# Colloquia 2012-2013

# Mathematics Colloquium

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

## Spring 2013

## Fall 2012

## Abstracts

### Sept 14: Jordan Ellenberg (UW-Madison)

*FI-modules: an introduction*
(joint work with T Church, B Farb, R Nagpal)

In topology and algebraic geometry one often encounters phenomena of _stability_. A famous example is the cohomology of the moduli space of curves M_g; Harer proved in the 1980s that the sequence of vector spaces H_i(M_g,Q), with g growing and i fixed, has dimension which is eventually constant as g grows with i fixed.

In many similar situations one is presented with a sequence {V_n}, where the V_n are not merely vector spaces, but come with an action of S_n. In many such situations the dimension of V_n does not become constant as n grows -- but there is still a sense in which it is eventually "always the same representation of S_n" as n grows. The preprint

http://arxiv.org/abs/1204.4533

shows how to interpret this kind of "representation stability" as a statement of finite generation in an appropriate category; we'll discuss this set-up and some applications to the topology of configuration spaces, the representation theory of the symmetric group, and diagonal coinvariant algebras. Finally, we'll discuss recent developments in the theory of FI-modules over general rings, which is joint work with (UW grad student) Rohit Nagpal.

### Sept 21: Joyce R. McLaughlin (Rensselaer Polytechnic Institute)

*Mathematics for Imaging Biomechanical Parameters in Dynamic Elastography*

Elastography, the imaging of biomechanical parameters in tissue, is motivated by the doctor’s palpation exam where the doctor presses against the skin to detect stiff and abnormal tissue changes. In dynamic elastography experiments, the tissue is in motion with displacement amplitudes on the order of tens of microns. The displacement(s) are determined with sequences of MR data sets or sequences of ultrasound RF/IQ data sets within the tissue; and the data shows a dispersive effect indicating that tissue is viscoelastic. A choice of viscoelastic model must be made. For each model the biomechanical parameters satisfy a first order, linear or nonlinear, partial differential equation (system) with real or complex coefficients. We discuss the mathematical properties of these equations and how those properties lead to successful interpretation of the data, and to successful algorithms and images. We show biomechanical images of breast cancer and prostate cancer and compare those images to ultrasound images and histology slides with marked cancerous inclusions.

### Sept 28: Eric Marberg (MIT)

*Supercharacters for algebra groups: applications and extensions*

The group U_n(F_q) of unipotent upper triangular matrices over a finite field belongs to the same list of fundamental examples as the symmetric or general linear groups. It comes as some surprise, therefore, that the group's irreducible characters are unknown, and considered in some sense unknowable. In order to tackle problems normally requiring knowledge of a group's irreducible characters, Diaconis and Isaacs developed the notion of the supercharacters of an algebra group, generalizing work of Andre and Yan. Algebra groups form a well-behaved class of p-groups including U_n(F_q) as a prototypical example, and supercharacters are certain reducible characters which form a useful approximation to the set of irreducible characters. In this talk I will survey several equivalent definitions of the supercharacters of an algebra group, and discuss some applications and extensions of these approaches. On one end of things, I intend briefly to introduce the recent discovery of how certain representation theoretic operations on the supercharacters of U_n(F_q) naturally define a Hopf algebra structure, which has been studied under a different name by combinatorialists. In another direction, I will explain how one can view the supercharacters of an algebra group as the first step in a more general reduction process, which can be used to shed light on some mysterious properties of U_n(F_q).

### Oct 04 Howard Masur (Chicago)

*Winning games for badly approximable real numbers and billiards in polygons*

Wolfgang Schmidt invented the notion of a winning subset of Euclidean space in a game between two players. Winning sets have nice properties such as full Hausdorff dimension. The basic example of a winning set considered by Schmidt are those reals badly approximated by rationals. An equivalent formulation by Artin is that badly approximable reals correspond to geodesics that stay in a bounded set in the modular curve. There is also an equivalent formulation in terms of sets of directions for the linear flow on a square torus or equivalently, billiard trajectories in a square. In joint work with Yitwah Cheung and Jon Chaika we extend this notion of winning to flows on flat surfaces of higher genus with applications to billiards in rational angled polygons. My intention in this talk is to give the background on the Schmidt game, describe the classical results before introducing the more recent work.

