# Applied Mathematics Seminar

Seminars in Semester 2, 2024 will be held in **F07 Carslaw Lecture Theatre 373** unless otherwise specified.

To be added to the mailing list, please contact Lindon Roberts.

## Semester 2, 2024

### November

** Monday** November 4 at 1pm

Luc Vinet (University of Montreal) - **please note unusual day**

** Title:** Entanglement of fermionic systems, signal processing and algebraic combinatorics

** Abstract:** This seminar will offer a review of recent studies on the entanglement of free-fermion systems on graphs where advantage is taken of methods pertaining to signal processing and algebraic combinatorics. On the one hand, a parallel with time and band limiting problems will be made to obtain a tridiagonal matrix that commutes with the entanglement Hamiltonian for a broad class of situations and on the other hand, the irreducible decomposition of the Terwilliger algebra arising in the context of P-polynomial association schemes will be shown to bring significant computational insights.

### October

** Tuesday** October 29 at 1pm

Jon Chapman (University of Oxford)

** Title:** Blowup in the nonlinear Schrodinger equation

** Abstract:** We systematically derive a normal form for the emergence of radially symmetric blowup solutions from stationary ones in the nonlinear Schrodinger equation. The derivation uses the methodology of asymptotics beyond all orders, and the resulting normal form applies when either the power law or dimension is used as the bifurcation parameter. It yields excellent agreement with numerics in both leading and higher-order effects, is applicable to both infinite and finite domains, and is valid in both critical and supercritical regimes.

** Thursday** October 24 at 10am

Giacomo Nannicini (University of Southern California) - **please note unusual day and time. This talk will be online-only.**

** Title:** Optimization and machine learning on quantum computers: yes/no/maybe?

** Abstract:** We would like to solve difficult optimization problems -- perhaps to allocate resources, perhaps to train a machine learning model -- and we would like to do it quickly. Is it reasonable to hope that quantum computers can help us do that? In this talk we discuss the current state of quantum optimization and quantum machine learning research, for problems that are classically defined: the problems are described on a classical (i.e., non-quantum) computer, and the solution that we are looking for is also classical. We will see that for some problems quantum algorithms are promising, even if only as an alternative to classical algorithms with different tradeoffs, but in other cases there has been no convincing evidence of their usefulness so far.

** Tuesday** October 8 at 1pm

Johannes Lohmann (University of Copenhagen)

** Title:** Multistability and tipping points in "real" systems by exploration of a climate model

** Abstract:** The prevailing notion of a tipping point (TP) in a complex system is the saddle-node bifurcation. This makes idealizing assumptions, such that the system displays fixed-point dynamics, is homogeneous and "well-mixed", and posesses one dominant positive feedback leading to bi-stability. A benefit of these assumptions is the existence of early-warning signals (EWS) due to the phenomenon of critical slowing down, which in principle allow the detection of an impending TP during a quasi-adiabatic parameter shift.
Many recent efforts exploit this and determine from historical observations whether and when TPs in real-world systems, such as the climate, are being crossed. While the statistical issue of EWS is continuously refined, the underlying assumptions remain uncontested due to the complex and unknown nature of real-world systems. In ongoing research, we try to close this gap by scrutinizing the TP of a collapse of the Atlantic Meridional Overturning Circulation (AMOC) in a climate model using brute-force computing power and various other techniques.
This led to the observation of rate-induced tipping before crossing the bifurcation in the model for non-adiabatic rates of a parameter shift, with a loss of predictability of final state due to chaos. Moreover, due to its spatially-extendedness and multiple feedbacks, the system is far beyond bi-stable, displaying a multitude of coexisting states differing subtly in their spatio-temporal variability. Hence, the TP is not a singular event easily targeted by EWS based on local stability. Finally, we argue that even with solid physical knowledge it may be impossible to know what degrees of freedom carry the critical dynamics of a TP. Thus, to really anticipate a TP it may be necessary to determine the real tipping mechanism by knowledge of the unstable edge state, and if possible mean action paths (instantons) between attractors. This is explored here using edge tracking and rare-event algorithms.

** Thursday** October 3 at 1pm

Katharine Turner (Australian National University) - **please note unusual day**

** Title:** A new computation paradigm for computing the persistent homology of Rips filtrations

** Abstract:** Given a point cloud in Euclidean space, and a fixed length scale, we can create simplicial complexes (called Rips complexes) to represent that point cloud using the pairwise distances between the points. By tracking how the homology classes as we increase that length scale we summarise the topology and the geometry of the "shape" of the point cloud, in what is called the persistent homology of its Rips filtration. A major obstacle to more widespread take up of persistent homology as a data analysis tool is the long computation time and, more importantly, the large memory requirements needed to store the filtrations of Rips complexes and compute its persistent homology. We bypass these issues by finding a "Reduced Rips Filtration" which has the same degree-1 persistent homology but with dramatically fewer simplices. The talk is based off joint work is with Musashi Koyama, Facundo Memoli and Vanessa Robins.

### September

** Tuesday** September 24 at 1pm

Clément Canonne (University of Sydney)

** Title:** Distribution Testing: Hypothesis Testing from Very Little (or Very Private) Data

** Abstract:** This talk will serve as an introduction to the (theoretical computer science) area of distribution testing, a subfield of property testing, and discuss some tricks, techniques, and ideas useful to design or analyze distribution testing algorithms. Focusing on the simple yet fundamental example of uniformity testing, the talk will then venture into differential private testing, and discuss a few tools allowing one to (easily?) obtain testing algorithms under various privacy threat models.

** Tuesday** September 10 at 1pm

John Sader (California Institute of Technology)

** Title:** Nonlinear flows generated by nanomechanical devices and their application

** Abstract:** A vibrating nanomechanical device immersed in a viscous fluid is known to generate an oscillatory flow that can dictate its performance in a range of applications. This includes its sensitivity to mass adsorption and environmental changes. The commonly characterized small-amplitude motion of these devices may generate a nonlinear flow which can modify their response. In this talk, I will discuss experimental and theoretical work aimed at elucidating this nonlinear response, its underlying physical mechanism and exploring its application. This will include the fluid-structure interaction driving the autonomous propulsion of nanorods and the presence of this nonlinear phenomenon in nanomechanical sensing. I will also present a data-driven approach that enables mass measurements using advanced nanoelectromechanical devices with uncharacterized vibrational modes.

### August

** Tuesday** August 27 at 1pm

Leticia Cugliandolo (Sorbonne University)

** Title:** (Non equilibrium) thermodynamics of classical Integrable models in their thermodynamic limit

** Abstract:** Motivated by recent experimental developments in atomic physics, a large
theoretical effort has been devoted to the analysis of the dynamics of
quantum isolated systems after a sudden quench. In this talk I will
describe the evolution of a family of classical many-body integrable
(Neumann) models after instantaneous quenches of the same kind. The
asymptotic dynamics of these models can be fully elucidated, and the
stationary properties (in the thermodynamic limit) compared to the ones
obtained exactly using a Generalised Gibbs Ensemble. The latter can not
only be built but also used to evaluate analytically all relevant
observables, a quite remarkable fact for an interacting integrable system
with a non-trivial phase diagram.

** Tuesday** August 20 at 1pm

Nisha Chandramoorthy (University of Chicago)

** Title:** A dynamical systems approach to sampling and surrogate modeling

** Abstract:** A neural surrogate model of a dynamical system learned from time series data may fail to reproduce its true long-term behavior. In other words, vanilla generalization does not determine the statistical accuracy of a neural surrogate model. When the Jacobian data of the true system is added to the regression problem however, the physical invariant distribution — ensemble/long term behavior — is reproduced by the neural dynamical model. We combine statistical learning theory with ergodic theory of dynamical systems to explain these observations. Our new generalization bounds characterize when a neural ODE model can learn the physical distribution as well as the short-term dynamics. Such a dynamics-aware generalization theory provides a principled basis for constructing new loss functions and implies that purely data-driven, as opposed to hybrid, approaches can also lead to accurate statistical modeling of chaotic systems. Further, we observe that these surrogate models, are able to sample the physical distribution, even though they are not explicitly trained to be generative models and rather use a supervised learning setup with a smaller sample complexity than dynamical generative models. In the second half, we present an infinite-dimensional Newton-Raphson method for transport of a tractable source distribution to a generic target distribution. The Newton-Raphson method finds a zero of a score-residual operator. This Score Operator Newton (SCONE) transport is a composition of transport maps obtained from each Newton iteration, which involves solving an elliptic PDE. The PDE requires a black box function that can evaluate the score of the target distribution. A natural application of SCONE is when the target distribution is a posterior in a Bayesian inference problem with a known likelihood and prior that can be sampled easily. We present another powerful application in Bayesian data assimilation in certain dynamical systems where the score of the target distribution can be computed without explicit knowledge of the prior distribution. Both in learning probability distributions and ODEs, we gather supporting evidence for our hypothesis that dynamical systems and ergodic theory can fruitfully intersect with statistical learning to improve our understanding and implementation of ML algorithms. The work on sampling is joint with Youssef Marzouk (MIT) and on learning dynamics is with Jeongjin Park (Georgia Tech).

