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Series: PDE Seminar

An interesting problem in gas and fluid dynamics is to understand the
behavior of vacuum states, namely the behavior of the system in the presence
of vacuum. A particular interest is so called physical vacuum which naturally
arises in physical problems. The main difficulty lies in the fact that the physical
systems become degenerate along the boundary. I'll present the well-
posedness result of 3D compressible Euler equations for polytropic gases. This
is a joint work with Nader Masmoudi.

Series: PDE Seminar

Motivated by applications to vehicular traffic, supply chains and others,
various continuous models for traffic flow on networks were recently
proposed. We first present some results for theory of conservation laws on graphs.
Then we focus on recent mixed models, involving continuous-discrete spaces
and ode-pde systems. Then a time evolving measures approach is showed, with applications to crowd
dynamics.

Series: PDE Seminar

The dissipative mechanism of Schroedinger equation
is mathematically described by the decay estimate of solutions.
In this talk I mainly focused on the use of harmonic analysis
techniques to obtain suitable time decay estimates and then
prove the local wellposedness for semilinear
Schroedinger equation in certain external magnetic field.
It turns out that the scattering with a potential may lead to understanding of the wellposedness of NLS in the presence of
nonsmooth or large initial data.
Part of this talk is a joint work with Zhenqiu Zhang.

Series: PDE Seminar

In the world of moderate Reynolds number, everyday turbulence of fluids
flowing across planes and down pipes a velvet revolution is taking place.
Experiments are almost as detailed as the numerical simulations, DNS is
yielding exact numerical solutions that one dared not dream about a decade
ago, and dynamical systems visualization of turbulent fluid's state space
geometry is unexpectedly elegant.
We shall take you on a tour of this newly breached, hitherto inaccessible
territory. Mastery of fluid mechanics is no prerequisite, and perhaps a
hindrance: the talk is aimed at anyone who had ever wondered why - if no
cloud is ever seen twice - we know a cloud when we see one? And how do we
turn that into mathematics? (Joint work with J. F.
Gibson)

Series: PDE Seminar

We discuss how to solve a Hamilton-Jacobi-Bellman equation ``at resonance." Our characterization is in terms of invariant measures and is analogous to the Fredholm alternative in the linear case.

Series: PDE Seminar

Consider a nonlinear Schrodinger
equation in $R^3$
whose linear part has three or more eigenvalues
satisfying some resonance
conditions. Solutions which are initially small in
$H^1 \cap
L^1(R^3)$ and inside a neighborhood of the first excited state
family are shown to converge to either a first excited state or a
ground state at time infinity. An essential part of our analysis
is on the linear and nonlinear estimates near nonlinear excited
states, around which the linearized operators have eigenvalues
with nonzero real parts and their corresponding eigenfunctions
are
not uniformly localized in space. This is a joint work with Kenji Nakanishi
and Tuoc Van Phan.The preprint of the talk is available at
http://arxiv.org/abs/1008.3581

Series: PDE Seminar

Series: PDE Seminar

In the simplest form, our result gives a characterization of bounded,divergence-free vector fields on the plane such that the Cauchyproblem for the associated continuity equation has a unique boundedsolution (in the sense of distribution).Unlike previous results in this directions (Di Perna-Lions, Ambrosio,etc.), the proof does not rely on regularization, but rather on adimension-reduction argument which allows us to prove uniqueness usingwell-known one-dimensional results (it is indeed a variant of theclassical method of characteristics).Note that our characterization is not given in terms of functionspaces, but using a qualitative property which is completelynon-linear in character, namely a suitable weak formulation of theSard property.This is a joint work with Giovanni Alberti (University of Pisa) andStefano Bianchini (SISSA, Trieste).

Series: PDE Seminar

Fokker-Planck equation is a linear parabolic equation which describes
the time evolution of of probability distribution of a stochastic
process defined on a Euclidean space. Moreover, it is the gradient flow
of free energy functional. We will present a Fokker-Planck equation
which is a system of ordinary differential equations and describes the
time evolution of probability distribution of a stochastic process on a
graph with a finite number of vertices. It is shown that there is a
strong connection but also substantial differences between the ordinary
differential equations and the usual Fokker-Planck equation on Euclidean
spaces. Furthermore, the ordinary differential equation is in fact a
gradient flow of free energy on a Riemannian manifold whose metric is
closely related to certain Wasserstein metrics. Some examples will also be discussed.

Series: PDE Seminar

Landau damping is a collisionless stability result of considerable
importance in plasma physics, as well as in galactic dynamics.
Roughly speaking, it says that spatial waves are damped in time
(very rapidly) by purely conservative mechanisms, on a time scale
much lower than the effect of collisions.
We shall present in this talk a recent work (joint with C. Villani) which
provides the first positive mathematical result for this effect in the
nonlinear regime, and qualitatively explains its robustness over
extremely long time scales. Physical introduction and implications
will also be discussed.