# Department of Mathematics and Statistics

## Colloquia

Colloquia for the Department of Mathematics and Statistics are normally held in University Hall 4010 on Fridays at 4:00 p.m. Any departures from this are indicated below. Light refreshments are served before the colloquia in 2040 University Hall. Driving directions, parking information and maps are available on the university website.

For the 2021-2022 academic year the colloquia will be held online due to COVID-19.
Here is the link: Blackboard Collaborate Colloquia Meeting Room

## 2021-2022 Colloquia

Below is a list of speakers, talk titles and abstracts for the current academic year. Abstracts for the talks are also posted in the hallways around the departmental offices.

### Spring Semester

April 29
Chanaka Kottegoda (The University of Toledo)
Complex Dynamics and Bifurcations of Predator-prey Systems with Generalized Holling Type Functional Responses and Allee Effects in Prey

Zoom Link Meeting ID: 614 048 1628

Abstract: This talk is devoted to high codimension bifurcations of a predator-prey system with generalized Holling type response function and Allee effects in prey. The nilpotent cusp singularity of order 3 and degenerate Hopf bifurcation of codimension 3 are completely analyzed. Remarkably it is the first time that three limit cycles are discovered in predator-prey models with multiplicative Allee effects. Moreover, a new unfolding of nilpotent saddle of codimension 3 with a fixed invariant line is discovered and fully developed, and the existence of codimension 2 heteroclinic bifurcation is proven. Our work extends the existing results of predator-prey systems with Allee effects. The bifurcation analysis and diagram allow us to give biological interpretations of predator-prey interactions.

April 22
Gregg Musiker (University of Minnesota)
From Cluster Algebras and Snake Graphs to Teichmueller Spaces and Super Teichmueller Spaces

Abstract: This talk will provide an introduction to cluster algebras from surfaces, and their generators, known as cluster variables. We will also describe how certain combinatorial objects known as snake graphs yield generating functions that agree with Laurent expansions of said cluster variables. From the point of view of decorated Teichmueller theory, these generating functions also provide formulas for lambda lengths, also known as Penner coordinates, with respect to a reference triangulation. We will then switch gears and describe formulas for super lambda lengths in the context of decorated super Teichmeuller space of a marked disc or an annulus. This latter work generalizes an earlier interpretation of dimers on snake graphs for ordinary lambda lengths defined with R. Schiffler and L. Williams. For the case of super lambda-lengths, this is joint work with N. Ovenhouse and S. Zhang.

April 15
A briefing on dilation theory

Abstract: This talk is mostly a historical recount of dilation theory in the complex plane. We provide a wide range of dilation-related theorems in different function spaces such as the Hardy space, Bergman space, super-harmonically weighted Dirichlet spaces, Bloch space, model spaces, de Branges-Rovnyak spaces, etc. We finish with some recent results and open questions, emerged in the last decade, to highlight the need for novel tools in dealing the remaining problems.

April 8
Anton Zeitlin (Louisiana State University)
Super-Teichmueller spaces and Penner coordinates

Abstract: The Teichmüller space parametrizes Riemann surfaces of fixed topological type and is fundamental in various contexts of mathematics and physics. It can be defined as a component of the moduli space of flat G=PSL(2,R) connections on the surface. Higher Teichmüller space extends these notions to appropriate higher rank classical Lie groups G, and N=1 super Teichmüller space likewise studies the extension to the super Lie group G=OSp(1|2). In this talk, I will discuss the solution to the problem of producing Penner-type coordinates on super-Teichmüller space and its higher analogues. I will also talk about some applications of these coordinates.

April 1 - In person in UH 4010
Mark Pengitore (University of Virginia)
Coarse Embeddings and Homological Filling Functions

Abstract: In this talk, we will relate homological filling functions with coarse embeddings. We will demonstrate that a coarse embedding of a group into a group of geometric dimension 2 induces an inequality on homological Dehn functions in dimension 2. As an application of this, we can show that if a finitely presented group coarsely embeds into a hyperbolic group of geometric dimension 2, then it is hyperbolic. Another application is a characterization of subgroups of groups with quadratic Dehn function. If there is enough time, we will talk about various higher-dimensional generalizations of our main result.

March 18
Yao Li (University of Massachusetts Amherst)
The Fokker-Planck equation: theory, computation, and data-driven method

Abstract: The time evolution of the probability density function of a stochastic differential equation is described by the Fokker-Planck equation, which has lots of applications in various areas. In this talk I will discuss some recent progress about the Fokker-Planck equation. On the theoretical side, I’ll introduce some refined estimates about how the invariant probability density function concentrates at the vicinity of the attractor. On the computational side, I will discuss a few different numerical methods that solve low (up to 4) and medium (up to 20) dimensional Fokker-Planck equations. Finally, I will introduce how to use a data-driven approach to study the speed of convergence of the Fokker-Planck equation to its steady state, and (if time permits) its application to the theoretical foundation of deep learning.