** Tuesday** August 13 at 1pm

Angela Reynolds (Virginia Commonwealth University)

** Title:** Modeling the Role of Innate Immune Cells in Diseases and Diet-induced Gut inflammation

** Abstract:** During an inflammatory response there is a complex cascade of reactions, which may lead to health or sustained inflammation during many diseases and processes. In order to understand how the immune cells involved in the inflammatory response contribute to the disease progression, we have developed various models for the immune cell dynamics. Using parameter estimation, sensitivity analysis, and/or classification methods we will explore predictors of outcome and how modulating the immune response dynamics can alter disease progression. In this talk, I will focus on 1) a model for sequential influx of immune cells following a bacterial stimulus and 2) the role of intestinal Alkaline Phosphatase (IAP) in the leaky gut phenomenon and the associated pro-inflammatory signal to the immune system.

** Tuesday** August 6 at 1pm

Matthew Roughan (University of Adelaide)

** Title:** Metagraphs, Policy and Security in Comms Networks

** Abstract:** Recent events have highlighted that cybersecurity is very difficult, and a purported solution can itself become the problem. We need better tools to reason about security solutions. Formal, mathematical tools, including graph theoretical constructs, provide a means to support network managers to reason about their network policies to create secure-by-design networks. In this talk, I will present one such tool that we have been using recently -- metagraphs -- which are closely related to hypergraphs. I will show how they can be used to represent security policy, and some of the techniques that make them valuable for reasoning about cybersecurity policies.

## Semester 1, 2024

### June

### May

** Tuesday** May 21 at 2pm

Greg Berkolaiko (Texas A&M University)

** Title:** Duistermaat index and eigenvalue interlacing for perturbations in boundary conditions

** Abstract:** Eigenvalue interlacing is a tremendously useful tool in linear algebra and spectral analysis. In its simplest form, the interlacing inequality states that a rank-one positive perturbation shifts the eigenvalue up, but not further than the next unperturbed eigenvalue. For different types of perturbations, this idea is known as the "Weyl interlacing" (additive perturbations), "Cauchy interlacing" (for principal submatrices of Hermitian matrices), "Dirichlet-Neumann bracketing" and so on.
We discuss the extension of this idea to general "perturbations in boundary conditions", encoded as interlacing between eigenvalues of two self-adjoint extensions of a fixed symmetric operator with finite (and equal) defect numbers. In this context, even the terms such as "signature of the perturbation" are not immediately clear, since one cannot take the difference of two operators with different domains. However, it turns out that definitive answers can be obtained, and they are expressed most concisely in terms of the Duistermaat index, an integer-valued topological invariant describing the relative position of three Lagrangian planes in a symplectic space. Two of the planes describe the two self-adjoint extensions being compared, while the third one corresponds to the distinguished Friedrichs extension.
We will illustrate our general results with simple examples, avoiding technicalities as much as possible and giving intuitive explanations of the Duistermaat index, the rank and signature of the perturbation in the self-adjoint extension, and the curious role of the third extension (Friedrichs) appearing in the answers.
Based on a work in progress with Graham Cox, Yuri Latushkin and Selim Sukhtaiev.

** Tuesday** May 14 at 2pm

Hung Phan (University of Massachusetts Lowell)

** Title:** Splitting algorithms: Convergence Analysis and applications

** Abstract:** In this talk, we discuss a conical extension of averaged non-expansive operators and its role in analyzing the convergence of several splitting algorithms: the proximal point algorithm, the forward-backward algorithm, the adaptive Douglas-Rachford algorithm, the 3-operator algorithm, and the adaptive ADMM. We also present an inspiring application of splitting algorithms in spatial design problems.

** Tuesday** May 7 at 2pm

Gary Froyland (University of New South Wales)

** Title:** Spectral analysis of climate dynamics with operator-theoretic approaches

** Abstract:** An important problem in modern applied science is to characterize the behaviour of systems with complex internal dynamics subjected to external forcings. Many existing approaches rely on ensembles to generate information from the external forcings, making them unsuitable to study natural systems where only a single realization is observed. A prominent example is climate dynamics, where an objective identification of signals in the observational record attributable to natural variability and climate change is crucial for making climate projections for the coming decades. I will show that the spectral theory of dynamical systems, combined with techniques from data science, provides an effective means for extracting slowly decaying modes of climate variability, nonlinear trends and persistent cycles, from a single trajectory of a high-dimensional model or observed time series. We apply our framework to real-world examples from climate dynamics: the El Nino Southern Oscillation and variability of sea surface temperature over the industrial era, and the mid-Pleistocene transition of Quaternary glaciation cycles.

### April

** Tuesday** April 30 at 2pm

Emily Stone (University of Montana-Missoula)

** Title:** Neuromodulation of Hippocampal Microcircuits: Some Modeling and Some Math

** Abstract:** In this talk I will first give an overview of oscillations in the voltage of neuron assemblies, and models thereof. We use these to study neurons in the hippocampus, a part of the brain thought to be central in learning and memory functions. These neurons are connected via electrochemical synapses, which use neurotransmitter released from the presynaptic neuron to change the voltage of the postsynaptic neuron. Inhibitory neurons cause the voltage of their target to decrease. Oscillations in inhibitory-to-inhibitory (I-I) coupled neurons in the hippocampus have been studied extensively numerically, and with analytic continuation methods. Neuromodulation on short time scales, in the form of presynaptic short-term plasticity (STP), can dynamically alter the connectivity of neurons in such a microcircuit. I will discuss the mechanism of STP, and a model for it parameterized from experimental data for a specific synapse in the hippocampus. The goal of the project is to understand the effect of adding this plasticity to the (I-I) microcircuit, both through numerical simulation and bifurcation analysis of a discrete dynamical system.

** Tuesday** April 23 at 2pm

Jordan Pitt (University of Sydney)

** Title:** Ocean wave propagation in the marginal ice zone: On the transition from consolidated to broken ice covers

** Abstract:** The surfaces of the high latitude oceans are frozen into a layer of ``sea ice”, which plays an important role in the global climate by reflecting the sun’s rays. Ocean surface waves propagate from the open ocean into the sea ice covered ocean and break up the ice cover, leaving it more vulnerable to melting. The ice cover attenuates wave energy over distance, so that the breakup is confined to a region known as the marginal ice zone. Field observations have been interpreted as indicating the non-intuitive behaviour of attenuation decreasing following breakup. I will present a mathematical model that explains the observations in terms of a combination of attenuation and ice-edge reflection, backed by laboratory experiments.

** Tuesday** April 16 at 2pm

Owen Dearricott (La Trobe University)

** Title:** Integrable systems, Painlevé VI and explicit solutions to the anti-self-dual Einstein equation via radicals

** Abstract:** Though Einstein’s equation is well studied, relatively few Einstein metrics have been written in terms of explicit formulae via radicals. In this talk we discuss many such examples that occur as anti-self dual Einstein metrics and describe their singularities. The construction heavily relies upon the theory of isomonodromic deformation and related algebraic geometry developed by N.J. Hitchin in the 1990s and the equivalence of the anti-self-dual Einstein equation to a certain Painlevé VI equation under some symmetry assumptions discovered by K.P. Tod. The solution to Painlevé VI is achieved through a relation of its solution to pairs of conics obeying the Poncelet’s porism by exploiting Cayley’s criterion. In this talk we discuss some important cases that are not well fleshed out in the literature, such as the solution of Painlevé VI associated with the Poncelet porism where the inscribing-circumscribing polygons have an even number of sides. Moreover, we provide some explicit metrics with neutral signature and others with unusual cone angle singularities along a singular real projective plane that were speculated about by Atiyah and LeBrun.