**Wed, Oct 10**: Bas Lemmens (University of Kent)

*From hyperbolic geometry to nonlinear Perron-Frobenius theory*

In a letter to Klein Hilbert remarked that the logarithm of the cross-ratio is a metric on any open, bounded, convex set in Euclidean space. These metric spaces are nowadays called Hilbert geometries. They are a natural non-Riemannian generalization of Klein's model of the hyperbolic plane, and play a role in the solution of Hilbert's fourth problem.

In the nineteen fifties Garrett Birkhoff and Hans Samelson independently discovered that one can use Hilbert's metric and the contraction mapping principle to prove the existence and uniqueness of a positive eigenvector for a variety of linear operators that leave a closed cone in a Banach space invariant. Their results are a direct extension of the classical Perron-Frobenius theorem concerning the eigenvectors and eigenvalues of nonnegative matrices. In the past decades this idea has been further developed and resulted in strikingly detailed nonlinear extensions of the Perron-Frobenius theorem. In this talk I will discuss the synergy between metric geometry and (nonlinear) operator theory and some of the recent results and open problems in this area.

**Thur, Oct 11**: Irene Gamba (University of Texas-Austin)

*Analytical and numerical issues associated with the dynamics of the non-linear Boltzmann equation*

The non-linear Boltzmann equation models the evolution of a statistical flow associated with particle systems in a rarefied or mesoscopic regimes. Its analytical local as well as long time behavior depends strongly on the growth conditions of the collisional kernels, as functions of the intermolecular potentials and scattering mechanisms. We will present recent analytical results, such as classical convolutional inequalities and sharp moments estimates, which imply propagation of $W^{k,p}$ norms and exponential decay of high energy tails, and their consequences on the existence, regularity, and stability of solutions for initial value problems as well as control of decay rates to equilibrium. In particular we present a numerical approximation to the non-linear Boltzmann problem by a conservative spectral scheme, and show spectral accuracy as well as error estimates.

### Oct 12: Joachim Rosenthal (Univ of Zurich)

*Linear Random Network Codes, a Grassmannian Approach*

Elastography, the imaging of biomechanical parameters in A novel framework for random network coding has been introduced by Koetter and Kschischang. In this framework information is encoded in subspaces of a given ambient space over a finite field. A natural metric is introduced where two subspaces are `close to each other' as soon as their dimension of intersection is large. This framework poses the challenge to come up with new codes with optimal or near optimal distance and to develop efficient decoding algorithms.

In a first part of the talk we will provide a survey. In a second part of the talk we report on progress constructing spread codes and orbit codes. The decoding problem of orbit codes can be interpreted as a problem in Schubert calculus over a finite field.

**Fri, Oct 19**: Saverio E. Spagnolie (Madison)

*Elastic slender bodies in fluids and slender bodies in elastic fluids*

Abstract:

The scientific study of elastic materials dates back to Galileo, and fluid mechanics to Archimedes, but the interaction of elastic bodies and viscous fluids remains a topic at the frontier of modern research. We will discuss two problems on this topic of recent interest. First, when a flexible filament is confined to a fluid interface, the balance between capillary attraction, bending resistance, and tension from an external source can lead to a self-buckling instability. We will walk through an analysis of this elastocapillary instability, and analytical formulae will be shown that compare favorably with the results of detailed numerical computations. Second, we will discuss the motility of a swimming helical body in a viscoelastic fluid, wherein the fluid itself exhibits an elastic response to deformation. The helical geometry is exploited to generate a highly accurate numerical method, and we will show that the introduction of viscoelasticity can either enhance or retard the swimming speed depending on the body geometry and the properties of the fluid (through a dimensionless Deborah number). Our findings bridge the gap between studies showing situationally dependent enhancement or retardation of swimming speed, and may help to clarify phenomena observed in systems from spermatozoan swimming to mechanical drilling.

**Fri, Oct 26**: Luke Oeding (UC Berkeley)

*The Trifocal Variety*

Abstract: In Computer Vision one considers many cameras looking at the same scene. From this setup many interesting geometric and algebraic questions arise. In this talk we will focus on the case of 3 cameras and study the so called trifocal tensors. Trifocal tensors are constructed from a bilinear map defined using the trifocal setup. A natural question is, given a particular tensor, how can one determine if it is a trifocal tensor? This question can be answered by finding implicit defining equations for the trifocal variety. From an algebraic standpoint, it is also interesting to know the minimal generators of the defining ideal of the trifocal variety.

