# 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) TBA 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) TBA Wasow lecture
Nov 18 Benjamin Recht (UW-Madison, CS Department) TBA Jordan
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 Tentatively Scheduled Street
Feb 10 Temporarily reserved 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 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 $\displaystyle{ E_7 }$ 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 $\displaystyle{ n }$ cameras. Our varieties are threefolds that vary in a family of dimension $\displaystyle{ 11n-15 }$ 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.

### 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).