** Tuesday** April 9 at 2pm

Michael Small (University of Western Australia)

** Title:** Dynamics of Machine Learning

** Abstract:** What is old is new again. Machine learning can be understood as attempts to apply data driven techniques to uncover underlying deterministic dynamics, or to approximate it through stochastic methods. In this talk I will describe three approaches to understand machine learning from the perspective of dynamical systems. First, recurrent neural networks will be shown to perform an embedding of time series data in the sense of Takens' theorem. That is, the internal state of the neural network is diffeomorphic to the underlying (presumed determinsitic) dynamical system. Second, while generative Artificial Intelligence achieves sentient-like performance through a carefully orchestrated random walk we will see how this can be construed as a stochastic dynamical system represented by a walk on a graph (or Markov chain). Thirdly, I will describe the application of learning techniques to estimate the state of a network dynamical system from observation of the node dynamics. Along the way, the utility of these methods will be demonstrated with application to industrial maintenance, music and detection of ventricular fibrillation.

### March

** Tuesday** March 26 at 2pm

Jared Bronski (University of Illinois)

** Title:** Stability and Global Attracting for the KdV-Burgers traveling wave

** Abstract:** The KdV-Burgers equation was proposed by Whitham as a model for the propagation of tidal bores, and represents one of the simplest partial differential equations to incorporate nonlinearity, dispersion and dissipation. The existence and uniqueness (modulo translation) of traveling wave was proven by Bona and Schonbek, and the stability to small perturbations was proven by Pego in the case where the traveling wave is monotone. We prove that under a certain spectral condition the traveling wave is a global attractor for a range of parameter values that includes the monotone case. For a portion of the parameter set the proof is purely analytic, over the rest of the range we rely on techniques of rigorous numerics. This is joint work with Blake Barker (BYU), Vera Hur (University of Illinois), and Zhao Yang (Chinese Academy of Sciences).

** Tuesday** March 19 at 2pm

Sumeetpal Singh (University of Wollongong)

** Title:** On resampling schemes for particle filters with weakly informative observations

** Abstract:** We consider particle filters with weakly informative observations (or 'potentials') relative to the latent state dynamics. The particular focus of this work is on particle filters to approximate time-discretisations of continuous-time Feynman-Kac path integral models --- a scenario that naturally arises when addressing filtering and smoothing problems in continuous time --- but our findings are indicative about weakly informative settings beyond this context too. We study the performance of different resampling schemes, such as systematic resampling, SSP (Srinivasan sampling process) and stratified resampling, as the time-discretisation becomes finer and also identify their continuous-time limit, which is expressed as a suitably defined `infinitesimal generator.' By contrasting these generators, we find that (certain modifications of) systematic and SSP resampling `dominate' stratified and independent `killing' resampling in terms of their limiting overall resampling rate. The reduced intensity of resampling manifests itself in lower variance in our numerical experiment. This efficiency result, through an ordering of the resampling rate, is new to the literature. The second major contribution of this work concerns the analysis of the limiting behaviour of the entire population of particles of the particle filter as the time discretisation becomes finer. We provide the first proof, under general conditions, that the particle approximation of the discretised continuous-time Feynman-Kac path integral models converges to a (uniformly weighted) continuous-time particle system. Joint work with N. Chopin, T. Soto and M. Vihola. DOI: 10.1214/22-AOS2222.

** Tuesday** March 12 at 2pm

Joseph Lizier (University of Sydney)

** Title:** Analytic relationship of relative synchronizability to network structure and motifs

** Abstract:** Synchronization phenomena on networks have attracted much attention in studies of neural, social, economic, and biological systems, yet we still lack a systematic understanding of how relative synchronizability relates to underlying network structure. Indeed, this question is of central importance to the key theme of how dynamics on networks relate to their structure more generally. We present an analytic technique to directly measure the relative synchronizability of noise-driven time-series processes on networks, in terms of the directed network structure. We consider both discrete-time autoregressive processes and continuous-time Ornstein–Uhlenbeck dynamics on networks, which can represent linearizations of nonlinear systems such as the Kuramoto model. Our technique builds on computation of the network covariance matrix in the space orthogonal to the synchronized state, enabling it to be more general than previous work in not requiring either symmetric (undirected) or diagonalizable connectivity matrices and allowing arbitrary self-link weights. More importantly, our approach quantifies the relative synchronization specifically in terms of the contribution of process motif (walk) structures. We demonstrate that in general the relative abundance of process motifs with convergent directed walks (including feedback and feedforward loops) hinders synchronizability. We also reveal subtle differences between the motifs involved for discrete or continuous-time dynamics. Our insights analytically explain several known general results regarding synchronizability of networks, including that small-world and regular networks are less synchronizable than random networks..

** Tuesday** March 5 at 2pm

Shane Henderson (Cornell University)

** Title:** COVID-19 Modeling to Keep Cornell University Open Throughout the Pandemic

** Abstract:** Unlike most universities, Cornell University reopened its Ithaca campus for in-person instruction in the Fall of 2020 during the COVID period and did so safely through the use of pooled testing. This decision and many others at the top levels of the university were guided by our mathematical modeling group. I'll discuss some of the questions we explored, the models we built, the data that informed our models, and how we dealt with several central data issues. Our work "under fire" motivated our current work in epidemiological modeling.

### February

### January

## Semester 2, 2023

### December

### November

** Wednesday ** November 1 at 1pm

Pieter Roffelsen (University of Sydney)

** Title:** On connection problems for Painlevé transcendents and affine Del Pezzo surfaces

** Abstract:** For ordinary differential equations, explicitly relating the behaviours of solutions near distinct points, constitutes an important but often completely intractable problem, known as a connection problem. It is thus a remarkable fact that, for the highly transcendental and nonlinear Painlevé equations, certain connection problems are solvable. In this talk, I will discuss some of the history and mathematics involved in solving these problems, as well as recent extensions to the q-difference setting, and how the beautiful geometry of affine Del Pezzo surfaces plays a central role in their solutions.

### October

** Wednesday ** October 25 at 1pm

Renjie Feng (University of Sydney)

** Title:** Determinantal point processes on spheres: Multivariate linear statistics

** Abstract:** I will talk about the multivariate linear statistics (also known as U-statistics) of determinantal point processes on unit spheres. I will first present a graphical representation for the cumulants of the multivariate linear statistics, extending the famous Soshnikov's formula for the univariate case. Then I will explain how we derive the 1st and 2nd Wiener chaos using this graphical representation. We computed sphere cases as introductory examples, but the method can be applied to any other determinantal point processes. This is based on the joint work with F. Goetze and D. Yao.

### September

** Wednesday ** September 20 at 1pm

Matthew Tam (University of Melbourne)

** Title:** Splitting algorithms for training GANs

** Abstract:** Generative adversarial networks (GANs) are an approach to fitting generative models over complex structured spaces. Within this framework, the fitting problem is posed as a zero-sum game between two competing neural networks which are trained simultaneously. Mathematically, this problem takes the form of a saddle-point problem; a well-known example of the type of problem where the usual (stochastic) gradient descent-type approaches used for training neural networks fail. In this talk, we rectify this shortcoming by proposing a new method for training GANs that has: (i) a sounds theoretical foundation, and (ii) does not increase the algorithm's per iteration complexity (as compared to gradient descent). The theoretical analysis is performed within the framework of monotone operator splitting.

** Wednesday ** September 13 at 1pm

Peter Harrowell (University of Sydney)

** Title:** Introduction to the Problem of Glass

** Abstract:** Amorphous solids – glasses, plastics, ceramics and ‘glassy’ metal alloys – make up an important part of the material world but they remain poorly understood at the microscopic level. The difficulties arise from the non-uniqueness of the solid state. This talk will address the issues of the complexity of the energy landscape associated with disordered configurations, the diversity of geometric structures, the localized response to strain (non-affine displacements) and thermal fluctuations (dynamic heterogeneities). Related phenomenon such as jamming will also be discussed.

** Wednesday ** September 6 at 1pm

Upanshu Sharma (University of New South Wales)

** Title:** Coarse-graining of Markov chains

** Abstract:** Coarse-graining is the procedure of approximating large and complex systems by simpler and lower-dimensional ones. It is typically characterised by a mapping which projects the full state of the system onto a smaller set; this mapping captures the relevant (often slow) features of the system. Starting from a continuous-time Markov chain and such a mapping, I will discuss an effective dynamics which approximates the true projected Markov chain and present error estimates in relative entropy on the approximation error. This talk is based on joint work with Bastian Hilder.

