# Department of Mathematics

## Department Colloquia

The UChicago math department hosts colloquia to introduce its members, and anyone else who wishes to attend, to recent developments and important ideas in mathematics.

### Fedor Manin (Ohio State): Growth, distortion, and isoperimetry in topology

Wednesday, May 29, 2019, 3:30pm-4:30pm, Eckhart 206

Abstract: The contractibility of a loop in our universe becomes almost irrelevant if it takes longer than the age of the universe to contract it; in other words, the simple connectivity of our experience is ultimately a geometric, not a topological fact. In the 1990's, this and other considerations led Gromov to propose a program of quantitative topology: asking about the "size" or "complexity" of the objects (a homotopy between two maps; a filling of a nullcobordant manifold) whose existence is implied by the results of algebraic and geometric topology. I will discuss the questions and the motivations behind them, as well as some answers, most of them recent.

### Song Sun (UC Berkeley): Complex structure degeneration and metric collapsing of Calami-Yau manifolds

Wednesday, May 8, 2019, 3:30pm-4:30pm, Eckhart 206

Abstract: A Calabi-Yau manifold is a compact Kahler manifold with trivial canonical bundle. Yau’s solution to the Calabi conjecture yields canonical Ricci-flat Kahler metrics (Calabi-Yau metric) on such a manifold, and this has deep applications in many areas of mathematics. It is a longstanding question to understand how the Ricci-flat metrics develop singularities when the complex structure degenerates. An especially intriguing phenomenon is that these metrics can collapse to lower dimensions and exhibit very non-algebraic features, and it is challenging to describe the corresponding geometric behavior. In this talk I will review the status of this problem and explain some recent progress.

### Toby Colding (MIT): A new approach to higher codimension mean curvature flow

Wednesday, May 1, 2019, 3:30pm-4:30pm, Eckhart 206

Abstract: We will discuss very recent joint work with Bill Minicozzi about a new approach to higher codimension mean curvature flow. This is a subject that has been notoriously difficult and where much less is known than for hypersurfaces. Some of the inspiration for this new approach comes from function theory on manifolds.

### Jennifer Wilson (University of Michigan): Representation Stability

Friday, April 26, 2019, 3:30pm - 4:30pm, Eckhart 206

Abstract: This talk will give an overview of the recent field of 'representation stability'. I will discuss how we can use representation theory to illuminate the structure of certain families of groups and topological spaces with actions of the symmetric groups $$S_n$$, focusing on configuration spaces as an illustrative example.

### Toti Daskalopoulos (Columbia): Ancient solutions to geometric Flows

Wednesday, April 17, 2019, 3:00pm-4:00pm, Eckhart 206

Abstract: Some of the most important problems  in  geometric flows  are related to the understanding of  singularities.  This usually happens through a blow up procedure near the  potential singularity which uses the scaling properties of the partial differential equation  involved.  In the case of a  parabolic  equation the blow up analysis often leads to ancient solutions which are  defined for all time  $$- \infty < t \leq T$$ for some $$T \leq +\infty$$. The classification of such solutions often sheds new insight to  the  singularity analysis. In some flows  it is also important for performing  surgery  near a singularity.  In this lecture we will give an overview of   {\em uniqueness theorems}  for ancient solutions to geometric partial differential equations  such as the Mean curvature flow, the  Ricci flow and the Yamabe flow.

### Mihnea Popa (Northwestern): D-modules in birational and complex geometry

Friday, March 15, 2019, 3:00pm-4:00pm, E206

Abstract: I will give an overview of techniques based on the theory of mixed Hodge modules, which lead to a number of applications of a rather elementary nature in birational and complex geometry, as well as in the study of singularities.  One of the main points I will emphasize is the existence (and usefulness) of a package of vanishing and positivity theorems in the context of filtered D-modules of Hodge theoretic origin.

### Simion Filip (Clay Math Institute and IAS): Hodge theory and dynamical systems

Thursday, March 14, 3:30pm-4:30pm, Eckhart 206

Abstract: I will explain some interactions between dynamics and Hodge theory, with applications going in both directions. For certain low-dimensional systems, such as flows on surfaces, Hodge theory provides the necessary analytic tools. It also connects the subject to arithmetic questions such as real multiplication and torsion points on abelian varieties. In the reverse direction, I will describe dynamical considerations that provide tools for a global study of variations of Hodge structure. One consequence is a new uniformization result for special families of algebraic 3-manifolds.

