Colloquia 2012-2013

Mathematics Colloquium

All colloquia are on Fridays at 4:00 pm in Van Vleck B239, unless otherwise indicated.

Fall 2011

date	speaker	title	host(s)
Sep 9	Manfred Einsiedler (ETH-Zurich)	Periodic orbits on homogeneous spaces	Fish
Sep 16	Richard Rimanyi (UNC-Chapel Hill)	Global singularity theory	Maxim
Sep 23	Andrei Caldararu (UW-Madison)	The Pfaffian-Grassmannian derived equivalence	(local)
Sep 30	Scott Armstrong (UW-Madison)	Optimal Lipschitz extensions, the infinity Laplacian, and tug-of-war games	(local)
Oct 7	Hala Ghousseini (University of Wisconsin-Madison)	Developing Mathematical Knowledge for Teaching in, from, and for Practice	Lempp
Oct 14	Alex Kontorovich (Yale)	On Zaremba's Conjecture	Shamgar
oct 19, Wed	Bernd Sturmfels (UC Berkeley)	Convex Algebraic Geometry	distinguished lecturer	Shamgar
oct 20, Thu	Bernd Sturmfels (UC Berkeley)	Quartic Curves and Their Bitangents	distinguished lecturer	Shamgar
oct 21	Bernd Sturmfels (UC Berkeley)	Multiview Geometry	distinguished lecturer	Shamgar
Oct 28	Roman Holowinsky (OSU)	Equidistribution Problems and L-functions	Street
Nov 4	Sijue Wu (U Michigan)	Wellposedness of the two and three dimensional full water wave problem	Qin Li
Nov 7, Mo, 3pm, SMI 133	Sastry Pantula (NSCU and DMS/NSF)	Opportunities in Mathematical and Statistical Sciences at DMS	Joint Math/Stat Colloquium
Nov 11	Henri Berestycki (EHESS and University of Chicago)	Reaction-diffusion equations and propagation phenomena	Wasow lecture
Nov 16, Wed	Henry Towsner (U of Conn-Storrs)	An Analytic Approach to Uniformity Norms	Steffen
Nov 18	Benjamin Recht (UW-Madison, CS Department)	The Convex Geometry of Inverse Problems	Jordan
Nov 22, Tue, 2:30PM, B205	Zhiwei Yun (MIT)	TBA	Tonghai
Nov 30, Wed, 2:30PM	Isaac Goldbring (UCLA)	Nonstandard methods in Lie theory	Steffen
Nov 30, Wed, 4PM	Bing Wang (Simons Institute)	TBA	Sean
Dec 2	Robert Dudley (University of California, Berkeley)	From Gliding Ants to Andean Hummingbirds: The Evolution of Animal Flight Performance	Jean-Luc
dec 9	Xinwen Zhu (Harvard University)	TBA	Tonghai

Spring 2012

date	speaker	title	host(s)
Jan 26, Thu	Peter Constantin (University of Chicago)	TBA	distinguished lecturer
Jan 27	Peter Constantin (University of Chicago)	TBA	distinguished lecturer
Feb 3	Scheduled		Street
Feb 24	Malabika Pramanik (University of British Columbia)	TBA	Benguria
March 2	Guang Gong (University of Waterloo)	TBA	Shamgar
March 16	Charles Doran (University of Alberta)	TBA	Matt Ballard
March 23	Martin Lorenz (Temple University)	TBA	Don Passman
March 30	Paolo Aluffi (Florida State University)	TBA	Maxim
April 6	Spring recess
April 13	Ricardo Cortez (Tulane)	TBA	Mitchell
April 18	Benedict H. Gross (Harvard)	TBA	distinguished lecturer
April 19	Benedict H. Gross (Harvard)	TBA	distinguished lecturer
April 20	Robert Guralnick (University of South California)	TBA	Shamgar
April 27	Tentatively Scheduled		Street
May 4	Mark Andrea de Cataldo (Stony Brook)	TBA	Maxim
May 11	Tentatively Scheduled		Shamgar

Abstracts

Fri, Sept 9: Manfred Einsiedler (ETH-Zurich)

Periodic orbits on homogeneous spaces

We call an orbit xH of a subgroup H<G on a quotient space Gamma \ G periodic if it has finite H-invariant volume. These orbits have intimate connections to a variety of number theoretic problems, e.g. both integer quadratic forms and number fields give rise periodic orbits and these periodic orbits then relate to local-global problems for the quadratic forms or to special values of L-functions. We will discuss whether a sequence of periodic orbits equidistribute in Gamma \ G assuming the orbits become more complicated (which can be measured by a discriminant). If H is a diagonal subgroup (also called torus or Cartan subgroup), this is not always the case but can be true with a bit more averaging. As a theorem of Mozes and Shah show the case where H is generated by unipotents is well understand and is closely related to the work of M. Ratner. We then ask about the rate of approximation, where the situation is much more complex. The talk is based on several papers which are joint work with E.Lindenstrauss, Ph. Michel, and A. Venkatesh resp. with G. Margulis and A. Venkatesh.