### August

** Wednesday ** August 23 at 1pm

Priya Subramanian (University of Auckland)

** Title:** Rogue bursting as an effect of broken symmetry

** Abstract:** The formation of rogue waves is of interest, from North sea waves and waves in tanks to waves in nonlinear optics. Most common models used to investigate rogue bursts use the nonlinear Schroedinger (NLS) equation and its variants. However, such integrable settings and analytical solutions are rare in higher dimensions. So we propose to use the model of a dissipative system: which describes interaction between standing waves in domains of moderate aspect ratio. When spatial reflection symmetry is broken, the left and right running waves can interact strongly producing a temporally localised extremely large amplitude event.

## Semester 1, 2023

### June

** Wednesday ** June 28 at 1pm

Taylor Klotz (University of Hawai`i at Mānoa)

** Title:** Using Symmetry to Construct Dynamic Feedback Linearizations of Nonlinear Control Systems

** Abstract:** It is often handy in trajectory planning problems to have a linear control system or linearizable control system. In the case that a control system is ``intrinsically nonlinear" the next best thing is flat outputs or a dynamic feedback linearization. It is also often the case that control systems inspired by nature/engineering have symmetries that may provide insight into many questions about a given control system. It turns out we can use symmetries to probe the existence of dynamic feedback linearizations and even construct them explicitly! I'll present this procedure via an explicit example. If time permits I'll mention a possible application to Darboux integrable PDE. This approach is known as cascade feedback linearizability and is joint work with Peter J. Vassiliou and Jeanne N. Clelland.

** Wednesday ** June 14 at 1pm

Andrew Krause (Durham)

** Title:** Pattern Formation via Blackboards and Web Browsers

** Abstract:** Motivated by a range of problems in embryology and ecology, I will present recent extensions to Turing's
classical reaction-diffusion paradigm for pattern formation. This will start by reviewing reaction-diffusion systems and their
analysis via classical linear instability theory, followed by a range of generalizations to more realistic scenarios of
reaction-transport models in complex domains. Such extensions are motivated by the evolving and heterogeneous landscapes of pattern
formation in nature. Throughout this discussion, numerical simulations will play key roles in validating and extending the
near-equilibrium theory. To drive home this last point, I will present VisualPDE, a new web-based simulator for lightning-fast
interactive explorations of these systems. Such accessible numerical tools are invaluable for rapidly prototyping models of complex
biological phenomena. Importantly, accessible simulations underscore the need for sound theory which goes beyond phenomenological
modelling in biology.

### May

** Wednesday ** May 24 at 1pm

Oscar Fajardo Fontiveros (Sydney)

** Title:** Transitions in Bayesian model selection problems: link prediction on complex networks and symbolic regression

** Abstract:** In this talk, I want to show you the effects of the prior and the likelihood in Bayesian inference problems
applied to model selection problems and how it can help us understand some aspects of the results that we get. To do that, first I
am going to show you the importance of both terms in the Bayesian inference process by showing you how the balance in the likelihood
and the prior are relevant in the models that we choose. After that I am going to show you a couple of applications of this framework:
a link prediction problem in complex networks and a symbolic regression problem. In the link prediction, I show how nodes metadata
(extra information such as the age, ethnicity, gender...) produce a crossover of the types of models that we get with the observed
links, affecting accuracy. In symbolic regression, I show that the noise of the data and its size, produce a transition of the
learnability of the ground true model that generated our dataset.

** Wednesday ** May 17 at 1pm

Bob Rink (VU Amsterdam)

** Title:** Thermalisation and integrability in the Fermi-Pasta-Ulam-Tzingou problem

** Abstract:** In the early 1950s, Fermi, Pasta, Ulam and Tzingou decided to investigate the dynamics of large degree of freedom
physical systems, by performing some of the first numerical simulations in history. Their study of a conservative chain of nonlinearly
interacting oscillators revealed quite a surprise: instead of the expected evolution to a "thermal equilibrium", they observed recurrent
behavior. Since then, different approaches were able to explain features of their observations, using for instance integrable PDE
approximations and perturbation theory. In this talk I will give an overview of some cornerstone results on the analysis of the problem,
including their shortcomings, and I will present some recent work of Antonio Ponno (Padova), Matteo Gallone (SISSA Trieste) and myself
on the near-integrable evolution of unidirectional waves.

** Wednesday ** May 10 at 1pm

Noa Kraitzman (Macquarie)

** Title:** Mathematical modelling of sea ice

** Abstract:** In this talk I will delve into the behaviour of sea ice, examining both its microstructure and its effective properties. Firstly, I will introduce a thermodynamically consistent model for the liquid-solid phase change in sea ice that incorporates the effects of salt, using multiscale analysis to derive a quasi-equilibrium Stefan-type problem. Secondly, I will investigate the thermal conduction in sea ice in the presence of fluid flow, using a new Stieltjes integral representation for the effective conductivity and present rigorous bounds on the conductivity obtained through Padé approximates.

** Wednesday ** May 3 at 1pm

Harini Desiraju (Sydney)

** Title:** Orthogonal polynomials on elliptic curves and Painlevé VI

** Abstract:** Elliptic orthogonal polynomials are a family of special functions that satisfy a certain
orthogonality condition with respect to a weight function on an elliptic curve. Building up on several recent
works on the topic, we establish a framework using Riemann-Hilbert problems to study such polynomials. When
the weight function is constant, these polynomials relate to the elliptic form of the sixth Painleve equation.
This talk is based on an ongoing work with Tomas Latimer and Pieter Roffelsen.

### April

** Wednesday ** Apr 26 at 1pm

Mark Tanaka (UNSW)

** Title:** Extinction or adaptation in a changing environment (and implications for microbial evolution and the evolution of recombination)

** Abstract:** What forces drive populations to extinction? Mathematical biologists have explored a number of mechanisms that increase the probability
of extinction in small populations - especially those facing environmental change. Populations can undergo adaptation to escape extinction, in a process known
as evolutionary rescue. By studying population models we show that Darwinian evolution can, ironically, reduce the chance of surviving environmental change.
The extinction risk is unexpectedly more pronounced in moderate to large populations. Genetic linkage is at the core of these effects. Our results therefore
have implications for the evolution of sex and recombination. They also raise questions about whether and how asexual species - e.g. bacterial species - can
go extinct. We have begun to explore these questions with population models.

** Wednesday ** Apr 19 at 1pm

*No speaker -- School meeting*

** Wednesday ** Apr 12 at 1pm

*No speaker -- Mid-semester break*

** Thursday ** Apr 6 at 11am in the AGR (NOTE: different schedule and location)

Andrew Bernoff (Harvey Mudd)

** Title:** Using Field Data to Inform Agent-Based and Continuous Models of Locust Hopper Bands

** Abstract:** An outstanding problem in mathematical biology is using laboratory and field observations to tune a model’s functional form and parameter values. In this talk I will discuss an ongoing project developing models of the Australian plague locust for which excellent field and experimental data is available. Under favorable environmental conditions flightless locust juveniles may aggregate into coherent, aligned swarms referred to as hopper bands. We will develop two models of hopper bands in tandem; an agent-based model that tracks the position of individuals and a partial differential equation model that describes locust and resource density. By examining 4.4 million parameter combinations, we identify a set of the problem’s ten parameters that reproduce field observations.

I will then discuss two ongoing efforts to improve this model. The first uses ideas from dynamical systems and continuum mechanics to extend this model into two dimensions by modeling the known tendency of locusts to align using ideas from the Kuramoto model of oscillator synchronization. The second, firmly based in data science, uses motion tracking of tens of thousands of locusts to shed light on how locust movement is informed by interactions with other individuals.

### March

** Wednesday ** Mar 22 at 1pm

Christof Melcher (RWTH Aachen)

** Title:** Skyrmions and emergent spin orbit coupling in a spherical magnet

** Abstract:** We discuss solitonic field configurations on a spherical magnet.
Exploiting the Hamiltonian structure and concepts of angular momentum, we present a new family of
localized solutions to the Landau–Lifshitz equation that are topologically distinct from the ground state
and break rotational symmetry. The approach illustrates emergent spin-orbit coupling arising from the loss
of individual rotational invariance in spin and coordinate space – a common feature of condensed matter
systems with topological phases.

** Wednesday ** Mar 15 at 1pm

Samuel Jelbart (TU Munich)

** Title:** Modulation theory for dynamic bifurcations

** Abstract:** Classical modulation theory can be viewed as a weaker alternative to center manifold theory
which can be used to study instabilities associated with the crossing of continuous spectra into the right-half
plane. This approach is often applied to the study of pattern formation close to linear instabilities. In this
talk, we propose and apply an extension to the case of dynamic bifurcations, where the control parameters are
allowed to evolve slowy in time. The key analytical method is a novel extension of the so-called geometric blow-up
technique, which has been successfully applied to the study of dynamic bifurcations in ODEs for many years now,
to the PDE setting. We show that the classical multi-scale ansatz in modulation theory can be reformulated as
a geometric blow-up transformation, after which modulation equations can be derived in the dynamic setting using
an adaptation of the formal method of multiple scales. We conclude by demonstrating the method for model problems
featuring dynamic Turing and dynamic Hopf bifurcations.