In this talk I will explain our use of symbolic and numerical computations aided by Representation Theory and Numerical Algebraic Geometry to find the minimal generators of the ideal of the trifocal variety. This is joint work with Chris Aholt (Washington). Our work builds on the work of others (such as Hartley-Zisserman, Alzati-Tortora and Papadopoulo-Faugeras) who have already considered this problem set-theoretically.

My goal is to make most of the talk accessible to anyone with a modest background in Linear Algebra.

**Tues, Oct 30**: Andrew Majda (Courant)

*Data Driven Methods for Complex Turbulent Systems*

An important contemporary research topic is the development of physics constrained data driven methods for complex, large-dimensional turbulent systems such as the equations for climate change science. Three new approaches to various aspects of this topic are emphasized here. 1) The systematic development of physics constrained quadratic regression models with memory for low-frequency components of complex systems; 2) Novel dynamic stochastic superresolution algorithms for real time filtering of turbulent systems; 3) New nonlinear Laplacian spectral analysis (NLSA) for large-dimensional time series which capture both intermittency and low-frequency variability unlike conventional EOF or principal component analysis. This is joint work with John Harlim (1,2), Michal Branicki (2), and Dimitri Giannakis (3).

**Thurs, Nov 1**: Lenya Ryzhik (Chicago)

*The role of a drift in elliptic and parabolic equations*

The first order partial differential equations are closely connected to the underlying characteristic ODEs. The second order elliptic and parabolic equations are as closely connected to the Brownian motion and more general diffusions with a drift. From a variety of points of view, the drift does not really matter - all diffusions look more or less similar. As a caveat to this reasonable line of thought, I will describe a menagerie of problems: linear and nonlinear, steady and time-dependent, compressible and incompressible, where the drift and diffusion confirm the Mayakovsky thesis "Woe to one alone!" Together, they lead to enhanced mixing and improved regularity that are impossible for each one of them to attain.

### Nov 2: Vladimir Sverak (Minnesota)

*On scale-invariant solutions of the Navier-Stokes equations*

The solutions of the Navier-Stokes equations which are invariant under the scaling symmetry of the equations provide an interesting window into non-linear regimes which are not accessible by perturbation theory. They appear to give valuable hints concerning the old question about the uniqueness of weak solutions. In the lecture we outline a recent proof of the result that for every scale-invariant initial data there is a global scale-invariant solution (smooth for positive times), and we explain connections to the uniqueness problem. This is joint work with Hao Jia.

### Nov 9: Uri Andrews (Madison)

*Computable Stability Theory*

Stability theory attempts to classify the underlying structure of mathematical objects. The goal of computable mathematics is to understand when mathematical objects or constructions can be demonstrated computably. I'll talk about the relationship between underlying structure and computation of mathematical objects.

### Monday, Nov 12: Charles Smart (MIT)

*Regularity and stochastic homogenization of fully nonlinear equations without uniform ellipticity*

I will discuss regularity of fully nonlinear elliptic equations when the usual uniform upper bound on the ellipticity is replaced by bound on its $L^d$ norm, where $d$ is the dimension of the ambient space. Our estimates refine the classical theory and require several new ideas that we believe are of independent interest. As an application, we prove homogenization for a class of stationary ergodic strictly elliptic equations.

### Wednesday, Nov 28: Cameron Hill (Notre Dame)

*Genericity in Discrete Mathematics*

I will discuss the intriguing role that ``generic* objects can play in finitary discrete mathematics. Using the framework of model theory, we will see that there are at least two reasonable but very different notions of genericity relative to a class of finite structures (like a class of finite graphs or a class of finite groups). Through discussions of zero-one laws and structural Ramsey theory, I will try to illustrate the importance of generically-categorical classes -- those for which the various notions of genericity coincide. To finish, I will state some results that follow from the assumption of generic-categoricity.*

### Nov 30: Nam Le (Columbia)

*Boundary regularity for solutions to the linearized Monge-Ampere equations and applications*

In this talk, I will discuss boundary regularity of solutions to the linearized Monge-Ampere equations, and their applications to nonlinear, fourth order, geometric Partial Differential Equations (PDE). First, I will present my regularity results in joint work with O. Savin and T. Nguyen including: boundary Holder gradient estimates and global $C^{1,\alpha}$ estimates, global Holder estimates and global $W^{2,p}$ estimates. Then, I will describe applications of the above regularity results to several nonlinear, fourth order, geometric PDE such as: global second derivative estimates for the second boundary value problem of the prescribed affine mean curvature and Abreu's equations; and global regularity for minimizers having prescribed determinant of certain convex functionals motivated by the Mabuchi functional in complex geometry.