January 21
Yun Kang (Arizona State University)
How Does Mathematics Save Honeybees?

Abstract: The honeybee, Apis mellifera, is not only crucial in maintaining biodiversity by pollinating 85% plant species but also is the most economically valuable pollinator of agricultural crops worldwide with value between $15 and$20 billion annually as commercial pollinators in the U.S. Unfortunately, the recent sharp declines in honeybee population have been considered a global crisis. In this talk, we will demonstrate how we develop different types of mathematical trackable models to explore how the crucial feedback mechanisms linking disease, parasitism, nutrition, and foraging behavior might be responsible for the colony growth dynamics and survival in a dynamical environment. Rigorous mathematics from those models that use nonlinear nonautonomous and/or delayed differential equations within metapopulation frameworks have been integrated with data to explore the contributing factors to the mysterious and dramatic loss of honeybees as well as provide a basis for new strategies for controlling Varroa and reducing colony losses for beekeepers, and benefit land managers.

### Fall Semester

December 10
Mingji Zhang (New Mexico Institute of Mining and Technology)
Qualitative properties of ionic flows via Poisson-Nernst-Planck models with nonzero but small permanent charges and multiple cations

Abstract: A quasi-one-dimensional Poisson-Nernst-Planck system for ionic flow through a membrane channel is studied. Nonzero but small permanent charge, the major structural quantity of an ion channel, is included in the model. The system includes three ion species, two cations with the same valences and one anion, which provides more correlations/interactions between ions compared to the case included only two oppositely charged particles. The cross-section area of the channel is included in the system, which provides certain information of the geometry of the three-dimensional channel. This is crucial for our analysis. Under the framework of geometric singular perturbation theory, more importantly, the specific structure of the model, the existence and local uniqueness of solutions to the system for small permanent charges is established. Furthermore, treating the permanent charge as a small parameter, through regular perturbation analysis, we are able to derive approximations of the individual fluxes and current-voltage relations explicitly, and this allows us to examine the small permanent charge effects on ionic flows in detail.

December 3
Radakrishnan 'Kit' Nair (University of Liverpool, UK)
On Weyl’s theorem on uniform distribution of polynomials

Suppose $P(x) = \alpha _k x^k + \ldots + \alpha _1 x + \alpha$ is a polynomial with one of $\alpha _k , \ldots , \alpha _1$ irrational. Suppose $( k_l )_{l\geq 1}$ is a class of highly regular sequences of natural numbers, to be specified. Then if a function $f: [0,1) \to \Bbb{C}$ is continuous, we have $$\lim _{l\to \infty}{1\over l}\sum _{i=1}^{l}f(P({k_l})) = \int _0^1f(t)dt.$$ In the case of $k_l=l \ (l=1,2, \ldots )$ this is a seminal theorem of Herman Weyl, that underlies much of modern analytic number theory ergodic theory. We will prove this theorem and give some arithmetic applications.

November 5
Emil Straube (Texas A&M University)
d-bar methods in complex analysis

Abstract: In this lecture, we will indicate some typical applications of $\overline{\partial}$-methods in complex analysis of one and several variables. We start with the solution of the inhomogeneous $\overline{\partial}$-equation in a planar domain, which lends itself to a simple proof of the Mittag--Leffler theorem on holomorphic functions with prescribed poles. Next, we describe how one variable methods give the solution of the inhomogeneous Cauchy-Riemann equations in $\mathbb{C}^{2}$ when the right hand side has compact support, and how this simple observation already leads to a phenomenon that is radically different from the one variable case: holomorphic functions in certain domains automatically extend holomorphically to a bigger domain. This leads to the question of domains of existence of holomorphic functions, and we indicate how these domains are characterized by the solvability of the inhomogeneous $\overline{\partial}$- equations. Finally, we add some remarks about solving the inhomogeneous equations with regularity estimates up to the boundary (time permitting).

October 29
Hoi Nguyen (The Ohio State University)
Random matrices: universality of the spectrum and cokernels

Abstract: Random Matrix Theory is a rich area with many applications. In this talk I will give a brief survey on some recent developments in the area, focusing mainly on the universality aspect of the spectrum and cokernels.

October 22
Pablo Roldan (Yeshiva University)
Continuation of relative equilibria in the n–body problem to spaces of constant curvature

Abstract: The curved n–body problem is a natural extension of the planar Newtonian n–body problem to surfaces of non-zero constant curvature. We prove that all non-degenerate relative equilibria of the planar problem can be continued to spaces of constant curvature k, positive or negative, for small enough values of this parameter. We also compute the extension of some classical relative equilibria to curved spaces using numerical continuation. For example, we extend Lagrange’s triangle configuration with different masses to both positive and negative curvature spaces.

October 8