** Wednesday ** Mar 8 at 1pm in F11 Chemistry Lecture Theatre 4

Michael Griebel (Bonn)

** Title:** Generalized sparse grid methods and applications

** Abstract:** High-dimensional problems appear in various mathematical models. Their numerical approximation involves
the well-known curse of dimension, which renders any direct discretization obsolete. One approach to circumvent
this issue, at least to some extent, is the use of generalized sparse grid methods, which can exploit additional
smoothness properties if present in the underlying problem.

In this talk, we will discuss the main principles and basic features of generalized sparse grids and show their
application in such diverse areas as econometrics, fluid dynamics, quantum chemistry, uncertainty quantification
and machine learning.

** Wednesday ** Mar 1 at 1pm in F11 Chemistry Lecture Theatre 4

Yumiko Takei (Ibaraki National College of Technology)

** Title:** WKB analysis via topological recursion for (confluent) hypergeometric differential equations

** Abstract:** The exact WKB analysis is a method to analyze differential equations with a small parameter h. The main ingredient of the exact WKB analysis is a formal solution for h, called a WKB solution. When we study differential equations by using the exact WKB analysis, Voros coefficients provide important quantities for describing global behavior of solutions of differential equations. The Voros coefficient is defined as a contour integral of the logarithmic derivative of WKB solutions.

On the other hand, the topological recursion introduced by B. Eynard and N. Orantin is a recursive algorithm to construct a formal solution to the loop equations that the correlation functions of the matrix model satisfy.

The quantization scheme connects WKB solutions with the topological recursion. It is found that WKB solutions can be constructed via the topological recursion.

In this talk, we prove that the Voros coefficients for hypergeometric differential equations are described by the generating functions of free energies defined in terms of the topological recursion. Furthermore, as its applications we show the following objects can be explicitly computed for hypergeometric equations: (i) three-term difference equations that the generating function of free energies satisfies, (ii) explicit forms of the free energies, and (iii) explicit forms of Voros coefficients.

### February

** Wednesday ** Feb 22 at 1pm in the AGR (Carslaw 829)

Warren Hare (UBC)

** Title:** Positive Basis and their use in Derivative Free Algorithms

** Abstract:** A positive basis is a set that non-negatively spans $R^n$ and
contains no proper subsets with the same property. These attributes make
positive bases a useful tool in derivative-free algorithms and an
interesting concept in mathematics. In this talk, we examine some
properties of positive bases, including how to check if something is a
positive basis and how to construct positive bases with nice structures.

## Semester 1, 2021

### May

** Thursday ** May 20

Ryan Goh (Boston)

** Title:** Growing oblique stripes

** Schedule:** Online at 9am (Zoom link to be sent to mailing list)

** Abstract:** Spatial growth plays an important role in controlling pattern formation in many different types of physical systems.
Specific examples include directional quenching in alloy melts, growing interfaces in biological systems, moving masks in ion milling,
eutectic lamellar crystal growth, and traveling reaction fronts, and such growth processes have been shown to select and control spatially
periodic patterns, while mediating defects. One of the simplest mechanisms of growth is a directional "quench" which travels across a
domain, suppressing patterns in one part of the domain, and exciting them in the other. In this talk we will discuss how a linear phase diffusion equation
with a nonlinear boundary condition posed on the half-plane can be used to characterize the formation of stripes oblique to the quenching interface.
We use rigorous analysis, formal asymptotics, and numerical continuation to characterize stripe selection for various quenching speeds and stripe angles.
Of particular interest, we find that the slow-growth, small-angle regime is governed by the glide-motion of a dislocation defect at the quench interface.
Finally, we compare our results numerically to stripe formation in a quenched anisotropic Swift-Hohenberg equation, a prototypical model of pattern formation.

** Thursday ** May 13

Reinout Quispel (La Trobe)

** Title:** How to discover properties of differential equations, and how to preserve those properties under discretization

** Schedule:** Carslaw 374 at 10:30am (In-person AND online! Zoom link to be sent to mailing list)

** Abstract:** This talk will be in two parts.
The first part will be introductory, and will address the question:
Given an ordinary differential equation (ODE) with certain
physical/geometric properties (for example a preserved measure, first
and/or second integrals), how can one preserve these properties under
discretization?
The second part of the talk will cover some more recent work, and
address the question:
How can one deduce hard to find properties of an ODE from its
discretization?

** Thursday ** May 6

Jie Yen Fan (Sydney)

** Title:** Multi-type age-structured population model

** Schedule:** Carslaw AGR at 10:30am (In-person AND online! Zoom link to be sent to mailing list)

** Abstract:** Population process in general setting, where each individual reproduces and dies depending on the state
(such as age and type) of the individual as well as the entire population, offers a more realistic framework to population modelling.
Formulating the population dynamics as a measure-valued stochastic process allows us to incorporate such dependence. The asymptotics,
namely the law of large numbers and the central limit theorem, can be obtained. Some examples, including sexual reproduction and the
spread of viral infection will be given.

### April

** Thursday ** April 29

Claire Postlethwaite (Auckland)

** Title:** A heteroclinic network model of Rock-Paper-Scissors-Lizard-Spock

** Schedule:** Online at 9am. Zoom link to be sent to mailing list.

** Abstract:** The well-known game of Rock-Paper-Scissors can be used as a simple model of competition between three species.
When modelled in continuous time using differential equations, the resulting system contains a heteroclinic cycle between three
equilibrium solutions. The game can be extended in a symmetric fashion by the addition of two further strategies (‘Lizard’ and ‘Spock’):
now each strategy is dominant over two of the four other strategies, and is dominated by the remaining two. The differential equation
now contains a set of coupled heteroclinic cycles forming a heteroclinic network. In this talk I will discuss how we study the dynamics
near this heteroclinic network. In particular, I will show how we are able to identify regions of parameter space in which arbitrarily
long periodic sequences of visits are made to the neighbourhoods of the equilibria, and how these regions form a complicated pattern in parameter space.

** Thursday ** April 22

Ryan Goh (Boston)

** Title:**

** Schedule:** (POSTPONED TO MAY 20)

** Abstract:**

** Thursday ** April 15

Yury Stepanyants (USQ)

** Title:** The asymptotic approach to the description of two-dimensional soliton patterns in the oceans

** Schedule:** 3:30pm at Quad S227 (In-person AND online! Zoom link to be sent to mailing list)

** Abstract:** The asymptotic approach is suggested for the description of
interacting surface and internal oceanic solitary waves. This approach allows
one to describe a stationary moving wave patterns consisting of two plane
solitary waves moving at an angle to each other. The results obtained within
the approximate asymptotic theory is validated by comparison with the
exact two-soliton solution of the Kadomtsev–Petviashvili equation. The
suggested approach is equally applicable to a wide class of non-integrable
equations too. As an example, the asymptotic theory is applied to the
description of wave patterns in the 2D Benjamin–Ono equation describing
internal waves in the infinitely deep ocean containing a relatively thin one of
the layers.

** Thursday ** April 1

Alex Townsend (Cornell)

** Title:** The art and science of low-rank techniques

** Schedule:** Online at 9am. Zoom link to be sent to mailing list.

** Abstract:** Matrices and tensors that appear in computational mathematics are so often
well-approximated by low-rank objects. Since random ("average") matrices are almost surely of full rank,
mathematics needs to explain the abundance of low-rank structures. We will give various methodologies that
allow one to begin to understand the prevalence of compressible matrices and tensors and we hope to reveal
underlying links between disparate applications. We will also show how the appearance of low-rank structures
can be used in function approximation, fast transforms, and partial differential equation (PDE) learning.

### March

** Thursday ** March 18

Jared Field (Melbourne)

** Title:** Gamilaraay Kinship Dynamics

** Schedule:** Carslaw 350 at 3:30pm (In-person AND online! Zoom link to be sent to mailing list)

** Abstract:** Traditional Indigenous marriage rules have been
studied extensively since the mid-1800s. Despite this, they
have historically been cast aside as having very little utility.
Here, I will walk through some of the interesting mathematics of
the Gamilaraay system and show that, instead, they are in fact a
very clever construction. Indeed, the Gamilaraay system dynamically
trades off kin avoidance -- to minimise incidence of recessive diseases
-- against pairwise cooperation, as understood formally through Hamilton's rule.

