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Series: School of Mathematics Colloquium

The KLS conjecture says that the Cheeger constant of any logconcave density is achieved to within a universal, dimension-independent constant factor by a hyperplane-induced subset. Here we survey the origin and consequences of the conjecture (in geometry, probability, information theory and algorithms) and present recent progress resulting in the current best bound, as well as a tight bound for the log-Sobolev constant (both with Yin Tat Lee). The conjecture has led to several techniques of general interest.

Wednesday, March 7, 2018 - 14:00 ,
Location: Atlanta ,
Agniva Roy ,
GaTech ,
Organizer: Anubhav Mukherjee

Three dimensional lens spaces L(p,q) are well known as the first examples of 3-manifolds that were not known by their homology or fundamental group alone. The complete classification of L(p,q), upto homeomorphism, was an important result, the first proof of which was given by Reidemeister in the 1930s. In the 1980s, a more topological proof was given by Bonahon and Hodgson. This talk will discuss two equivalent definitions of Lens spaces, some of their well known properties, and then sketch the idea of Bonahon and Hodgson's proof. Time permitting, we shall also see Bonahon's result about the mapping class group of L(p,q).

Series: Analysis Seminar

An overarching problem in matrix weighted theory is the so-called A2 conjecture, namely the question of whether the norm of a Calderón-Zygmund operator acting on a matrix weighted L2 space depends linearly on the A2 characteristic of the weight. In this talk, I will discuss the history of this problem and provide a survey of recent results with an emphasis on the challenges that arise within the setup.

Series: Job Candidate Talk

I will discuss a recent line of research that uses properties of real rooted polynomials to get quantitative estimates in combinatorial linear algebra problems. I will start by discussing the main result that bridges the two areas (the "method of interlacing polynomials") and show some examples of where it has been used successfully (e.g. Ramanujan families and the Kadison Singer problem). I will then discuss some more recent work that attempts to make the method more accessible by providing generic tools and also attempts to explain the accuracy of the method by linking it to random matrix theory and (in particular) free probability. I will end by mentioning some current research initiatives as well as possible future directions.

Series: CDSNS Colloquium

The restricted three body problem models the motion of a body of zero mass under the influence of the Newtonian gravitational force caused by two other bodies, the primaries, which describe Keplerian orbits. In 1922, Chazy conjectured that this model had oscillatory motions, that is, orbits which leave every bounded region but which return infinitely often to some fixed bounded region. Its existence was not proven until 1960 by Sitnikov in a extremely symmetric and carefully chosen configuration. In 1973, Moser related oscillatory motions to the existence of chaotic orbits given by a horseshoe and thus associated to certain transversal homoclinic points. Since then, there has been many atempts to generalize their result to more general settings in the restricted three body problem.In 1980, J. Llibre and C. Sim\'o, using Moser ideas, proved the existence of oscillatory motions for the restricted planar circular three body problem provided that the ratio between the masses of the two primaries was arbitrarily small. In this talk I will explain how to generalize their result to any value of the mass ratio. I will also explain how to generalize the result to the restricted planar elliptic three body problem. This is based on joint works with P. Martin, T. M. Seara. and L. Sabbagh.

Series: Combinatorics Seminar

How many triangles are needed to make the new graphs not look like random graphs?
I am trying to answer this question.
(The talk will be during 12:05-1:15pm; please note the room is *Skiles 256*)

Monday, March 5, 2018 - 13:55 ,
Location: Skiles 005 ,
Nick Dexter ,
University of Tennessee ,
ndexter@utk.edu ,
Organizer: Wenjing Liao

We present and analyze a novel sparse polynomial approximation method
for the solution of PDEs with stochastic and parametric inputs. Our
approach treats the parameterized problem as a problem of joint-sparse
signal reconstruction, i.e.,
the simultaneous reconstruction of a set of signals sharing a common
sparsity pattern from a countable, possibly infinite, set of
measurements. Combined with the standard measurement scheme developed
for compressed sensing-based polynomial approximation, this
approach allows for global approximations of the solution over both
physical and parametric domains. In addition, we are able to show that,
with minimal sample complexity, error estimates comparable to the best
s-term approximation, in energy norms, are achievable,
while requiring only a priori bounds on polynomial truncation error. We
perform extensive numerical experiments on several high-dimensional
parameterized elliptic PDE models to demonstrate the superior recovery
properties of the proposed approach.

Series: CDSNS Colloquium

A trajectory is quasiperiodic if the trajectory lies on and is dense in some d-dimensional torus, and there is a choice of coordinates on the torus for which F has the form F(t) = t + rho (mod 1) for all points in the torus, and for some rho in the torus. There is an extensive literature on determining the coordinates of the vector rho, called the rotation numbers of F. However, even in the one-dimensional case there has been no general method for computing the vector rho given only the trajectory (u_n), though there are plenty of special cases. I will present a computational method called the Embedding Continuation Method for computing some components of r from a trajectory. It is based on the Takens Embedding Theorem and the Birkhoff Ergodic Theorem. There is however a caveat; the coordinates of the rotation vector depend on the choice of coordinates of the torus. I will give a statement of the various sets of possible rotation numbers that rho can yield. I will illustrate these ideas with one- and two-dimensional examples.

Friday, March 2, 2018 - 15:05 ,
Location: Skiles 271 ,
Adrian P. Bustamante ,
Georgia Tech ,
Organizer:

Given a one-parameter family of maps of an interval to itself, one can observe period doubling bifurcations as the parameter is varied. The aspects of those bifurcations which are independent of the choice of a particular one-parameter family are called universal. In this talk we will introduce, heuristically, the so-called Feigenbaun universality and then we'll expose some rigorous results about it.

Series: Combinatorics Seminar

This is Lecture 3 of a series of 3 lectures. See the abstract on Tuesday's ACO colloquium of this week.(Please note that this lecture will be 80 minutes' long.)