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# Past Colloquium Talks

## 2023-2024 Colloquium Talks

Speaker: Dr. Nilima Nigam (Simon Fraser University) | October 24, 2023

### Dr. Nilima Nigam (Simon Fraser University)

*Whimsical excursions in spectral geometry*

October 24, 2023

12:30pm – 1:30pm

ASB 10900 (Big Data Hub – Presentation Studio)

Most humans encounter instances of eigenvalues and eigenfunctions - we can tell apart the tones of a small bird from those of a large mammal; we've probably watched standing waves develop on clamped strings. Engineers compute resonances for all manners of applications. In our own courses, we extensively talk about the spectra of familiar mathematical objects - Hermitian matrices, differential operators, graphs and networks. The spectra of differential operators depend intricately on the geometry of the domains the operators are defined on, and this is the focus of spectral geometry.

In this talk, I'll describe some of the classical questions which arise - can you hear the shape of a drum? Can you design a drum to have a given tone? How do we compute these spectra?

I'll provide a whimsical collection of examples showing how numerical analysis and spectral geometry interact and enrich each other. And I'll describe some open problems in the field, which are hopefully of interest to some.

Speaker: Dr. Natasha Morrison (University of Victoria) | November 14, 2023

### Dr. Natasha Morrison (University of Victoria)

*Bootstrap percolation and related graph processes*

November 14, 2023

12:30pm – 1:30pm

ASB 10900 (Big Data Hub – Presentation Studio)

Bootstrap percolation processes are a family of cellular automata that were originally introduced in 1979 by the physicists Chalupa, Leith and Reich as a model of ferromagnetism. Since then, they have been used throughout a variety of disciplines to model real world phenomena such as the spread of influence in a social network, information processing in neural networks or the spread of a computer virus.

The r-neighbour bootstrap percolation process on a graph G starts with an initial set A of infected vertices and, at each step of the process, a healthy vertex becomes infected if it has at least r infected neighbours (once a vertex becomes infected, it remains infected forever). If every vertex of the graph eventually becomes infected, we say that A percolates.

In this talk I will discuss results concerning this and other related processes, including graph saturation, weak saturation, and rainbow saturation.

Speaker: Dr. Mark Lewis (University of Victoria) | January 30, 2024

### Dr. Mark Lewis (University of Victoria)

**Mathematical models for the neutral genetics of populations under climate change**

**Mathematical models for the neutral genetics of populations under climate change**

January 30, 2024

12:30pm - 1:30pm

Location: ASB 10900 (Big Data Hub - Presentation Studio)

In this talk I will discuss the genetic structure of populations subject to climate change and undergoing range expansion. The models and analyses are based on reaction diffusion and integrodifference equations for the asymptotic neutral genetic structure of populations. We decompose solutions into neutral genetic components called neutral fractions. The "inside dynamics" then describe the spatiotemporal evolution of these fractions and can be used to predict changes in genetic diversity. Results are compared with small-scale experimental systems that have been developed to test the mathematical theory.

## 2022-2023 Colloquium Talks

Speaker: Dr. Caroline Colijn (Simon Fraser University) | October 25, 2022

### Dr. Caroline Colijn (Simon Fraser University)

*Mathematical method and models to learn about virus transmission from sequence data*

October 25, 2022

3:30pm – 4:30pm

ASB 10900 (Big Data Hub – Presentation Studio)

The capacity to read the RNA or DNA from large numbers of viral or bacterial genomes in principle offers a lot of information about how these pathogens spread and how to control them. But there is a wide gap between genetic sequences and interpretable information that can be used for onward modelling, to understand pathogen evolution and for public health interventions. In this talk, I will describe how new mathematical tools help fill this gap, using a combination of discrete structures, estimation and dynamic modelling. Mathematical innovations offer new ways to describe and summarize the information in genetic data, new methods to use those data to learn how pathogens move from person to person and around the world, and new ways to learn where the highest levels of transmission are occuring.

Speaker: Dr. Weiran Sun (Simon Fraser University) | November 29, 2022

### Dr. Weiran Sun (Simon Fraser University)

*Kinetic Inverse Problems*

November 29, 2022

12:30pm – 1:30pm

ASB 10900 (Big Data Hub – Presentation Studio)

This talk aims at a general audience and it consists of two parts: the first part is an overview of my research program which is roughly divided into four subjects. The second part will focus on one of the subjects on kinetic inverse problems. In particular, we will explain the application of the singular decomposition method. This method has been successfully applied to the linear transport equations to recover their absorption and scattering coefficients. Our interest is to extend this method to more general kinetic inverse problems. As examples, we will show various reconstructions of parameters in nonlinear transport equations, linear transport with an interface and the nonlinear Vlasov-Poisson equation.

Speaker: Dr. Marni Mishna (Simon Fraser University) | January 31, 2023

### Dr. Marni Mishna (Simon Fraser University)

*Towards a combinatorial understanding of transcendental functions*

January 31, 2023

12:30pm – 1:30pm

ASB 10900 (Big Data Hub – Presentation Studio)

Mathematical transcendance refers to objects (usually numbers or functions) that do not satisfy any polynomial equation, that is, they are not algebraic. The numbers $\pi$ and $e$ are famous transcendental numbers, and $e^x$ and the Gamma function are examples of transcendental functions. The problem of understanding the structure of transcendental objects has fascinated mathematicians for well over a century. Combinatorics provides an intuitive framework to study power series. A combinatorial family is associated to a power series in $\mathbb{R}[[x]]$ via its enumerative generating function wherein the number of objects of size $n$ is the coefficient of $x^n$. Twentieth century combinatorics and theoretical computer science provided characterizations of classes with rational and algebraic generating functions. Finding natural extensions of these correspondences has been a motivating goal of enumerative combinatorics for several decades.

This talk will focus on two well studied classes of transcendental functions: the differentiably finite and differentially algebraic. We will illustrate how a geometrical approach allows us to classify the generating functions of families of walks on lattices. Lattice path and random walk models are in bijection with a striking number of classes with transcendental generating functions: from pattern avoiding permutations, to Young tableaux of bounded height and so this program and has led to progress in characterizing differential transcendence of other combinatorial generating functions arising in the literature, and indeed generally.

Speaker: Dr. Nathan Ilten (Simon Fraser University) | March 07, 2023

### Dr. Nathan Ilten (Simon Fraser University)

*Polynomials, Polytopes, and Geometry*

March 07, 2023

12:30pm – 1:30pm

ASB 10900 (Big Data Hub – Presentation Studio)

Algebraic geometry is the systematic study of solution sets of systems of polynomials equations. Questions in this field range from enumeration (how many solutions are there?) to classification (what geometric objects arise this way?). In this talk, I will give a brief introduction to algebraic geometry in general, and then focus on a special class of equations that arise by considering relations between integral points in lattice polytopes. The corresponding solution sets, toric varieties, form an important bridge between algebraic geometry and combinatorics. I will discuss several questions involving toric varieties, and how toric varieties can be used to study more general systems of equations.