## Second Semester 2020

### December

** Wednesday ** December 2nd

Samuel Jelbart (Sydney)

** Title:** Extending the Scope of Geometric Singular Perturbation Theory

** Abstract:** Multi-scale phenomena in e.g., biology, engineering, and neuroscience are frequently
described by singularly perturbed ordinary differential equations with solutions varying over vastly
separated timescales, making their analysis a challenging problem. In recent decades, significant progress
has been made via the development of Geometric Singular Perturbation Theory (GSPT), which provides a powerful
theoretical framework for the analysis of such problems. When combined with a geometric method for the desingularisation
of singularities known as blow-up, GSPT can provide a remarkably detailed and geometrically informative understanding of the dynamics.

However, GSPT in its standard form has several limitations which restrict the scope of its applicability. Discontinuity,
exponential nonlinearities and 'hidden scales' all present significant challenges to the theory. In this talk,
we will explore these limitations in the context of a simple electrical oscillator model - the Le Corbeiller
oscillator - and show how they can be overcome using a combination of tools that are adapted from GSPT and the
theory of piecewise-smooth (PWS) systems. Our aim will be to understand the onset of multi-scale 'relaxation
oscillations' which converge to PWS cycles as a singular perturbation parameter tends to zero. Our main analytical
tool is the blow-up method, which must be adapted to resolve degeneracy stemming from (i) the loss of smoothness
and (ii) the presence of an essential singularity.

The talk will conclude with a brief discussion on the systematic implementation of these ideas, which have so far been developed only in the context of applications.

### November

** Wednesday ** November 18th

James Meiss (UC Boulder)

** Title:** Computing Invariant Tori and Resonances using Birkhoff Averages

** Abstract:** Invariant tori are prominent features of Hamiltonian systems.
In particular, integrable systems are foliated by tori with half the dimension
of the phase space. KAM theory implies that many of these tori persist under smooth
perturbations, but as the perturbations grow, only the ``robust'’ tori persist in
the face of increasing chaos. Perhaps the simplest example of such dynamics is the
case of area-preserving maps. John Greene conjectured that the locally most robust
rotational circles have rotation numbers that are noble, i.e., have continued fractions
with a tail of ones, and that, of these circles, the most robust has golden mean
rotation number. For higher dimensional cases, the number theoretic properties of
the robust tori are still unknown, though it has been conjectured that those with
frequency vectors in cubic irrational field should be most robust.

We develop a method based on a weighted Birkhoff average (an idea due to, Das, Saiki, Sander, and Yorke) to identify chaotic orbits, resonances, and rotational invariant tori. It can quickly compute frequency vectors of the latter to machine precision. Variants of Chirikov’s standard map are used as 2D test cases. As a higher-dimensional test, we study a ``standard’' family of 3D, volume-preserving maps---to which KAM theory applies, and attempt to identify the most robust two-tori.

(This research is in collaboration with Evelyn Sander of George Mason University)

### October

** Wednesday ** October 28th

Sandro Vaienti (Toulon)

** Title:** On some recent applications of extreme value theory to dynamical systems

** Abstract:** We review a few applications of extreme value theory to:

(i) open systems;

(ii) give the distribution of observables defined along the temporal
evolution of a dynamical system.

Applications are given for the class of prevalent observables.

** Wednesday ** October 21st

Guo Deng (Macquarie)

** Title:** Generation, propagation and interaction of solitary waves in integrable versus non-integrable nonlinear
lattices

** Abstract:** The study of lattice dynamics, i.e., the motion of a spatially discrete system governed by a system
of differential-difference equations, is a classical subject. Of particular interest are lattices that support the
propagation of solitary waves. In this talk, we will compare the properties of two kinds of lattices, one integrable
and one non-integrable: the Toda lattice and the Hertzian chain. As is well known, the Toda lattice is an integrable
system and has exact soliton solutions. In contrast, the Hertzian chain, which has many physical and engineering
applications, is a non-integrable system and no exact solitary-wave solutions are known.
Here we will analyze the similarities and differences between the solitary waves in these two systems, we will
discuss how each of these systems respond to a velocity perturbation, and we will compare the interaction dynamics
of solitary waves. This is a joint work with Dr. Gino Biondini and Dr. Surajit Sen.

** Wednesday ** October 14th

Erika Camacho (Arizona State)

** Title:** Analyzing two different mathematical models of cone metabolism

** Abstract:** Cell degeneration, including that resulting in retinal diseases
such as retinitis pigmentosa and AMD, is linked to metabolic issues. In the retina,
photoreceptor degeneration can result in disturbances of glucose levels and metabolic
processes. To identify the key mechanisms in metabolism that may be culprits of this
degeneration, we develop and investigate two mathematical models of the metabolic pathway
of aerobic glycolysis in a healthy cone photoreceptor. We develop two different
nonlinear systems of enzymatic functions and differential equations.
In one case, we mathematically model cone molecular and photoreceptor cellular interactions.
In the other case, within a single cone cell, we consolidate some of the metabolic
processes in the glycolytic pathway and focus on the glucose, lactate, and pyruvate levels.
We perform numerical simulations, use available metabolic data to estimate parameters and
fit the models to this data. We conduct uncertainty and sensitivity analysis to identify
the processes that have the largest impact on each system.

In the molecular and cellular level model, we consider the case of a healthy cone, a
cone with low levels of glucose, and a cone with low and no rod-derived cone viability factor
(RdCVF). The three key processes identified are metabolism of fructose-1,6-bisphosphate,
production of glycerol-3-phosphate and competition that rods exert on cone resources.
The first two processes are proportional to the partition of the carbon flux between
glycolysis and the pentose phosphate pathway or the Kennedy pathway, respectively. The
last process is the rods’ competition for glucose, which may explain why rods also provide
the RdCVF signal to compensate.

In the other model, we use bifurcation techniques and identify a bistable regime, biologically
corresponding to a healthy versus a pathological state. The system exhibits a saddle node
bifurcation and hysteresis. Model simulations reveal the modulating effect of external lactate
in bringing the system to steady state; the bigger the difference between external lactate and
initial internal lactate concentrations, the longer the system takes to achieve steady state.
Sensitivity analysis reveals that the rate of b-oxidation of ingested outer segment fatty acids
consistently plays an important role in the concentration of glucose, G3P, and pyruvate, whereas
the extracellular lactate level consistently plays an important role in the concentration of
lactate and acetyl coenzyme A.

The ability of these mechanisms to affect key metabolites’ levels (as revealed in our analyses)
signifies the importance of inter-dependent and inter-connected feedback processes modulated
by and affecting cone’s metabolism.

### September

** Wednesday **, Sept 30 at 9am

Sarah Iams (Harvard)

** Title:** Patterned vegetation in drylands: satellite imagery and models

** Abstract:** Banded vegetation patterns are surprisingly common in drylands.
In these ecosystems, water is a limiting nutrient, and ecohydrological processes are
considered to be relevant to the formation and maintenance of patterns. Modeling
challenges for capturing and predicting the evolution of these patterns include a
lack of first principles mechanisms around which to build a model, and the range of
timescales that are relevant to the system. Timescales include the rapid timescales
of rainfall and runoff (minutes to hours), the seasonal timescales of plant growth
and death (days to months), and the longer time scales associated with pattern evolution
(years to decades). In this talk, I will discuss results from studies based on
satellite data, I will provide a brief overview of reaction-advection-diffusion modeling
approaches, and I will present models where we have worked to incorporate topographic
variation and fast-slow switching to capture the dynamics of the system.

## First Semester 2020

** Wednesday ** Mar 11, 2pm in Carslaw 373

Ian Melbourne (Warwick)

** Title:** Deterministic homogenization for fast-slow dynamical systems (DIFFERENT LOCATION: CARSLAW 175)

** Abstract:** We consider deterministic fast-slow systems where the fast dynamics is
assumed to be (non)uniform hyperbolic. The aim is to prove that the slow dynamics converges
to a stochastic differential equation (with the correct interpretation of the stochastic integrals).
Various results in this direction will be described.