### Dec 14: Amanda Folsom (Yale)

*q-series and quantum modular forms*

While the theory of mock modular forms has seen great advances in the last decade, questions remain. We revisit Ramanujan's last letter to Hardy, and prove one of his remaining conjectures as a special case of a more general result. Surprisingly, Dyson's combinatorial rank function, the Andrews-Garvan crank functions, mock theta functions, and quantum modular forms, all play key roles. Along these lines, we also show that the Rogers-Fine false theta functions, functions that have not been well understood within the theory of modular forms, specialize to quantum modular forms. This is joint work with K. Ono (Emory U.) and R.C. Rhoades (Stanford U.).

### Tues, Jan 15, B139: Lillian Pierce (Oxford)

*A new twist on the Carleson operator*

Must the Fourier series of an L^2 function converge pointwise almost everywhere? In the 1960's, Carleson answered this question in the affirmative, by studying a particular type of maximal singular integral operator, which has since become known as a Carleson operator. In the past 40 years, a number of important results have been proved for generalizations of the original Carleson operator. In this talk we will introduce the Carleson operator and survey several of its generalizations, and then describe new joint work with Po Lam Yung that introduces curved structure to the setting of Carleson operators.

### Thurs, Jan 17, 2pm, 901VV: Jonah Blasiak (Michigan)

*Positivity, complexity, and the Kronecker problem*

Positivity problems in algebraic combinatorics ask to find positive combinatorial formulae for nonnegative quantities arising in geometry and representation theory like cohomological dimensions and dimensions of algebras and their irreducible representations. A famous open positivity problem in representation theory is the Kronecker problem, which asks for a positive combinatorial formula for decomposing tensor products of irreducible representations of the symmetric group. We will begin with a general discussion of positivity problems and an intriguing new motivation for these problems from complexity theory. We will then present our solution to a special case of the Kronecker problem that substantially improves on previous results.

### Fri, Jan 25: Alexander Fish (Sydney)

*Product sets in amenable groups through a dynamical approach*

We will describe a new correspondence between product sets in a countable amenable group and product sets in compact groups. This approach enables us to obtain quantitative results concerning product sets in amenable groups. The talk is for general mathematical audience. (based on a joint work with M. Bjorklund)

### Fri, Feb 1: Tullia Dymarz (Madison)

*Quasisymmetric vs Bi-Lipschitz maps*

On a metric space, there are various classes of functions which respect aspects of the metric space structure. One of the most basic classes is the bi-Lipschitz maps (Lipschitz maps whose inverses are also Lipschitz). Another possibly much larger class consists of the so-called quasisymmetric maps (these are closely related quasiconformal maps). On both Euclidean space and the p-adics, there are many quasisymmetric maps which are not bi-Lipschitz. However, on the product of Euclidean space with the p-adics, we show that all quasisymmetric maps are bi-Lipschitz. Furthermore, our proof does not use any direct analysis but instead uses coarse topology and results from negative curvature.

### Feb 8: Jean-Luc Thiffeault (Madison)

*pseudo-Anosovs with small or large dilatation*

Homeomorphisms of a surface to itself can be classified using the well-known Thurston-Nielsen theorem. The most interesting topological class contains pseudo-Anosov mappings: they stabilize a pair of transverse singular foliations with a finite number of singularities. These foliations are called unstable and stable, and are respectively expanded and compressed by an algebraic constant called the dilatation. Characterizing the possible values of these dilatations for a given suface is an open problem. Here I discuss a method to find the minimum value of the dilatation on closed surfaces of a given genus, for the special case where the foliations are orientable. I will then address the opposite problem: how to find pseudo-Anosovs with large dilatations. Unlike the minimizer problem, this is not well-defined -- the answer is infinity -- unless we add a constraint. Constraints can arise from practical optimization problems in engineering, and I will show some optimal solutions that can be incorporated in devices called taffy pullers. (This is joint work with Erwan Lanneau and Matt Finn.)