### Laura DeMarco (Northwestern): Complex dynamics and arithmetic equidistribution

Wednesday, March 13, 4:15-5:15pm, E133

Abstract: I will explain a notion of arithmetic equidistribution that has recently found application in the study of complex dynamical systems.  It was first introduced about 25 years ago, by Szpiro-Ullmo-Zhang, to analyze the geometry and arithmetic of abelian varieties.  In 2011, Matt Baker and I used the theory to study periodic points of maps on $$\mathbb{P}^1$$.  In this talk, I will explain some dynamical questions that were inspired by questions about elliptic curves, and then how the dynamical results allowed us to solve problems in the original setting of abelian varieties.  The new results are joint with Holly Krieger and Hexi Ye.

### Persi Diaconis (Stanford): Adding Numbers and Shuffling Cards

Wednesday March 13, 2019, 3:00 pm - 4:00pm, Eckhart 206

Abstract: When numbers are added in the usual way 'carries' occur along the way. It turns out that the carries form a Markov chain with an 'amazing' transition matrix. This same matrix occurs in fractal geometry, the Veronese embedding AND in the analysis of shuffling cards. I will explain the connections 'in english'

### Bhargav Bhatt (University of Michigan): Interpolating p-adic cohomology theories

Monday, Feb. 25, 3-4pm, Eckhart 203

Abstract: Integration of differential forms against cycles on a complex manifold helps relate de Rham cohomology to singular cohomology, which forms the beginning of Hodge theory. The analogous story for p-adic manifolds, which is the subject of p-adic Hodge theory, is richer due to a wider variety of available cohomology theories (de Rham, etale, crystalline, and more) and torsion phenomena. In this talk, I will give a bird's eye view of this picture, guided by the recently discovered notion of prismatic cohomology that provides some cohesion to the story.

### Tristan Rivière (Institute for Mathematical Research, ETH Zurich): The Cost of the Sphere Eversion and the 16\pi Conjecture?

Wednesday February 6, 2019, 3:00pm - 4:00pm, Eckhart 206

Abstract: How much does it cost...to knot a closed simple curve ? To cover the sphere twice ? to realize such or such homotopy class ? ...etc. All these questions consisting of assigning a "canonical" number and possibly an optimal "shape" to a given topological operation are known to be mathematically very rich and to bring together notions and techniques from topology, geometry and analysis.

In this talk we will concentrate on the operation consisting of turning inside out the 2 sphere in the 3 dimensional space. Since Smale's proof in 1959 of the existence of such an operation the search for effective realizations of such eversions has triggered a lot of fascination and works in the math community. The absence in nature of matter that can interpenetrate and the quasi impossibility, up to the advent of virtual imaging, to experience this deformation is maybe the reason for the difficulty to develop an intuitive approach on the problem.

We will present the optimization of Sophie Germain conformally invariant elastic energy for the eversion. Our efforts will finally bring us to consider more closely an integer number together with a mysterious minimal surface.

### Pierre-Louis Lions (Collège de France): Mean Field Games: What? Why? How?

Friday February 1, 2019, 4:00pm - 5:00pm, Kent 120

Abstract: Mean Field Games (MFG) are recent mathematical models that aim to describe the collective behavior of a large number of agents/players. We shall briefly present in this talk some motivation and a few applications to economics, crowd motion, mobile communication networks, and machine learning. Without going into any (technical) mathematical detail, the existing mathematical toolbox will be described together with some recent developments and perspectives.

### Angus Macintyre (University of Edinburgh): The Complex and the Zilber Exponential

Monday October 29, 2018, 3:00 pm - 4:00pm, Ryerson 251

Abstract: In 2005 Boris Zilber published a very influential paper, giving a sophisticated model-theoretic construction of an exponential field satisfying Schanuel's Conjecture and a kind of Nullstellensatz relating to extensions preserving a dimension arising from Schanuel's Conjecture. He proved some very remarkable properties of such exponential fields, and boldly conjectured that the classical complex exponential field is one of these fields. The conjecture of course assumes that Schanuel's Conjecture is true for the complex exponential, and subsequent research has generally made this assumption.