** Wednesday ** Mar 4, 2pm in Carslaw 373

Bob Rink (VU Amsterdam)

** Title:** Bifurcations in networks - is synchrony just symmetry?

** Abstract:** Networks of coupled nonlinear dynamical systems arise as models
throughout the sciences. Such network systems may display unexpected collective
behaviour even if the individual dynamical systems that it is made of are quite simple.
This behaviour includes (partial) synchronisation, where (some of) the agents of the
network evolve in unison (think of the simultaneous firing of neurons). It has also
been observed that synchrony in networks often emerges and breaks through unusual
bifurcation scenarios.
This talk is motivated by the question how we can predict and
compute these synchrony breaking bifurcation scenarios from intrinsic geometric
properties of the network. One of these geometric properties is an algebraic structure
that we called "hidden network symmetry". This structure lets us invoke representation
theory to determine the generic local behaviour of network dynamical systems.
This is
joint work with Jan Sanders and Eddie Nijholt.

## Second Semester 2019

** Wednesday ** Nov 20, 2pm in Carslaw 373

Ivan Graham (Bath)

** Title:** Uncertainty quantification for PDEs

** Abstract:** In this talk I'll give an overview of work in the uncertainty quantification
of PDEs with random input data, where the main objective is to compute expected values of
quantities of interest derived from the solutions of the PDEs. I'll give some practical examples
and then I'll explain how, via parametrization, the random PDE can be written as a parametrized
family of deterministic PDEs with parameter lying in a possibly (very) high dimensional space.
Such problems can then be solved by sampling the PDE (often many times over) and then averaging,
to obtain expected values.
A successful algorithm then consists of (a) making good choices of points
in high-dimensional parameter space at which to sample the data, (b) computing the samples of
the data, and (c) fast computation of samples of the PDE, very many of which may be needed.
In
recent years there are many successful algorithms combining (a), (b) and (c) for some classes of
PDEs, particularly the diffusion equation, and I'll describe a method which uses quasi-Monte Carlo
for (a), circulant embedding for (b) and algebraic multigrid for (c). Recently I've been working
on the frequency domain wave equation, which arises in the study of waves in random media.
There the problems which arise are much more difficult particularly because there is no nice method
to achieve (c), and so there are many open problems. I'll present some recent progress in this area.

** Wednesday ** Nov 13, 2pm in Carslaw 373

Andrea Bertozzi (UCLA)

** Title:** A theory for undercompressive shocks in tears of wine

** Abstract:** We revisit the tears of wine problem for thin films in water-ethanol
mixtures and present a new model for the climbing dynamics. The new formulation includes
a Marangoni stress balanced by both the normal and tangential components of gravity as
well as surface tension which lead to distinctly different behavior. The combined physics
can be modeled mathematically by a scalar conservation law with a nonconvex flux and a
fourth order regularization due to the bulk surface tension. Without the fourth order term,
shock solutions must sastify an entropy condition - in which characteristics impinge on the
shock from both sides. However, in the case of a nonconvex flux, the fourth order term is a
singular perturbation that allows for the possibility of undercompressive shocks in which
characteristics travel through the shock. We present computational and experimental evidence
that such shocks can happen in the tears of wine problem, with a protocol for how to observe
this in a real life setting.

** Wednesday ** Oct 30, 2pm in Carslaw 373

Jan Obloj (Oxford)

** Title:** Optimal Transport with a Martingale Constraint: theory, applications and numerics

** Abstract:** Optimal transportation is a very rich and well-established field in mathematics.
I consider here its variant where the transport has a direction and an additional martingale, or
barycentre preservation, constraint. I will explain how this problem, called the Martingale Optimal
Transport (MOT), arises naturally in (robust) financial mathematics and how it links with the classical
Skorokhod embedding problem in probability. I will then discuss some recent results on structure of martingale
transports. Finally, I will present recent advances on numerical methods for such problems via Linear Programming
and/or deep Neural Networks methods.
Based on joint works with Pietro Siorpaes and with Gaoyue Guo.

** Wednesday ** Oct 25, 2pm in Carslaw 373

Jitesh Gajjar (Manchester)

** Title:** Problems with the numerical solution of initial value problems in triple-deck theory

** Abstract:** There are a number of example problems arising in triple-deck theory which we have
attempted to solve recently where the solution technique seems to generate unexplained behaviour.
For example in the classic supersonic compression ramp problem discussed in Logue, Gajjar & Ruban
(2014) wavepackets appear in the results which do not appear to be related to anything physical.
A similar situation arises in trying to solve the classic initial value vibrator problem of Terentev
(1981). In this talk we discuss a closely related problem of boundary layer flow past localised heating
elements. The linearised initial-value problem is solved analytically as well as numerically, and
we will discuss the strange wavepacket behaviour which is also present.

** Wednesday ** Oct 23, 2pm in the AGR room

Matthew Holden (Queensland)

** Title:** The Dynamics of Wildlife Crime

** Abstract:** Illegal harvest of wildlife (poaching) is one of the greatest threats to biodiversity.
Most countries try to reduce poaching by increasing law enforcement to catch and punish poachers. But
despite best efforts from police, poaching is more frequent now than ever. In this talk, we present
simple ordinary differential equation models of poachers and wildlife, to explore why law-enforcement
has failed to stem the poaching problem. We then use these models to project the performance of controversial,
alternative, management actions, such as, campaigns to reduce consumer demand for illegal wildlife products,
and legalising trade of these products.

** Wednesday ** Oct 16, 2pm in the AGR room

Nathan Duignan (Sydney)

** Title:** On the Simultaneous Binary Collision

** Abstract:** We explore the work of my recent PhD thesis, namely, theory surrounding the singularity
at a simultaneous binary collision in the 4-body problem. It is known that any attempt to remove the
singularity via block regularisation will result in a regularised flow that is no more than differentiable
with respect to initial conditions. Through an example based analysis of planar systems, this concept of
block regularisation is defined. Then, this curious loss of differentiability is investigated through a blow-up
procedure and a new proof of the regularity in the collinear problem is provided. In the process, it is revealed
that the critical manifold from the blow-up consists of two manifolds of normally hyperbolic saddle singularities
which are connected by a manifold of heteroclinics. By utilising recent work on transitions near such objects and
their normal forms, an asymptotic series of the transition past the singularity is explicitly computed. It becomes
remarkably apparent that the finite differentiability at is due to the inability to construct a set of integrals
local to the simultaneous binary collision.

** Wednesday ** Oct 9, 2pm in the AGR room

Christopher Lustri (Macquarie)

** Title:** The Role of Stokes lines in Physical Systems

** Abstract:** Systems with small parameters are often studied using asymptotic techniques.
Despite the ubiquity of these techniques, many classical asymptotic methods are unable to capture
behaviour that occurs on an exponentially small scale, which lies "beyond all orders" of power series
in the small parameter. Typically this does not cause any issues; this behavior is too small
to have a measurable impact on the overall behaviour of the system. I will showcase two systems
in which exponentially small contributions have a significant effect on the overall system behaviour.
The first system, which I will discuss in detail, will be nonlinear waves propagating through particle
chains with periodic masses. I will show that it is typically possible for Toda and FPUT lattices for
certain combinations of parameters - determined by the exponentially small system behaviour - to produce
solitary waves that propagate indefinitely. The second system, which I will discuss more briefly, will
be the shape of bubbles in a steadily translating Hele-Shaw cell. By studying exponentially small effects,
it is possible to construct exotic bubble shapes which correspond to recent laboratory experiments.

** Wednesday ** Sept 25, 2pm in the AGR room

Theo Vo (Monash)

** Title:** French ducks roam free across the brain!

** Abstract:** Rhythms in the brain are vital for all aspects of physiological development and function.
At the cellular level, neural rhythms typically manifest as electrical signals known as bursts, consisting
of long periods of inactivity interspersed with rapid trains of closely spaced action potentials. Bursts
are the basic units of neural information and have been proposed to support numerous functional roles, such
as synchronization between neuronal populations, attention, synaptic plasticity, and memory and awareness.
In this seminar, we examine the dynamics of bursting from the viewpoint of canard (French: duck) theory.
We explain the origins and properties of bursting in various contexts, such as in hormone and neurotransmitter
secretion in the pituitary gland. We also discuss how the predictions from canard theory can be tested in vitro.

** Wednesday ** Sept 18, 2pm in the AGR room

Anthony Roberts (Adelaide)

** Title:** Paradoxes across the scales: model reduction from fine-scale to coarse-scale dynamics

** Abstract:** Let's first prove that negative probabilities are OK! The question is when? and how? Answer:
when modelling the dynamics of a high-D system by a low-D system---such model reduction is the theme of this talk.
Then let's discuss how averaging is unsound despite many claiming it is exact! Of course, such averaging underpins
many conservation PDEs in space, and hence these PDEs may mislead!
Lastly, let's see how non-autonomous/stochastic systems
have to be modelled by uncertain variables! Examples discussed include quasi-stationary probability, thin fluid films,
shear dispersion, spatial birth and death, lattice systems, Brownian motion, and population models. Remember these
paradoxes and their resolution whenever you consider dynamics at multiple levels of model resolution.

