Seminars and Colloquia by Series

Wednesday, April 11, 2018 - 12:10 , Location: Skiles 006 , Albert Fathi , Georgia Tech , albert.fathi@math.gatech.edu , Organizer: Adrian Perez Bustamante
The goal of this lecture is to explain and motivate the connection between Aubry-Mather theory (Dynamical Systems), and viscosity solutions of the Hamilton-Jacobi equation (PDE).This connection is the content of weak KAM Theory.The talk should be accessible to the “generic” mathematician. No a priori knowledge of any of the two subjects is assumed.The set-up of this theory is classical mechanical systems, in its Lagrangian formulation to take advantage of the action principle. This is the natural setting for Celestial Mechanics. Today it is also the setting for motions of satellites in the solar system.Hamilton found a reformulation of Lagrangian mechanics in terms of position and momentum instead of position and speed. In this formulation appears the Hamilton-Jacobi equation. Although this is a partial differential equation, its solutions allow to find solutions of the Hamiltonian (or Lagrangian) systems which are, in fact, governed by an ordinary differential equation.KAM (Kolmogorov-Arnold-Moser) theorem addressed at its beginning (Kolomogorov) the problem of stability of the solar system. It came as a surprise, since Poincare ́’s earlier work pointed to instability. In fact, some initial conditions lead to instability (Poincare ́) and some others lead to stability(Kolomogorov).Aubry-Mather theory finds some more substantial stable motion that survives outside the region where KAM theorem applies.The KAM theorem also provides global differentiable solutions to the Hamilton-Jacobi equation.It is known that the Hamilton-Jacobi equation usually does not have smooth global solutions. Lions & Crandall developed a theory of weak solutions of the Hamilton-Jacobi equation.Weak KAM theory explains how the Aubry-Mather sets can be obtained from the points where weak solutions of the Hamilton-Jacobi equation are differentiable.
Wednesday, March 28, 2018 - 12:10 , Location: Skiles 006 , Wenjing Liao , Georgia Tech , wliao60@gatech.edu , Organizer: Adrian Perez Bustamante
Many data sets in image analysis and signal processing are in a high-dimensional space but exhibit a low-dimensional structure. We are interested in building efficient representations of these data for the purpose of compression and inference. In the setting where a data set in $R^D$ consists of samples from a probability measure concentrated on or near an unknown $d$-dimensional manifold with $d$ much smaller than $D$, we consider two sets of problems: low-dimensional geometric approximations to the manifold and regression of a function on the manifold. In the first case, we construct multiscale low-dimensional empirical approximations to the manifold and give finite-sample performance guarantees. In the second case, we exploit these empirical geometric approximations of the manifold and construct multiscale approximations to the function. We prove finite-sample guarantees showing that we attain the same learning rates as if the function was defined on a Euclidean domain of dimension $d$. In both cases our approximations can adapt to the regularity of the manifold or the function even when this varies at different scales or locations.
Wednesday, March 14, 2018 - 12:10 , Location: Skiles 006 , Elizabeth Holdsworth , Georgia Tech , elizabeth.holdsworth@library.gatech.edu , Organizer: Adrian Perez Bustamante
There is so much that the GT library can do for you, from providing research materials to assistance with data visualization to patent guidance. However, rather than trying to guess what you want from us, this year we asked!  Based on the response to a short ranking survey I sent out last month, this session will cover: 1. How to find grants, fellowships, and travel money with the sponsorship database, Pivot. There are opportunities for postdocs and non US citizens too!2. How to use MathSciNet. We will cover navigating its classification index to actually getting the article you want. 3. How to find and download articles from our systems, Google Scholar, and from other libraries. And if we have time:  4. How to make a poster and cheaply print it.  
Wednesday, February 14, 2018 - 12:10 , Location: Skiles 006 , Leonid Bunimovich , Georgia Tech , Organizer: Adrian Perez Bustamante
Some basic problems, notions and results of the Ergodic theory will be introduced. Several examples will be discussed.   It is also  a preparatory talk for the next day colloquium where finite time properties of dynamical and stochastic systems will be discussed rather than traditional questions all dealing with asymptotic in time properties.