### Feb 15: Eric Lauga (UCSD)

*Optimization in fluid-based locomotion*

The world of self-propelled low-Reynolds number swimmers is inhabited by a myriad of microorganisms such as bacteria, spermatozoa, ciliates, and plankton. In this talk, we focus on the locomotion of ciliated cells. Cilia are short slender whiplike appendages (a few microns long, one tenth of a micron wide) internally actuated by molecular motors (dyneins) which generate a distribution of bending moments along the cilium length and produce time-varying shape deformations. In most cases cilia are not found individually but instead in densely packed arrays on surfaces. In this work we will ask the question: can the individual and collective dynamics of cilia on the surface of an individual microorganism be rationalized as the solution to an optimization problem? We first address the deformation of individual cilia anchored on surfaces before characterizing the locomotion and feeding by surface distortions of swimmers covered by cilia array. We demonstrate, as solution to the optimization procedure, the appearance of the well-known two-stroke kinematics of an individual cilium, as well as waves in cilia array reminiscent of experimentally-observed metachronal waves.

### Feb 22: Svitlana Mayboroda (University of Minnesota)

*Elliptic PDEs, analysis, and potential theory in irregular media*

Elliptic boundary value problems are well-understood in the case when the boundary, the data, and the coefficients exhibit smoothness. However, perfectly uniform smooth systems do not exist in nature, and every real object inadvertently possesses irregularities (a sharp edge of the boundary, an abrupt change of the medium, a defect of the construction).

The analysis of general non-smooth elliptic PDEs gives rise to decisively new challenges: possible failure of maximal principle and positivity, breakdown of boundary regularity, lack of the classical L^2 estimates, to mention just a few. Further progress builds on an involved blend of harmonic analysis, potential theory and geometric measure theory techniques. In this talk we are going to discuss some highlights of the history, conjectures, paradoxes, and recent discoveries such as the higher-order Wiener criterion and maximum principle for higher order PDEs, solvability of rough elliptic boundary problems, as well as an intriguing phenomenon of localization of eigenfunctions -- within and beyond the limits of the famous Anderson localization.

Parts of the talk are based on joint work with S. Hofmann, M. Filoche, C. Kenig, V. Maz'ya, and J. Pipher.

### March 1: Kirsten Wickelgren (Harvard)

*Grothendieck's anabelian conjectures*

Grothendieck's anabelian conjectures predict that the solutions to certain polynomial equations over Q are determined by the loops on the corresponding space of all solutions, or more precisely that the etale fundamental group is a fully faithful functor from certain anabelian schemes to profinite groups with Galois action. This is analogous to an equivalence between fixed points and homotopy fixed points for Galois actions. We will introduce the anabelian conjectures and their topological analogues, and relate certain nilpotent obstructions to the existence of rational points introduced by Jordan Ellenberg to higher cohomology operations. These cohomology operations encode when the existence of particular Galois extensions implies the existence of others, and are connected with the formality of the etale cochains. As a corollary one has that the order n Massey product <x,x,..x,1-x,x,...x> vanishes, where x denotes the image of x in k* under the Kummer map k* -> H^1(Gal(kbar/k), Z_l(1)).

### March 8: Dan Negrut (UW-Mechanical Engineering)

*Using Advanced Computing in Applied Dynamics: From the Dynamics of Granular Material to the Motion of the Mars Rover*

This talk outlines numerical solution methods and their implementation in a high performance computing-enabled software infrastructure aimed at supporting physics-based simulation for virtual design in Engineering. The applications of interest include granular dynamics, rigid/flexible many-body dynamics, and fluid-solid interaction problems. CHRONO, the software infrastructure developed as part of this ongoing effort, partitions the problem of interest into a number of sub-problems that are solved in parallel using Graphics Processing Unit (GPU) cards, or multi-core CPUs. The five components at the cornerstone of the vision that eventually led to CHRONO are: (a) modeling support for multi-physics phenomena; (b) scalable numerical methods for multi-GPU and multi-core hardware architectures; (c) methods for proximity computation and collision detection; (d) support for domain decomposition and load balance; and (e) tools for carrying out visualization and post-processing in a distributed manner. Several engineering applications will be used to demonstrate how these five components are implemented to leverage a heterogeneous CPU/GPU supercomputer available at the Wisconsin Applied Computing Center. The talk will conclude with a brief discussion of current trends in high performance computing and how they are poised to change the field of Computational Science in the near future.

