Magnetohydrodynamic fluids with zero magnetic diffusivity

Speaker: 

Xianpeng Hu

Institution: 

Courant Institute

Time: 

Tuesday, May 20, 2014 - 3:00pm to 4:00pm

Host: 

Location: 

RH306

 

Understanding the incompressible/compressible fluid is a fundamental, but
challenging, project not only in numerical analysis, but also in
theoretical analysis, especially when extra effects, such as the elastic
deformation or the magnetic field, interact with the flow. In this talk,
the incompressible fluid and its associated flow map will be reviewed first.
The main object of this talk devotes to a recent work in understanding
incompressible/compressible magnetohydrodynamic fluids with zero magnetic
diffusivity (which is equivalent to infinite conductivity). This is a
joint work with Fanghua Lin.

Tunneling in graphene: magic angles and their origins. (On a joint work with M. Katsnelson, A. Okunev, I. Schurov, D. Zubov.)

Speaker: 

Victor Kleptsyn

Institution: 

CNRS, Institute of Mathematical Research of Rennes

Time: 

Thursday, May 15, 2014 - 11:00am to 12:00pm

Host: 

Location: 

RH 440R

My talk will be devoted to a joint work with M. Katsnelson, A. Okunev, I. Schurov and D. Zubov.

Graphene is a layer of carbon (forming a hexagonal lattice) of thickness of one or several atoms. One of its remarkable properties is that the behavior of electrons on it is described by the Dirac equation, the same equation that describes the behavior of ultrarelativistic particles. A corollary of this is the Klein tunneling: an electron (or, as it is much more appropriate to say, an wave or quasiparticle) that falls orthogonally on a flat potential barrier on a single-layer graphene, not only has a positive chance of tunneling through it (what is quite natural in quantum mechanics), but passes through it with probability one(!).

Reijnders, Tudorovskiy and Katsnelson, while modeling a transition through an n-p-n junction, have discovered the presence of other, nonzero "magic" angles, under which the falling particle (of given energy) passes through the barrier with probability one.

There are a several interesting problems that arise out of this work. On the one hand, a zero probability of reflection is a codimension two condition (the coefficient before the reflected wave is a complex coefficient that should be equal to zero). Thus, we have a system of two equations on one variable (the incidence angle) that has nonempty set of solutions, what one would not normally expect. And it is interesting to explain their origins.

On the other hand, there is a question that is interesting from the point of view of potential applications: can one invent a potential that "closes well" the transition probabilities (in particular, that has no magic angles)? This question comes from construction of transistors: that is what we should observe for a transistor in the "closed" state.

I will speak about our advances in all these problems. In particular, the tunneling problem on bilayer graphene turns out to be (vaguely) connected to the slow-fast systems on the 2-torus.

Self-genericity axioms VII

Speaker: 

Andres Forero

Institution: 

UCI

Time: 

Monday, May 12, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

We complete the exposition on self-genericity axioms for ideals on P(Z) (Club Catch, Projective Catch and Stationary Catch). We have established some relations with forcing axioms and with the existence of certain regular forcing embeddings and projections, and also point out connections with Precipitousness. We give an rough overview of the method used for proving the existence of models with Woodin cardinals coming from these axioms, using the Core Model Theory. In this talk we finish explaining the mechanism of absorbing extenders in the core model, and lifting iterability from countable models to models of large cardinality.

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Slopes of modular forms

Speaker: 

Liang Xiao

Institution: 

UC Irvine

Time: 

Tuesday, May 27, 2014 - 2:00pm

Location: 

RH 340P

I will explain several conjectures and results regarding the slope distribution of Up operator action on the space of modular forms.  Most notably, we prove that the slopes of modular forms with a highly p-divisible characters roughly form unions of arithmetic progressions.  This is a joint work with Daqing Wan and Jun Zhang.

Sparse Time-Frequency Representation Method for Adaptive Data Analysis

Speaker: 

Peyman Tavallali

Institution: 

CalTech

Time: 

Monday, May 19, 2014 - 4:00pm to 5:00pm

Host: 

Location: 

RH306

The Sparse Time-Frequency Representation (STFR) method is based on the assumption that many physical signals inherently contain AM-FM representations. We propose a sparse optimization method to extract the AM-FM representations of such signals. We prove the convergence of the method for periodic signals under certain assumptions and provide practical algorithms specifically for the non-periodic STFR, which extends the method to tackle problems that former STFR methods could not handle, including stability to noise and non-periodic data analysis. This is a significant improvement since many adaptive and non-adaptive signal processing methods are not fully capable of handling non-periodic signals. Moreover, we propose a new STFR algorithm to study intrawave signals with strong frequency modulation and analyze the convergence of this new algorithm for periodic signals. Such signals have previously remained a bottleneck for all signal processing methods. Furthermore, we propose a modified version of STFR that facilitates the extraction of intrawaves that have overlaping frequency content. We show that the STFR methods can be applied to the realm of dynamical systems and cardiovascular signals. In particular, we present a simplified and modified version of the STFR algorithm that is potentially useful for the diagnosis of some cardiovascular diseases. We further explain some preliminary work on the nature of Intrinsic Mode Functions (IMFs) and how they can have different representations in different phase coordinates. This analysis shows that the uncertainty principle is fundamental to all oscillating signals.

Spectral packing dimension for 1-d discrete quasi-periodic Schrödinger operators

Speaker: 

Shiwen Zhang

Institution: 

University of California, Irvine, Math. Department

Time: 

Thursday, May 29, 2014 - 10:00am

Location: 

Rowland Hall 440R

Advancement committee:

Chair: Svetlana Jitomirskaya

 

          Aleksander Figotin

          Anton Gorodetski

          Abel Klein

 

outside member(Chemistry): Kieron Burke

Self-genericity axioms VI

Speaker: 

Andres Forero

Institution: 

UCI

Time: 

Monday, May 5, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

We complete the exposition on self-genericity axioms for ideals on P(Z) (Club Catch, Projective Catch and Stationary Catch). We have established some relations with forcing axioms and with the existence of certain regular forcing embeddings and projections, and also point out connections with Precipitousness. We give an rough overview of the method used for proving the existence of models with Woodin cardinals coming from these axioms, using the Core Model Theory. In this talk we explain the mechanism of absorbing extenders in the core model, and lifting iterability from countable models to models of large cardinality.

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