Seminars and Colloquia by Series

Thursday, November 6, 2014 - 11:00 , Location: Skiles 005 , Ken Ono , Emory University , Organizer: Kirsten Wickelgren
The speaker will discuss recent work on Moonshine and the Rogers-Ramanujan identities. The Rogers-Ramanujan identities are two peculiar identities which express two infinite product modular forms as number theoretic q-series. These identities give rise to the Rogers-Ramanujan continued fraction, whose values at CM points are algebraic integral units. In recent work with Griffin and Warnaar, the speaker has obtained a comprehensive framework of identities for infinite product modular forms in terms of Hall-Littlewood q-series. This work characterizes those integral units that arise from this theory. In a related direction, the speaker revisits the classical Moonshine Theorem which asserts that the coefficients of the modular j-functions are dimensions of virtual characters for the Monster, the largest of the simple sporadic groups. There are 194 irreducible representations of the Monster, and it has been a longstanding open problem to determine the distribution of these representations in Moonshine. In joint work with Griffin and Duncan, the speaker has obtained exact formulas for these distributions.
Thursday, October 30, 2014 - 11:00 , Location: Skiles 005 , Professor Albert Fathi , ENS-Lyon & IUF , Organizer: Martin Short
This is a joint work with Pierre Pageault. For a homeomorphism h of a compact space, a Lyapunov function is a real valued function that is non-increasing along orbits for h. By looking at simple dynamical systems(=homeomorphisms) on the circle, we will see that there are systems which are topologically conjugate and have Lyapunov functions with various regularity. This will lead us to define barriers analogous to the well known Peierls barrier or to the Maסי potential in Lagrangian systems. That will produce by analogy to Mather's theory of Lagrangian Systems an Aubry set which is the generalized recurrence set introduced in the 60's by Joe Auslander (via transfinite induction) and a Maסי set which is essentially Conley's chain recurrent set. No serious knowledge of Dynamical Systems is necessary to follow the lecture.
Thursday, October 23, 2014 - 11:00 , Location: Skiles 005 , Professor Igor Pritsker , Oklahoma State University , Organizer: Martin Short
The area was essentially originated by the general question: How many zeros of a random polynomials are real? Kac showed that the expected number of real zeros for a polynomial with i.i.d. Gaussian coefficients is logarithmic in terms of the degree. Later, it was found that most of zeros of random polynomials are asymptotically uniformly distributed near the unit circumference (with probability one) under mild assumptions on the coefficients. Thus two main directions of research are related to the almost sure limits of the zero counting measures, and to the quantitative results on the expected number of zeros in various sets. We give estimates of the expected discrepancy between the zero counting measure and the normalized arclength on the unit circle. Similar results are established for polynomials with random coefficients spanned by various bases, e.g., by orthogonal polynomials. We show almost sure convergence of the zero counting measures to the corresponding equilibrium measures for associated sets in the plane, and quantify this convergence. Random coefficients may be dependent and need not have identical distributions in our results.
Friday, September 19, 2014 - 11:00 , Location: Skiles 005 , Professor Alberto Bressan , Penn State University , Organizer: Martin Short
The talk will survey the main definitions and properties of patchy vector fields and patchy feedbacks, with applications to asymptotic feedback stabilization and nearly optimal feedback control design. Stability properties for discontinuous ODEs and robustness of patchy feedbacks will also be discussed.
Wednesday, June 11, 2014 - 15:30 , Location: Skiles 006 , Ravi Vakil , Stanford University , , Organizer: Joseph Rabinoff
Given some class of "geometric spaces", we can make a ring as follows. (i) (additive structure) When U is an open subset of such a space X, [X] = [U] + [(X \ U)] (ii) (multiplicative structure) [X x Y] = [X] [Y].In the algebraic setting, this ring (the "Grothendieck ring of varieties") contains surprising structure, connecting geometry to arithmetic and topology.  I will discuss some remarkable statements about this ring (both known and conjectural), and present new statements (again, both known and conjectural).  A motivating example will be polynomials in one variable. (This talk is intended for a broad audience.)  This is joint work with Melanie Matchett Wood.
Thursday, May 1, 2014 - 11:00 , Location: Skiles 006 , Gunnar Carlsson , Stanford University , Organizer: Joseph Rabinoff
The general problem of extracting knowledge from large and complex data sets is a fundamental one across all areas of the natural and social sciences, as well as in most areas of commerce and government. Much progress has been made on methods for capturing and storing such data, but the problem of translating it into knowledge is more difficult. I will discuss one approach to this problem, via the study of the shape of the data sets, suitably defined. The use of shape as an organizing problems permits one to bring to bear the methods of topology, which is the mathematical field which deals with shape. We will discuss some different topological methods, with examples.
Thursday, April 10, 2014 - 11:00 , Location: Skiles 006 , Jan Draisma , Eindhoven University of Technology , Organizer: Joseph Rabinoff
In this age of high-dimensional data, many challenging questions take the following shape: can you check whether the data has a certain desired property by checking that property for many, but low-dimensional data fragments? In recent years, such questions have inspired new, exciting research in algebra, especially relevant when the property is highly symmetric and expressible through systems of polynomial equations. I will discuss three concrete questions of this kind that we have settled in the affirmative: Gaussian factor analysis from an algebraic perspective, high-dimensional tensors of bounded rank, and higher secant varieties of Grassmannians. The theory developed for these examples deals with group actions on infinite-dimensional algebraic varieties, and applies to problems from many areas. In particular, I will sketch its (potential) relation to the fantastic Matroid Minor Theorem.
Thursday, April 3, 2014 - 11:00 , Location: N/A , N/A , N/A , Organizer: Joseph Rabinoff
Thursday, March 27, 2014 - 11:05 , Location: Skiles 006 , Don Zagier , MPI Bonn and College de France , Organizer: Stavros Garoufalidis
An introduction for non-experts on real and finite Euler sums, also known as multiple zeta values.
Thursday, March 6, 2014 - 11:00 , Location: Skiles 006 , Annette Werner , Johann Wolfgang Goethe-Universität (Frankfurt) , Organizer: Joseph Rabinoff
Drinfeld's upper half-spaces over non-archimedean local fields are the founding examples of the theory of period domains. In this talk we consider analogs of Drinfeld's upper half-spaces over finite fields. They are open subvarieties of a projective space. We show that their automorphism group is the group of automorphisms of the ambient projective space. This is a problem in birational geometry, which we solve using tools in non-archimedean analytic geometry.