Seminars and Colloquia by Series

Tuesday, April 2, 2019 - 11:00 , Location: Skiles 006 , John Baez , UC Riverside , , Organizer: Matt Baker

A classical particle moving in an inverse square central force, like a planet in the gravitational field of the Sun, moves in orbits that do not precess. This lack of precession, special to the inverse square force, indicates the presence of extra conserved quantities beyond the obvious ones. Thanks to Noether's theorem, these indicate the presence of extra symmetries. It turns out that not only rotations in 3 dimensions, but also in 4 dimensions, act as symmetries of this system. These extra symmetries are also present in the quantum version of the problem, where they explain some surprising features of the hydrogen atom. The quest to fully understand these symmetries leads to some fascinating mathematical adventures.

Thursday, March 28, 2019 - 11:00 , Location: Skiles 006 , Eugenia Malinnikova , Norwegian University of Science and Technology , Organizer: Mayya Zhilova

The Remez inequality for polynomials quantifies the way the maximum of a polynomial over an interval is controlled by its maximum over a subset of positive measure. The coefficient in the inequality depends on the degree of the polynomial; the result also holds in higher dimensions. We give a version of the Remez inequality for solutions of second order linear elliptic PDEs and their gradients. In this context, the degree of a polynomial is replaced by the Almgren frequency of a solution. We discuss other results on quantitative unique continuation for solutions of elliptic PDEs and their gradients and give some applications for the estimates of eigenfunctions for the Laplace-Beltrami operator. The talk is based on a joint work with A. Logunov.

Thursday, February 7, 2019 - 11:00 , Location: Skiles 006 , Prof. Lexing Ying , Stanford University , Organizer: Molei Tao

Interpolative decomposition is a simple and yet powerful tool for approximating low-rank matrices. After discussing the theory and algorithms, I will present a few new applications of interpolative decomposition in numerical partial differential equations, quantum chemistry, and machine learning.

Friday, December 7, 2018 - 16:00 , Location: Skiles 006 , Tara Brendle , University of Glasgow , Organizer: Dan Margalit
The Burau representation plays a key role in the classical theory of braid groups. When we let the complex parameter t take the value -1, we obtain a symplectic representation of the braid group known as the integral Burau representation. In this talk we will give a survey of results on braid congruence subgroups, that is, the preimages under the integral Burau representation of principal congruence subgroups of symplectic groups. Along the way, we will see the (perhaps surprising) appearance of braid congruence subgroups in a variety of other contexts, including knot theory, homotopy theory, number theory, and algebraic geometry.
Friday, November 16, 2018 - 11:00 , Location: Skiles 006 , Mark Rudelson , University of Michigan , , Organizer: Galyna Livshyts

please note special time!

Random matrices arise naturally in various contexts ranging from theoretical physics to computer science. In a large part of these problems, it is important to know the behavior of the spectral characteristics of a random matrix of a large but fixed size. We will discuss a recent progress in this area illustrating it by problems coming from combinatorics and computer science:

Condition number of “full” and sparse random matrices. Consider a system of linear equations Ax = b where the right hand side is known only approximately. In the process of solving this system, the error in vector b gets magnified by the condition number of the matrix A. A conjecture of von Neumann that with high probability, the condition number of an n × n random matrix with independent entries is O(n) has been proven several years ago. We will discuss this result as well as the possibility of its extension to sparse matrices.

Random matrices in combinatorics. A perfect matching in a graph with an even number of vertices is a pairing of vertices connected by edges of the graph. Evaluating or even estimating the number of perfect matchings in a given graph deterministically may be computationally expensive. We will discuss an application of the random matrix theory to estimating the number of perfect matchings in a de- terministic graph.

Random matrices and traffic jams. Adding another highway to an existing highway system may lead to worse traffic jams. This phenomenon known as Braess’ paradox is still lacking a rigorous mathematical explanation. It was recently explained for a toy model, and the explanation is based on the properties of the eigenvectors of random matrices.

Thursday, October 25, 2018 - 11:00 , Location: Skiles 006 , Gábor Lugosi , Pompeu Fabra University, Barcelona , Organizer: Mayya Zhilova
In these lectures we discuss some statistical problems with an interesting combinatorial structure behind. We start by reviewing the "hidden clique" problem, a simple prototypical example with a surprisingly rich structure. We also discuss various "combinatorial" testing problems and their connections to high-dimensional random geometric graphs. Time permitting, we study the problem of estimating the mean of a random variable.
Thursday, September 27, 2018 - 11:00 , Location: Skiles 006 , Igor Pak , UCLA , Organizer: Mayya Zhilova
Given a convex polytope P, what is the number of integer points in P? This problem is of great interest in combinatorics and discrete geometry, with many important applications ranging from integer programming to statistics. From computational point of view it is hopeless in any dimensions, as the knapsack problem is a special case. Perhaps surprisingly, in bounded dimension the problem becomes tractable. How far can one go? Can one count points in projections of P, finite intersections of such projections, etc? We will survey both classical and recent results on the problem, emphasizing both algorithmic and complexity aspects. Some elegant hardness results will make an appearance in dimension as small as three. If time permits, we will discuss connections to Presburger Arithmetic and decidability problems for irrational polyhedra. Joint work with Danny Nguyen.
Thursday, September 20, 2018 - 11:00 , Location: Skiles 006 , Blair Sullivan , Department of Computer Science, NC State University , Organizer: Christine Heitsch
Techniques from structural graph theory hold significant promise for
designing efficient algorithms for network science. However, their
real-world application has been hampered by the challenges of unrealistic
structural assumptions, hidden costs in big-O notation, and
non-constructive proofs. In this talk, I will survey recent results which
address many of these concerns through an algorithmic pipeline for
structurally sparse networks, highlighting the crucial role of certain
graph colorings, along with several open problems. For example, we give
empirical and model-based evidence that real-world networks exhibit a form
of structural sparsity known as "bounded expansion,'' and discuss
properties of several low-treedepth colorings used in efficient algorithms
for this class.
Based on joint works with E. Demaine, J. Kun, M. O'Brien, M. Pilipczuk, F.
Reidl, P. Rossmanith, F. Sanchez Villaamil, and S. Sikdar.
Tuesday, September 4, 2018 - 11:00 , Location: Skiles 006 , Sara van de Geer , ETH Zurich , Organizer: Mayya Zhilova
The colloquium will be the second lecture of the TRIAD Distinguished Lecture Series by Prof. Sara van de Geer. For further information please see
Wednesday, June 6, 2018 - 15:30 , Location: Skiles 006 , Sarah Koch , U Michigan , Organizer: Dan Margalit
Given two complex polynomials, we can try to mathematically paste them
together to obtain a rational function through a procedure known as
mating the polynomials. In this talk, we will begin by trying to
understand the "shape" of complex polynomials in general. We will then
discuss the mating of two quadratic polynomials: we explore examples
where the mating does exist, and examples where it does not. There will
be lots of movies and exploration in this talk.