Fri, Sept 16: Richard Rimanyi (UNC)

Global singularity theory

The topology of the spaces A and B may force every map from A to B to have certain singularities. For example, a map from the Klein bottle to 3-space must have double points. A map from the projective plane to the plane must have an odd number of cusp points.

To a singularity one may associate a polynomial (its Thom polynomial) which measures how topology forces this particular singularity. In the lecture we will explore the theory of Thom polynomials and their applications in enumerative geometry. Along the way, we will meet a wide spectrum of mathematical concepts from geometric theorems of the ancient Greeks to the cohomology ring of moduli spaces.

Fri, Sept 23: Andrei Caldararu (UW-Madison)

The Pfaffian-Grassmannian derived equivalence

String theory relates certain seemingly different manifolds through a relationship called mirror symmetry. Discovered about 25 years ago, this story is still very mysterious from a mathematical point of view. Despite the name, mirror symmetry is not entirely symmetric -- several distinct spaces can be mirrors to a given one. When this happens it is expected that certain invariants of these "double mirrors" match up. For a long time the only known examples of double mirrors arose through a simple construction called a flop, which led to the conjecture that this would be a general phenomenon. In joint work with Lev Borisov we constructed the first counterexample to this, which I shall present. Explicitly, I shall construct two Calabi-Yau threefolds which are not related by flops, but are derived equivalent, and therefore are expected to arise through a double mirror construction. The talk will be accessible to a wide audience, in particular to graduate students. There will even be several pictures!

Fri, Sept 30: Scott Armstrong (UW-Madison)

Optimal Lipschitz extensions, the infinity Laplacian, and tug-of-war games

Given a nice bounded domain, and a Lipschitz function defined on its boundary, consider the problem of finding an extension of this function to the closure of the domain which has minimal Lipschitz constant. This is the archetypal problem of the calculus of variations "in the sup-norm". There can be many such minimal Lipschitz extensions, but there is there is a unique minimizer once we properly "localize" this Lipschitz minimizing property. This minimizer is characterized by the infinity Laplace equation: the Euler-Lagrange equation for our optimization problem. This PDE is a very highly degenerate nonlinear elliptic equation which does not possess smooth solutions in general. In this talk I will discuss what we know about the infinity Laplace equation, what the important open questions are, and some interesting recent developments. We will even play a probabilistic game called "tug-of-war".

Fri, Oct 7: Hala Ghousseini (University of Wisconsin-Madison)

Developing Mathematical Knowledge for Teaching in, from, and for Practice

Recent research in mathematics education has established that successful teaching requires a specialized kind of professional knowledge known as Mathematical Knowledge for Teaching (MKT). The mathematics education community, however, is beginning to appreciate that to be effective, teachers not only need to know MKT but also be able to use it in interaction with students (Hill & Ball, 2010). Very few examples exist at the level of actual practice of how novice teachers develop such knowledge for use. I will report on my current work on the Learning in, from, and for Practice project to develop, implement, and study what mathematics teacher educators can do to support novice teachers in acquiring and using Mathematical Knowledge for Teaching.

Fri, Oct 14: Alex Kontorovich (Yale)

On Zaremba's Conjecture

It is folklore that modular multiplication is "random". This concept is useful for many applications, such as generating pseudorandom sequences, or in quasi-Monte Carlo methods for multi-dimensional numerical integration. Zaremba's theorem quantifies the quality of this "randomness" in terms of certain Diophantine properties involving continued fractions. His 40-year old conjecture predicts the ubiquity of moduli for which this Diophantine property is uniform. It is connected to Markoff and Lagrange spectra, as well as to families of "low-lying" divergent geodesics on the modular surface. We prove that a density one set satisfies Zaremba's conjecture, using recent advances such as the circle method and estimates for bilinear forms in the Affine Sieve, as well as a "congruence" analog of the renewal method in the thermodynamical formalism. This is joint work with Jean Bourgain.

Wed, Oct 19: Bernd Sturmfels (Berkeley)

Convex Algebraic Geometry

This lecture concerns convex bodies with an interesting algebraic structure. A primary focus lies on the geometry of semidefinite optimization. Starting with elementary questions about ellipses in the plane, we move on to discuss the geometry of spectrahedra, orbitopes, and convex hulls of real varieties.

Thu, Oct 20: Bernd Sturmfels (Berkeley)

Quartic Curves and Their Bitangents

We present a computational study of plane curves of degree four, with primary focus on writing their defining polynomials as sums of squares and as symmetric determinants. Number theorists will enjoy the appearance of the Weyl group [math]\displaystyle{ E_7 }[/math] as the Galois group of the 28 bitangents. Based on joint work with Daniel Plaumann and Cynthia Vinzant, this lecture spans a bridge from 19th century algebra to 21st century optimization.

