernoulli model on the integer lattice
Charles Smart, University of Chicago

I will give a brief mathematical introduction to Anderson localization followed by a discussion of my recent work with Jian ding. In our work we establish localization near the edge for the Anderson Bernoulli model on the two dimensional lattice. Our proof follows the program of Bourgain--Kenig and uses a new unique continuation result inspired by Buhovsky--Logunov--Malinnikova--Sodin. I will also discuss recent work of by Li and Zhang on the three dimensional case.

Wed Nov 20

PDE Seminar

3:35pm - Vincent Hall 570
Effective Poisson equation of density functional theory at positive temperature
Li Chen, MIT

Density functional theory (DFT) has been a very successful effective theory of many-body quantum mechanics. In particular, the Kohn-Sham (KS) equations of DFT serve as an accurate model for the electron densities. The KS equations are a case of the Schrodinger-Poisson equations whose electron-electron effective interaction potential only depends on the density of electrons. When the number of electrons are limited, the KS equation can be solved quickly by numerical method at temperature T = 0. Since physically interesting settings are at T > 0, we study the KS equations at positive temperature and give an iterative scheme to construct solutions.

One important class of electronic structures described by the KS equations is a crystalline lattice. At positive temperature, we show that a local perturbation to a crystalline structure induces an electric field governed by the Poisson equation. The latter equation emerges as an effective equation of the KS equations. This is a joint work with Israel M. Sigal.

Wed Nov 27

PDE Seminar

3:35pm - Vincent Hall 570
Quantitative Absolute Continuity of Harmonic Measure, and the Lp Dirichlet Problem
Steve Hofmann, University of Missouri

For a domain ? ? Rd, quantitative, scale-invariant absolute continuity (more precisely, the weak-A? property) of harmonic measure with respect to surface measure on ??, is equivalent to the solvability of the Dirichlet problem for Laplace’s equation, with data in some Lp space on ??, with p < ?. Drawing an analogy to the famous Wiener criterion, which characterizes the domains in which the classical Dirichlet problem, with continuous boundary data, can be solved, it is of interest to find criteria for Lp solvability, thus allowing for singular boundary data. We shall review known results in this direction, in which (within the past 18 months) a rather complete picture has now emerged.

Wed Dec 04

PDE Seminar

3:35pm - Vincent Hall 570
Multi-scale analysis of Jordan curves
Benjamin Jaye, Clemson University

In this talk we will describe how one can detect regularity in Jordan curves through analysis of associated geometric square functions. We will particularly focus on the resolution to a conjecture of L. Carleson. Joint work with Xavier Tolsa and Michele Villa (https://arxiv.org/abs/1909.08581).

Wed Dec 11

PDE Seminar

3:35pm - Vincent Hall 570
Quantitative stochastic homogenization via Malliavin calculus
Antoine Gloria, Sorbonne Université

Abstract: This talk is about stochastic homogenization of linear elliptic equations in divergence form. Let $a(x)=h(G(x))$ be a diffusion coefficient field, where $h$ is a Lipschitz function and $G$ is a Gaussian field (with possibly thick tail). Solutions $u_\varepsilon$ of elliptic equations $-\nabla \cdot a(\cdot/\varepsilon) \nabla u_\varepsilon = \nabla \cdot f$ in $\mathbb R^d$ with such random heterogeneous coefficients $a$ both oscillate spatially and fluctuate randomly at scale $\varepsilon >0$. I will show how suitable quantitative two-scale expansions allow one to reduce the analysis of oscillations and fluctuations of solutions to bounds on the corrector and fluctuations of the homogenization commutator, respectively. The main probabilistic ingredient is Malliavin calculus, and the main analytical ingredient is large-scale elliptic regularity. This is based on joint works with Mitia Duerinckx, Julian Fischer, Stefan Neukamm, and Felix Otto.