## RUTGERS MATHEMATICS DEPARTMENT COLLOQUIUM

### Rutgers UniversityDepartment of Mathematics

Co-organizers:
Angela Gibney (angela.gibney {at} gmail [dot] com)
Alex Kontorovich (alex.kontorovich {at} rutgers [dot] edu)
Danny Krashen (daniel.krashen {at} gmail [dot] com)
Chris Woodward (ctw {at} math [dot] rutgers [dot] edu)

### Forthcoming Talks

Unless otherwise specified, talks will be held online on the date indicated from 3:30pm to 4:30pm PM. Talks may not be held every week.

December 4: Wei Ho (Michigan)

### Fall 2020

Date: December 2 3:30pm-4:30
Speakers: Wei Ho
Title: The Hasse local-to-global principle for some genus one curves
Abstract:

The Hasse principle is a useful guiding philosophy in arithmetic geometry that relates "global" questions to analogous "local" questions, which are often easier to understand. A simple incarnation of the Hasse principle says that a given polynomial equation has a solution in the rational numbers (i.e., is "globally soluble") if and only if it has a solution in the real numbers and in the p-adic numbers for all primes p (i.e., is "everywhere locally soluble"). While this principle holds for many "simple" such polynomials, it is a very difficult question to classify the polynomials (or more generally, algebraic varieties) for which the principle holds or fails.

In this talk, we will discuss problems related to the Hasse principle for some classes of varieties, especially certain genus one curves. We will describe how to compute the proportion of these curves that are everywhere locally soluble (joint work with Tom Fisher and Jennifer Park), and we will explain why the Hasse principle fails for a positive proportion of these curves (joint work with Manjul Bhargava).

### Previous Talks

Date: November 18 3:30pm-4:30
Speakers: Rob Lazarsfeld
Title: How irrational is an irrational variety?
Abstract:

Recall that an algebraic variety is said to be rational if it contains a Zariski-open subset isomorphic to a Zariski-open subset of projective space. Subtle questions of rationality have seen a great deal of recent interest and progress, but most varieties aren't rational. I will survey a developing body of work in a complementary direction, centered around measures of irrationality for varieties whose non-rationality is known.

Date: November 11 3:30pm-4:30
Speakers: Rob Ghrist
Title: Opinion Dynamics on Sheaves
Abstract:

There is a long history of networked dynamical systems that models the spread of opinions over social networks, with the graph Laplacian playing a lead role. One of the difficulties in modelling opinion dynamics is the presence of polarization: not everyone comes to consensus. This talk will describe work joint with Jakob Hansen [OSU] introducing a new model for opinion dynamics using sheaves of vector spaces over social networks. The graph Laplacian is enriched to a Hodge Laplacian, and the resulting dynamics on *discourse sheaves* can lead to some very interesting and perhaps more realistic outcomes.

Date: November 4 3:30pm-4:30
Speakers: Chelsea Walton
Title: Quantum Symmetry
Abstract:

In this talk, I will discuss "quantum symmetry" from an algebraic viewpoint, especially for symmetries of algebras. The term "quantum" is used as algebras here are usually noncommutative. I will mention some interesting results on when symmetries of algebras must factor or do not factor through symmetries of classical gadgets (such as groups or Lie algebras), that is, when we must enter the realm of quantum groups (or Hopf algebras) to understand symmetries of a given algebra. This all fits neatly into the framework of studying algebras in monoidal categories, and if time permits, I will give some recent results in this direction. I aim to keep the level of the talk down-to-earth by including many basic definitions and examples.

Date: October 28 3:30pm-4:30
Speakers: Inna Zakharevich
Title: The Dehn complex: scissors congruence, K-theory, and regulators
Abstract:

Hilbert's third problem asks: do there exist two polyhedra with the same volume which are not scissors congruent? In other words, if $P$ and $Q$ are polyhedra with the same volume, is it alwayspossible to write $P = \bigcup_{i=1}^n P_i$ and $Q = \bigcup_{i=1}^nQ_i$ such that the $P$'s and $Q$'s intersect only on the boundaries and such that $P_i \cong Q_i$? In 1901 Dehn answered this question in the negative by constructing a second scissors congruence invariant now called the "Dehn invariant," and showing that a cube and a regular tetrahedron never have equal Dehn invariants, regardless of their volumes. We can then restate Hilbert's third problem: do the volume and Dehn invariant separate the scissors congruence classes? In 1965 Sydler showed that the answer is yes; in 1968 Jessen showed that this result extends to dimension 4, and in 1982 Dupont and Sah constructed analogs of such results in spherical and hyperbolic geometries. However, the problem remains open past dimension 4. By iterating Dehn invariants Goncharov constructed a chain complex, and conjectured that the homology of this chain complex is related to certain graded portions of the algebraic K-theory of the complex numbers, with the volume appearing as a regulator. In joint work with Jonathan Campbell, we have constructed a new analysis of this chain complex which illuminates the connection between the Dehn complex and algebraic K-theory, and which opens new routes for extending Dehn's results to higher dimensions. In this talk we will discuss this construction and its connections to both algebraic and Hermitian K-theory, and discuss the new avenues of attack that this presents for the generalized Hilbert's third problem.

