## Seminars and Colloquia by Series

Series: PDE Seminar
Tuesday, April 30, 2019 - 15:00 , Location: skiles 006 , Chenchen Mou , UCLA , , Organizer: Xukai Yan

In this talk we study master equations arising from mean field game
problems, under the crucial monotonicity conditions.
Classical solutions of such equations require very strong technical
conditions. Moreover, unlike the master equations arising from mean
field control problems, the mean field game master equations are
non-local and even classical solutions typically do not satisfy the
comparison principle, so the standard viscosity solution approach seems
infeasible. We shall propose a notion of weak solution for such
equations and establish its wellposedness. Our approach relies on a new
smooth mollifier for functions of measures, which unfortunately does not
keep the monotonicity property, and the stability result of master
equations. The talk is based on a joint work with Jianfeng Zhang.

Series: PDE Seminar
Tuesday, April 16, 2019 - 15:00 , Location: Skiles 006 , Li Chen , University of Mannheim , , Organizer: Ronghua Pan

I this talk I will summerize some of our contributions in the analysis of parabolic elliptic Keller-Segel system, a typical model in chemotaxis. For the case of linear diffusion, after introducing the critical mass in two dimension, I will show our result for blow-up conditions for higher dimension. The second part of the talk is concentrated in the critical exponent for Keller-Segel system with porus media type diffusion. In the end, motivated from the result on nonlocal Fisher-KPP equation, we show that the nonlocal reaction will also help in preventing the blow-up of the solutions.

Series: PDE Seminar
Tuesday, April 9, 2019 - 15:00 , Location: Skiles 006 , Professor Gieri Simonett , Vanderbilt University , , Organizer: Xukai Yan

I will consider the motion of a rigid body with an interior cavity that is completely filled with a viscous fluid. The equilibria of the system will be characterized and their stability properties are analyzed. It will be shown that the fluid exerts a stabilizing effect, driving the system towards a state where it is moving as a rigid body with constant angular velocity. In addition, I will characterize the critical spaces for the governing evolution equation, and I will show how parabolic regularization in time-weighted spaces affords great flexibility in establishing regularity and stability properties for the system. The approach is based on the theory of Lp-Lq maximal regularity. (Joint work with G. Mazzone and J. Prüss).

Series: PDE Seminar
Tuesday, April 2, 2019 - 15:00 , Location: skiles 006 , Professor Yan Guo , Brown University , , Organizer: Xukai Yan

In a joint work with Sameer Iyer, the validity of steady Prandtl layer expansion is established in a channel. Our result covers the celebrated Blasius boundary layer profile, which is based on uniform quotient estimates for the derivative Navier-Stokes equations, as well as a positivity estimate at the flow entrance.

Series: PDE Seminar
Tuesday, March 12, 2019 - 15:00 , Location: Skiles 006 , , University of South Carolina , , Organizer: Xukai Yan

The Euler-Alignment system arises as a macroscopic representation of the Cucker-Smale model, which describes the flocking phenomenon in animal swarms. The nonlinear and nonlocal nature of the system bring challenges in studying global regularity and long time behaviors. In this talk, I will discuss the global wellposedness of the Euler-Alignment system with three types of nonlocal alignment interactions: bounded, strongly singular, and weakly singular interactions. Different choices of interactions will lead to different global behaviors. I will also discuss interesting connections to some fluid dynamics systems, including the fractional Burgers equation, and the aggregation equation.

Series: PDE Seminar
Tuesday, March 5, 2019 - 15:00 , Location: Skiles 006 , Professor Shouhong Wang , Indiana University , , Organizer: Xukai Yan

First, we introduce a new field theoretical interpretation of quantum mechanical wave functions, by postulating that the wave function is the common wave function for all particles in the same class determined by the external potential V, of the modulus of the wave function represents the distribution density of the particles, and the gradient of phase of the wave function provides the velocity field of the particles. Second, we show that the key for condensation of bosonic particles is that their interaction is sufficiently weak to ensure that a large collection of boson particles are in a state governed by the same condensation wave function field under the same bounding potential V. For superconductivity, the formation of superconductivity comes down to conditions for the formation of electron-pairs, and for the electron-pairs to share a common wave function. Thanks to the recently developed PID interaction potential of electrons and the average-energy level formula of temperature, these conditions for superconductivity are explicitly derived. Furthermore, we obtain both microscopic and macroscopic formulas for the critical temperature. Third, we derive the field and topological phase transition equations for condensates, and make connections to the quantum phase transition, as a topological phase transition. This is joint work with Tian Ma.

Series: PDE Seminar
Tuesday, February 26, 2019 - 15:00 , Location: Skiles 006 , Professor Hongjie Dong , Brown University , , Organizer: Xukai Yan

I will first give a short introduction of the Navier-Stokes equations, then review some previous results on theconditional regularity of solutions to the incompressible Navier-Stokes equations in the critical Lebesguespaces, and finally discuss some recent work which mainly addressed the boundary regularity issue.

Series: PDE Seminar
Tuesday, February 12, 2019 - 15:00 , Location: skiles 006 , Ting Zhang , Zhejiang University , , Organizer: Xukai Yan

Abstract: In this talk, we consider the Cauchy problem of the N-dimensional incompressible viscoelastic fluids with N ≥ 2. It is shown that, in the low frequency part, this system possesses some dispersive properties derived from the one parameter group e∓itΛ. Based on this dispersive effect, we construct global solutions with large initial velocity concentrating on the low frequency part. Such kind of solution has never been seen before in the literature even for the classical incompressible Navier-Stokes equations. The proof relies heavily on the dispersive estimates for the system of acoustics, and a careful study of the nonlinear terms. And we also obtain the similar result for the isentropic compressible Navier-Stokes equations. Here, the initial velocity with arbitrary B⋅N 2 −1 2,1 norm of potential part P⊥u0 and large highly oscillating are allowed in our results. (Joint works with Daoyuan Fang and Ruizhao Zi)

Series: PDE Seminar
Tuesday, January 22, 2019 - 15:00 , Location: Skiles 006 , Matias Delgadino , Imperial College , Organizer: Xukai Yan

In this talk we will introduce two models for the movement of a small droplet over a substrate: the thin film equation and the quasi static approximation. By tracking the motion of the apparent support of solutions to the thin film equation, we connect these two models. This connection was expected from Tanner's law: the edge velocity of a spreading thin film on a pre-wetted solid is approximately proportional to the cube of the slope at the inflection. This is joint work with Prof. Antoine Mellet.

Series: PDE Seminar
Tuesday, November 27, 2018 - 15:00 , Location: skiles 006 , Yilun(Allen) Wu , The University of Oklahoma , , Organizer: Xukai Yan

A rotating star may be modeled as gas under self gravity with a given total mass and prescribed angular velocity. Mathematically this leads to the Euler-Poisson system. In this talk, we present an existence theorem for such stars that are rapidly rotating, depending continuously on the speed of rotation. No previous results using continuation methods allowed rapid rotation. The key tool for the result is global continuation theory via topological degree, combined with a delicate limiting process. The solutions form a connected set $\mathcal K$ in an appropriate function space. Take an equation of state of the form $p = \rho^\gamma$; $6/5 < \gamma < 2$, $\gamma\ne 4/3$. As the speed of rotation increases, we prove that either the density somewhere within the stars becomes unbounded, or the supports of the stars in $\mathcal K$ become unbounded. Moreover, the latter alternative must occur if $\frac43<\gamma<2$. This result is joint work with Walter Strauss.