# Stanford Applied Math Seminar

Wednesdays at 4:15 PM in 384H.

## Winter 2017 Schedule

January 11
Lenya Ryzhik, Stanford University
Regularity of the solutions of the Euler-Cucker-Smale system

The Cucker-Smale systems is a popular model of collective behavior of interacting agents, used, in particular, to model bird flocking and fish swarming. The underlying premise is the local alignment of the bird (or fish, or any other agent) velocities. The Euler-Cucker-Smale system is an effective macroscopic PDE limit of such particle systems. It has the form of the pressureless Euler equations with a non-linear density-dependent alignment term. The alignment term is a non-linear version of the fractional Laplacian to a power $\alpha\in(0,1)$. It is known that the corresponding Burgers' equation with a linear dissipation of this type develops shocks in a finite time. We show that nonlinearity enhances the dissipation, and the solutions stay globally regular for all $\alpha\in(0,1)$: the dynamics is regularized due to the nonlinear nature of the alignment.

January 18
Jack Xin, UC Irvine
G-equations and Front Speed in Fluid Flows

We first review G-equation (a level set Hamilton-Jacobi equation modeling interface motion) and front speeds in two dimensional cellular flow, then discuss front speeds in three-dimensional Arnold-Beltrami-Childress (ABC) flow. The ABC flow can be viewed as a perturbation of cellular flow, or a nearly integrable system when one of the ABC parameters is small. We show the existence of ballistic orbits where at least one component grows linearly in time, based on the Hamiltonian formulation and time reversal symmetries of the flows. The symmetry based construction works in the non-perturbative case when all three ABC parameters are equal to one. The linear growth law of front speeds of G-equation follows from the ballistic orbits. We end with open problems of front speeds in ABC flows for dissipative G-equations and reaction-diffusion-advection equations.

January 20, Friday, 12pm in 384H
Stefan Steinberger, Yale University
Parabolizing elliptic PDEs and applications

Given the solution u of an elliptic PDE, there is a naturally associated parabolic PDE for which u is a fixed point in time. We propose the application of parabolic techniques (Brownian motion via the stochastic interpretation) to study the elliptic equation; most of the talk will be spent on a variety of applications using this underlying philosophy and include a geometric inequality (improving on a classical result of Elliot Lieb), capacity interpretations of dimensionality reduction techniques with applications in big data, localization phenomena of Schrodinger operators and probabilistic techniques for bounding eigenvalues.

January 25,
Mark Alber, UC San Diego
TBA

February 1, ICME Colloquium, Distinguished Series
Ingrid Daubechies, Duke University
TBA

February 3,
Jianfeng Lu, Duke University
TBA

February 6, Monday, Joe Keller Lecture
Michael Weinstein, Columbia University
TBA

February 22
Fernando Guevara Vasquez, University of Utah
TBA

March 1,
Lisa Hartung, New York University
TBA

March 8
Lin Lin, UC Berkeley
TBA

For questions, contact