Fri, Oct 21: Bernd Sturmfels (Berkeley)

Multiview Geometry

The study of two-dimensional images of three-dimensional scenes is foundational for computer vision. We present work with Chris Aholt and Rekha Thomas on the polynomials characterizing images taken by [math]\displaystyle{ n }[/math] cameras. Our varieties are threefolds that vary in a family of dimension [math]\displaystyle{ 11n-15 }[/math] when the cameras are moving. We use toric geometry and Hilbert schemes to characterize degenerations of camera positions.

Fri, Oct 28: Roman Holowinsky (OSU)

Equidistribution Problems and L-functions

There are several equidistribution problems of arithmetic nature which have had shared interest between the fields of Ergodic Theory and Number Theory. The relation of such problems to homogeneous flows and the reduction to analysis of special values of automorphic L-functions has resulted in increased collaboration between these two fields of mathematics. We will discuss two such equidistribution problems: the equidistribution of Heegner points for negative quadratic discriminants and the equidistribution of mass of Hecke eigenforms. Equidistribution follows upon establishing subconvexity bounds for the associated L-functions and are fine examples as to why one might be interested in such objects.

Fri, Nov 4: Sijue Wu (U Michigan)

Wellposedness of the two and three dimensional full water wave problem

We consider the question of global in time existence and uniqueness of solutions of the infinite depth full water wave problem. We show that the nature of the nonlinearity of the water wave equation is essentially of cubic and higher orders. For any initial data that is small in its kinetic energy and height, we show that the 2-D full water wave equation is uniquely solvable almost globally in time. And for any initial interface that is small in its steepness and velocity, we show that the 3-D full water wave equation is uniquely solvable globally in time.

Mo, Nov 7: Sastry Pantula (DMS/NSF, NCSU)

Opportunities in Mathematical and Statistical Sciences at DMS

In this talk, I will give you an overview of the funding and other opportunities at DMS for mathematicians and statisticians. I will also talk about our new program in computational and data-enabled science and engineering in mathematical and statistical sciences (CDS&E-MSS).

Fri, Nov 11: Henri Berestycki (EHESS and University of Chicago)

Reaction-diffusion equations and propagation phenomena

Starting with the description of reaction-diffusion mechanisms in physics, biology and ecology, I will explain the motivation for this class of non-linear partial differential equations and mention some of the interesting history of these systems. Then, I will review classical results in the homogeneous setting and discuss their relevance. The second part of the lecture will be concerned with recent developments in non-homogeneous settings, in particular for Fisher-KPP type equations. Such problems are encountered in models from ecology. The mathematical theory will be seen to shed light on questions arising in this context.

Wed, Nov 16: Henry Towsner (U of Conn-Storrs)

An Analytic Approach to Uniformity Norms

The Gowers uniformity norms have proven to be a powerful tool in extremal combinatorics, and a number of "structure theorems" have been given showing that the uniformity norms provide a dichotomy between "structured" objects and "random" objects. While analogous norms (the Gowers-Host-Kra norms) exist in dynamical systems, they do not quite correspond to the uniformity norms in the finite setting. We describe an analytic approach to the uniformity norms in which the "correspondence principle" between the finite setting and the infinite analytic setting remains valid.

Fri, Nov 18: Ben Recht (UW-Madison)

The Convex Geometry of Inverse Problems

Deducing the state or structure of a system from partial, noisy measurements is a fundamental task throughout the sciences and engineering. The resulting inverse problems are often ill-posed because there are fewer measurements available than the ambient dimension of the model to be estimated. In practice, however, many interesting signals or models contain few degrees of freedom relative to their ambient dimension: a small number of genes may constitute the signature of a disease, very few parameters may specify the correlation structure of a time series, or a sparse collection of geometric constraints may determine a molecular configuration. Discovering, leveraging, or recognizing such low-dimensional structure plays an important role in making inverse problems well-posed.

In this talk, I will propose a unified approach to transform notions of simplicity and latent low-dimensionality into convex penalty functions. This approach builds on the success of generalizing compressed sensing to matrix completion, and greatly extends the catalog of objects and structures that can be recovered from partial information. I will focus on a suite of data analysis algorithms designed to decompose general signals into sums of atoms from a simple---but not necessarily discrete---set. These algorithms are derived in a convex optimization framework that encompasses previous methods based on l1-norm minimization and nuclear norm minimization for recovering sparse vectors and low-rank matrices. I will provide sharp estimates of the number of generic measurements required for exact and robust recovery of a variety of structured models. I will then detail several example applications and describe how to scale the corresponding inference algorithms to massive data sets.