### March 15: Kurusch Ebrahimi Fard (Madrid)

*Spitzer-type identities in non-commutative Rota-Baxter algebras*

Gian-Carlo Rota suggested in one of his last articles the problem of developing the notion of integration algebra, complementary to the already existing theory of differential algebras. This idea was mainly motivated by Rota's deep appreciation for Chen's fundamental work on iterated integrals. As a starting point for such a theory Rota proposed to consider a particular operator identity first introduced in 1960 by the mathematician Glen Baxter. It was later coined Rota-Baxter identity. Examples range from algebras with a direct decomposition into subalgebras to algebras of functions equipped with the ordinary Riemann integral or its discrete analogs. Rota-Baxter algebras feature a genuine factorization property. It is intimately related to linear fixpoint equations, such as those, for instance, appearing in the renormalization problem in perturbative quantum field theory. For arbitrary commutative Rota-Baxter algebras, proper exponential solutions of such fixpoint equations are described by what is known as the classical Spitzer identity. The similar classical Bohnenblust-Spitzer identity involves the symmetric group, and generalizes the simple observation that the n-fold iterated integral of a function is proportional to the n-fold product of the primitive of this function. Recently, the seminal Cartier-Rota theory of classical Spitzer-type identities has been generalized to noncommutative Rota-Baxter algebras. Pre-Lie algebras (also known as Vinberg or Gerstenhaber algebras) play a crucial role in this approach. In this talk we will provide a short introduction to Rota-Baxter algebras, and review recent work on Spitzer-type identities. This talk is based on joint work with Frederic Patras (CNRS, Nice, France) and Dominique Manchon (CNRS, Clermont-Ferrand, France).

### April 5: John Jones (ASU)

*Number fields with prescribed ramification*

Finite extensions of the rational numbers are basic objects of algebra and number theory. We discuss the interaction of two basic invariants associated to such a field, its set of ramified primes and its Galois group. After describing the basic problem, we present computational and theoretical results.

### April 12: Andrew Snowden (MIT)

In recent years, it has been realized that various large algebraic structures admitting large amounts of symmetry behave as if they were small. For example, when symmetries are taken into account, polynomial rings in infinitely many variables retain some of the favorable properties of polynomial rings in finitely many variables. This observation has been applied to obtain uniformity results in diverse fields. I will describe some of the recent work in this area.

### April 19: Moe Hirsch (Honorary Fellow -- UW Madison)

I will discuss various ideas about mathematics which many of its practitioners seem to believe, including the Myth of Truth, the Myth of Proof, the Myth of Certainty, and so forth.

This talk is available as a pdf file:

<http://sprott.physics.wisc.edu/chaos-complexity/hirsch12.pdf>.

### May 3: Davesh Maulik (Columbia)

**Plane curve singularities and knot invariants**

Given a polynomial function $f(x,y)$ in two complex variables, with a critical point at the origin, by studying the nearby points, one can associate a knot (or link) inside $\mathbb{R}^3$. In nice situations, we can try to understand analytic properties of the singularity in terms of topological properties of this knot. For example, Milnor showed that the codimension of the ideal generated by the partial derivatives is related to the degree of the Alexander polynomial of the link. I will explain a broad generalization of this example, first conjectured by Oblomkov and Shende, which relates the geometry of the Hilbert scheme of the singularity to the HOMFLY polynomial of the link. As time permits, I will discuss further extensions (some known, some conjectural).

### May 10: Steve Gelbart (Weizmann Institute, Israel)

*"Zeta"*

This will be an introductory (and incomplete) lecture on the story of “zeta”. Starting with the fundamental work of Euler, Kummer and Riemann, and the telling work of Artin, Hecke and Hasse-Weil, we end up with the ever growing mysteries of Iwasawa, Langlands, and beyond.