The conjecture survives, and one has been able to show that the two exponential fields share many properties, normally established by quite different proofs for the two fields. I will survey the situation, with special reference to a conjecture in complex analysis made sixty years ago by H. Shapiro.

### Anna Gilbert (University of Michigan): Sparse matrices in sparse analysis

Monday October 22, 2018, 3:00pm - 4:00pm, Ryerson 251

Abstract: In this talk, I will give two vignettes on the theme of sparse matrices in sparse analysis. The first vignette covers work from compressive sensing in which we want to design sparse matrices (i.e., matrices with few non-zero entries) that we use to (linearly) sense or measure compressible signals. We also design algorithms such that, from these measurements and these matrices, we can efficiently recover a compressed, or sparse, representation of the sensed data. I will discuss the role of expander graphs and error correcting codes in these designs and applications to high throughput biological screens. The second vignette flips the theme; suppose we are given a distance or similarity matrix for a data set that is corrupted in some fashion, find a sparse correction or repair to the distance matrix so as to ensure the corrected distances come from a metric; i.e., repair as few entries as possible in the matrix so that we have a metric. I will discuss generalizations to graph metrics, applications to (and from) metric embeddings, and algorithms for variations of this problem. I will also touch upon applications in machine learning and bio-informatics.

### Sylvia Serfaty (Courant Institute): Systems of points with Coulomb interactions

Wednesday, October 17, 2018, 3:00pm - 4:00pm, Eckhart 206

Abstract: Large ensembles of points with Coulomb interactions arise in various settings of condensed matter physics, classical and quantum mechanics, statistical mechanics, random matrices and even approximation theory, and they give rise to a variety of questions pertaining to analysis, Partial Differential Equations and probability. We will first review these motivations, then present the ''mean-field'' derivation of effective models and equations describing the system at the macroscopic scale. We then explain how to analyze the next order behavior, giving information on the configurations at the microscopic level and connecting with crystallization questions, and finish with the description of the effect of temperature.

### Hugh Woodin (Harvard): Beyond the age of independence by forcing?

Wednesday, May 31, 2017, 3:00pm–4:00pm, Eckhart 206

Abstract: Gödel’s consistency proof for the Axiom of Choice and the Continuum Hypothesis involves his discovery of the Constructible Universe of Sets. The axiom "$$V = L$$" is the axiom which asserts that every set is constructible. This axiom settles the Continuum Hypothesis and more importantly, Cohen’s method of forcing cannot be used in the context of the axiom "$$V = L$$".

However the axiom $$V = L$$ is false since it limits the fundamental nature of infinity. In particular the axiom refutes (most) strong axioms of infinity.

A key question emerges. Is there an "ultimate" version of Gödel’s constructible universe L yielding an axiom "$$V = \text{ultimate } L$$" which retains the power of the axiom "$$V = L$$" for resolving questions like that of the Continuum Hypothesis, which is also immune against Cohen’s method of forcing, and yet which does not refute strong axioms of infinity?

Until recently there seemed to be a number of convincing arguments as to why no such ultimate $$L$$ can possibly exist. But the situation is now changed.

### Emmy Murphy (Northwestern): Planar graphs, Legendrian surfaces, and contact homology

Wednesday, May 10, 2017, 3:00pm–4:00pm, Eckhart 206

Abstract: To any cubic planar graph, we can associate a surface inside $$\mathbb{R}^5$$, which is nicely compatible with a certain canonical geometry, called a contact structure. From here we can apply techniques from contact geometry to study graph theory; for example it was shown by Treumann-Zaslow that the space of constructable sheaves with singular support on the surface recovers the chromatic polynomial of the graph. Another strategy is to look at pseudo-holomorphic curves with boundary conditions on the surface, which defines an algebraic package known as Legendrian contact homology.

We'll explain what this gadget looks like in purely graph theoretical terms. In particular, the augmentation variety of this contact homology recovers the chromatic polynomial of the graph, though it does this in a highly non-trivial way and therefore yields a novel combinatorial definition of a graph coloring. But it also contains more subtle information about the graph, concerning how trajectories in the plane are required to interact with the graph. From here we can draw connections with mathematical physics, such as the spectral networks of Gaiotto-Moore-Neitzke and mirror symmetry in the style of Aganagic-Vafa.