Friday, February 2, 2018 - 15:53 , Location: Skiles 006 , Leonid Bunimovich , GA Tech , Organizer: Timothy Duff
Some basic problems, notions and results of the Ergodic theory will be introduced. Several examples will be discussed.   It is also  a preparatory talk for the next day colloquium where finite time properties of dynamical and stochastic systems will be discussed rather than traditional questions all dealing with asymptotic in time properties.
Wednesday, January 31, 2018 - 12:10 , Location: Skiles 006 , Greg Blekherman , GA Tech , Organizer: Timothy Duff
In recent years the problem of low-rank matrix completion received a tremendous amount of attention. I will consider the problem of exact low-rank matrix completion for generic data. Concretely, we start with a partially-filled matrix M, with real or complex entries, with the goal of finding the unspecified entries (completing M) in such a way that the completed matrix has the lowest possible rank, called the completion rank of M. We will be interested in how this minimal completion rank depends on the known entries, while keeping the locations of specified and unspecified entries fixed. Generic data means that we only consider partial fillings of M where a small perturbation of the entries does not change the completion rank of M.
Wednesday, December 6, 2017 - 13:10 , Location: Skiles 006 , Kelly Yancey and Matthew Yancey , Institute for Defense Analyses , kyancey@math.umd.edu , Organizer:
The Institute for Defense Analyses - Center for Computing Sciences is a nonprofit research center that works closely with the NSA.  Our center has around 60 researchers (roughly 30 mathematicians and 30 computer scientists) that work on interesting and hard problems.  The plan for the seminar is to begin with a short mathematics talk on a project that was completed at IDA-CCS and declassified, then tell you a little about what we do, and end with your questions.  The math that we will discuss involves symbolic dynamics and automata theory.  Specifically we will develop a metric on the space of regular languages using topological entropy.  This work was completed during a summer SCAMP at IDA-CCS.  SCAMP is a summer program where researchers from academia (professors and students), the national labs, and the intelligence community come to IDA-CCS to work on the agency's hard problems for 11 weeks.
Wednesday, December 6, 2017 - 12:10 , Location: Skiles 006 , John Etnyre , GT Math , Organizer:
Four dimensions is unique in many ways. For example $n$-dimensional Euclidean space has a unique smooth structure if and only if $n$ is not equal to  four. In other words, there is only one way to understand smooth functions on $R^n$ if and only if $n$ is not 4. There are many other way that smooth structures on 4-dimensional manifolds behave in surprising ways. In this talk I will discuss this and I will sketch the beautiful interplay of ideas (you got algebra, analysis and topology, a little something for everyone!) that go into proving $R^4$ has more that one smooth structure (actually it has uncountably many different smooth structures but that that would take longer to explain).    
Wednesday, November 29, 2017 - 12:10 , Location: Skiles 006 , Chongchun Zeng , Georgia Tech , Organizer:
In this talk, we consider the structure of a real $n \times n$ matrix in the form of $A=JL$, where $J$ is anti-symmetric and $L$ is symmetric. Such a matrix comes from a linear Hamiltonian ODE system with $J$ from the symplectic structure and the Hamiltonian energy given by the quadratic form $\frac 12\langle Lx, x\rangle$. We will discuss the distribution of the eigenvalues of $A$, the relationship between the canonical form of $A$ and the structure of the quadratic form $L$, Pontryagin invariant subspace theorem, etc. Finally, some extension to infinite dimensions will be mentioned.    
Wednesday, November 15, 2017 - 12:10 , Location: skiles 006 , Joseph Rabinoff , GT Math , Organizer:
A motivating problem in number theory and algebraic geometry is to find all integer-valued solutions of a polynomial equation.  For example, Fermat's Last Theorem asks for all integer solutions to x^n + y^n = z^n, for n >= 3. This kind of problem is easy to state, but notoriously difficult to solve.  I'll explain a p-adic method for attacking Diophantine equations, namely, p-adic integration and the Chabauty--Coleman method.  Then I'll talk about some recent joint work on the topic.

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