Date: October 11 3:30pm-4:30
Speakers: Kasso Okoudjou
Title: An introduction to time-frequency analysis
Abstract:

Time-frequency analysis seeks to study functions or signals in both space and frequency. In this talk, I will focus on one time-frequency tool: the Weyl-Heisenberg or Gabor systems. The origin of these systems can be partially attributed to Dennis Gabor, who, in 1946, claimed that any square Lebesgue integrable function can be written as an infinite linear combination of time and frequency shifts of the standard Gaussian. Since then, decomposition methods for larger classes of functions or distributions in terms of various elementary building blocks have led to an impressive body of work in harmonic analysis. In this talk, I will give a brief introduction to the theory of Gabor frames and will survey some related easily stated yet unresolved open problems.

Date: October 14 3:30pm-4:30
Speakers: Benson Farb
Title: Hilbert's 13th problem and geometry
Abstract:

Hilbert's 13th Problem (H13) is a fundamental open problem about polynomials in one variable. It is part of a beautiful (but mostly forgotten) story going back 3 thousand years. In this talk I will explain how H13 (and related problems) fits into a wider framework that includes problems in enumerative algebraic geometry and the theory of modular functions. I will then report on some recent progress, joint with Mark Kisin and Jesse Wolfson. While some fancy objects will appear in this talk, much of it should (I hope) be understandable to undergraduate math majors.

Date: October 7 4:15pm - 5:15pm
Speakers: Sol Friedberg
Title: Mathematicians and K-12 Math Education
Abstract:

This talk will offer information on K-12 math education in the US, how this connects with the job of a university math department, and roles that individual concerned mathematicians can play. The speaker, whose primary activity is math research, is also Vice Chair of the US National Commission on Math Instruction (NAS).

Date: September 30 3:30pm-4:30
Speakers: Anna Wienhard
Title: Where geometry meets dynamics: groups, entropy and Hausdorff dimension
Abstract:

I will discuss geometric and dynamical properties of actions of discrete groups on Riemannian symmetric spaces. I will highlight some aspects of the interplay between geometry and dynamics, and present some recent results which generalize theorems of Sullivan, Bridgeman-Taylor, McMullen for convex cocompact subgroups acting on hyperbolic space in the framework of discrete subgroups of Lie groups of higher rank.

Date: September 23 3:30pm-4:30
Speakers: Gunther Uhlmann
Title: Journey to the Center of the Earth
Abstract:

We will consider the inverse problem of determining the sound speed or index of refraction of a medium by measuring the travel times of waves going through the medium. This problem arises in global seismology in an attempt to determine the inner structure of the Earth by measuring travel times of earthquakes. It has also several applications in optics and medical imaging among others.

The problem can be recast as a geometric problem: Can one determine the Riemannian metric of a Riemannian manifold with boundary by measuring the distance function between boundary points? This is the boundary rigidity problem. The linearization of this problems involves the integration of a tensor along geodesics, similar to the X-ray transform.

We will also describe some recent results, joint with Plamen Stefanov and Andras Vasy, on the partial data case, where you are making measurements on a subset of the boundary.

No previous knowledge of Riemannian geometry will be assumed.

Date: September 9 3:30pm-4:30
Speakers: Abid Ali, Yunbai Cao, Chris Lutsko
Title: Short Talks By Post-Docs
Abstract:

The first meeting of the department colloquium is tomorrow at 3:30; please see the Zoom link below. The first few weeks will be short talks by new postdocs, to get to know and welcome them to the department (as best we can virtually). Tomorrow's speakers are: Abid Ali, Yunbai Cao, and Chris Lutsko. We invite all to meet at "tea" (gather.town) as usual at 3, and head over to zoom at 3:30. The schedule is then: 10 min lecture, 5 min questions/discussion, 5 mins break (if people like, they can go back into the "lounge", gather.town), repeating for each of the 3 speakers.
Best,
Alex Kontorovich, on behalf of co-organizers Angela Gibney, Danny Krashen, and Chris Woodward.

This page is maintained by Chris Lutsko.