### Ciprian Manolescu (UCLA): The Triangulation Conjecture

Wednesday, April 26, 2017, 3:00pm–4:00pm, Eckhart 206

Abstract: The triangulation conjecture stated that any $$n$$-dimensional topological manifold is homeomorphic to a simplicial complex. It is true in dimensions at most 3, but false in dimension 4 by the work of Casson and Freedman. In this talk I will explain the proof that the conjecture is also false in higher dimensions. This result is based on previous work of Galewski-Stern and Matumoto, who reduced the problem to a question in low dimensions (the existence of elements of order 2 and Rokhlin invariant one in the 3-dimensional homology cobordism group).

### James Lee (University of Washington): Discrete conformal metrics and spectral geometry on distributional limits

Wednesday, March 29, 2017, 3:00pm–4:00pm, Eckhart 206

Abstract: Random triangulations have been studied as a discrete model for 2D quantum gravity since the 1980s. While there are many conjectures about the emergent geometry of such models, mathematical progress has been somewhat slower. Toward this end, Benjamini and Schramm (2001) defined the distributional limit of a sequence of finite graphs; the limit object is a unimodular random graph in the sense of Aldous and Lyons. Benjamini and Schramm showed that every distributional limit of finite planar graphs with uniformly bounded degrees is almost surely recurrent (the random walk returns to its starting point infinitely often almost surely).

Their approach uses the Koebe-Andreev-Thurston circle packing theorem to uniformize the geometry of the limit graph. In contrast, we consider intrinsic deformations of the path metric by a (random) weighting of the vertices. This leads to the notion of the conformal growth exponent of a unimodular random graph, which is the best degree of volume growth of balls that can be achieved by such a weighting of "unit area." The conformal growth exponent carries information about the underlying geometry; in particular, it bounds the almost sure spectral dimension.

We show that distributional limits of finite graphs that can be sphere-packed in $$\mathbb{R}^d$$ have conformal growth exponent at most $$d$$, and thus the connection to spectral dimension yields $$d$$-dimensional lower bounds on the heat kernel. When the conformal growth exponent is bounded by 2, one obtains more precise information, including a conjectured generalization of the Benjamini-Schramm Recurrence Theorem to larger families of graphs.

These methods extend to models with unbounded degrees, giving new proofs of almost sure recurrence for the extensively studied uniform infinite planar triangulation (UIPT) and quadrangulation (UIPQ). The latter results were established only recently by Gurel-Gurevich and Nachmias (2013). Our approach yields quantitative lower bounds on the heat kernel, spectral measure, and speed of the random walk.

### Melanie Wood (UW Madison): Random groups from generators and relations

Wednesday, January 11, 3:00pm–4:00pm, Eckhart 206

Abstract: We consider a model of random groups that starts with a free group on $$n$$ generators and takes the quotient by $$n$$ random relations. We discuss this model in the case of abelian groups (starting with a free abelian group), and its relationship to the Cohen-Lenstra heuristics, which predict the distribution of class groups of number fields. We will explain a universality theorem, an analog of the central limit theorem for random groups, that says the resulting distribution of random groups is largely insensitive to the distribution from which the relations are chosen. Finally, we discuss joint work with Yuan Liu on the non-abelian random groups built in this way, including the existence of a limit of the random groups as $$n$$ goes to infinity.

### Nick Rozenblyum (UChicago): Shifted symplectic structures, quantization, and applications

Wednesday, October 26, 3:00pm–4:00pm, Eckhart 206

Abstract: Many moduli problems of interest, such as moduli spaces of local systems, come equipped with a natural symplectic structure. Moreover, quantization of these symplectic structures is closely related to counting problems in geometry and topology, such as the Casson invariant and its generalizations, as well as Feynman integration in physics. The theory of shifted symplectic structures, a vast generalization of algebraic symplectic geometry, provides a natural framework for constructing and studying such symplectic structures and their quantizations. I will give a brief overview of this theory and describe applications in geometry and physics.

### Doug Ravenel (Rochester): What is a $$G$$-spectrum?

Wednesday, May 25, 2016, 3:00pm–4:00pm, Eckhart 206

Abstract: Spectra in the sense of stable homotopy theory have been a major object of study in algebraic topology for half a century. During that time the basic definition has undergone some major revisions, including a major breakthrough in 1993 due to Peter May and three coauthors. Remarkably, these shifting foundations have not affected any of the computations made using earlier definitions. In the talk I will describe how the use of category theory has led to major simplifications.