** Wednesday ** Sept 4, 2pm in the AGR room

Martin Wechselberger (Sydney)

** Title:** Regularisation of shock waves in reaction-nonlinear diffusion models: a geometric singular perturbation theory approach

** Abstract:** Reaction-nonlinear diffusion models arising in the context of cell migration and population dynamics can exhibit the property of aggregation
– or backward diffusion. While this is physically relevant, mathematically it causes such models to break down. The aggregation causes shocks to form, and
the solutions are no longer computable.
To account for shocks, modellers have employed the technique of regularisation – adding additional small higher order
terms to these models to smooth out the shocks. These regularisation techniques have been widely employed in models of chemical phase-separation, though they
have gone relatively unnoticed in biological models until very recently.
We have developed techniques from the field of geometric singular perturbation theory
to resolve similar issues of shock formation in a different class of models, so-called advection-reaction models (hyperbolic balance laws). In this presentation,
we will tackle the question of existence and formation of shocks in regularised reaction-nonlinear diffusion models using geometric singular perturbation theory.

** Wednesday ** August 21, 2pm in the AGR room

Imene Khames (INSA Rouen)

** Title:** Nonlinear Network Wave Equation: Periodic Solutions and Graph Characterizations

** Abstract:** We study the discrete nonlinear wave equation in arbitrary finite networks.
This is a general model, where the usual continuum Laplacian is replaced by the graph Laplacian.
We consider such a wave equation with a cubic on-site nonlinearity which is the discrete \Phi^4 model,
describing a mechanical network of coupled nonlinear oscillators or an electrical network where the components
are diodes or Josephson junctions.
In the first part, we investigate the extension of the linear normal modes of the
graph Laplacian into nonlinear periodic orbits. Normal modes -whose Laplacian eigenvectors are composed uniquely of {1}, {-1,1} or {-1,0,1}-
give rise to nonlinear periodic orbits for the discrete \Phi^4 model. We perform a systematic linear stability (Floquet) analysis of these orbits
and show the modes coupling when the orbit is unstable. Then, we characterize graphs having Laplacian eigenvectors in {-1,1} and {-1,0,1} using graph spectral
theory.
In the second part, we investigate periodic solutions that are exponentially (spatially) localized. Assuming a large amplitude localized initial condition
on one node of the graph, we approximate its evolution by the Duffing equation. The rest of the network satisfies a linear system forced by the excited node.
This approximation is validated by reducing the discrete \Phi^4 equation to the discrete nonlinear Schrodinger equation and by Fourier analysis. These results
relate nonlinear dynamics to graph spectral theory.

** Wednesday ** August 7, 2pm in the AGR room

Vera Roshchina (UNSW)

** Title:** Faces of convex sets: dimensions and regularity

** Abstract:** The facial structure of convex sets can be surprisingly complex, and unexpected irregularities of the arrangements of faces give rise to badly behaved sets and various counterexamples.
In this talk I will focus on specific properties of facial structure that capture irregularities in the facial structure of the set (dimensions of faces, singularity degree, facial exposure and facial dual completeness).
I will also talk about some classic results related to faces of convex sets, mention some new results and counterexamples and will relate this to several open problems in convex algebraic geometry and the geometry of polytopes.

## Previous seminars

### 2019

#### First Semester

** Wednesday ** March 27, 2pm in the AGR room

Dr. Peter Cudmore (Systems Biology Laboratory, the University of Melbourne)

** Title:** On Emergence in Complex Physical Systems

** Abstract:** Many problems in biology, physics and engineering involve predicting and controlling complex systems, loosely defined as interconnected system-of-systems. Such systems can exhibit a variety of interesting non-equilibrium features such as emergence and phase transitions, which result from mutual interactions between nonlinear subsystems.
Modelling these systems is a task in-and-of itself, as systems can span many physical domains and evolve of multiple time scales. Nonetheless, one wishes to analyse the geometry of these models and relate both qualitative and quantitative insights back to the physical system.
Beginning with the modelling and analysis of a coupled optomechanical systems, this talk presents some recent results concerning the existence and stability of emergent oscillations. This forms the basis for a discussion of new directions in symbolic computational techniques for complex physical systems as a means to discuss emergence more generally.

** Wednesday ** May 8, 2pm in the AGR room

Dr. Heather McCreadie (Aberystwyth University, UK)

** Title:** Autonomous Curve Fitting of the Dst index during Geomagnetic Storms

** Abstract:** A technique has been developed to fit all types of geomagnetic storms identified in the Dst. A lognormal fitting procedure may be used to describe any storm by setting the lognormal standard deviation to greater than 0.9. The fit needs to be constrained around the peak time and the scaling factor determined. This will enable an autonomous method for fitting any type of storm within the Dst. The unique factor identifying the relationship between the main and recovery phase of a storm is the lognormal mean.

** Wednesday ** May 22, 2pm in the AGR room

Dr. Paul Griffiths (Coventry University, UK)

** Title:** Temperature dependent viscosity flows - analysis and applications

** Abstract:** In this talk we will consider two and three dimensional boundary layer flows. The stability of both the flat plate and rotating disk boundary layers will be discussed in the context of fluids that exhibit a variational viscosity. In particular, we will present linear stability results for fluids with viscosity that varies as a function of temperature. Numerical results (neutral curves, growth rates and energy analyses) will be supported by asymptotic predictions at large Reynolds numbers. The influence of an enforced axial flow will also be discussed in the context of Chemical Vapour Deposition (CVD).

** Wednesday ** June 5, 2pm in the AGR room

Prof. Vladimir Dragovic (UT Dallas, USA)

** Title: Triangular Schlesinger systems, Painleve VI equations, and superelliptic curves **

** Abstract:** We study the Schlesinger system in the case when the unknown matrices of arbitrary size (p×p) are triangular and the eigenvalues of each matrix form an arithmetic progression with a rational difference q, the same for all matrices. We show that such a system
possesses a family of solutions expressed via periods of meromorphic differentials on the Riemann surfaces of superelliptic curves. We determine the values of the difference q, for which our solutions lead to explicit polynomial or rational solutions of the Schlesinger system. As an application of
the (2 × 2)-case, we obtain explicit sequences of rational solutions and one-parameter families of rational solutions of Painleve VI equations. This is a joint work with Renat Gontsov and Vasilisa Shramchenko.

** Wednesday ** 12 of June 2019, 2pm in the AGR room

Dr. Vijay Rajagopal (Dept. of Biomedical Engineering, University of Melbourne)

** Title: ** Dissecting the role of the internal architecture of cardiac cells on calcium signaling in the heart using computational models.

** Abstract: ** Calcium plays a central role in how our hearts beat. Each heartbeat is governed by the cyclic rise and fall of calcium in the cell cytoplasm through various co-ordinated and tightly regulated electrical and chemical processes. Calcium also plays a crucial role in determining when our heart muscle will grow in order to increase the force with which the heart beats as long-term demand for blood supply is increased. This process of cell and heart muscle growth is termed hypertrophy and is analogous to how our skeletal muscles grow through weight training. Exactly how calcium can regulate beat-to-beat muscle contraction and also send a signal to the cell nucleus for long-term growth is unclear. In this talk I will present our research into how the spatial organisation of ion-channels that govern calcium concentration in the cytoplasm affect calcium dynamics in the cell for beat-to-beat contraction and hypertrophic growth.

### 2018

#### Second Semester

** Wednesday ** October 10, 2pm in the AGR room

Dr. Minh-Ngoc Tran (Business School, University of Sydney)

** Title:** Bayesian Deep Net GLM and GLMM

** Abstract: ** Deep feedforward neural networks (DFNNs) are a powerful tool for functional approximation. We describe flexible versions of generalized linear and generalized linear mixed models incorporating basis functions formed by a DFNN. Efficient computational methods for high-dimensional Bayesian inference are developed using Gaussian variational approximation, with a parsimonious but flexible factor parametrization of the covariance matrix. We implement natural gradient methods for the optimization, exploiting the factor structure of the variational covariance matrix in computation of the natural gradient. Our flexible DFNN models and Bayesian inference approach lead to a regression and classification method that has a high prediction accuracy, and is able to quantify the prediction uncertainty in a principled and convenient way. We also describe how to perform variable selection in our deep learning method. The proposed methods are illustrated in a wide range of simulated and real-data examples, and the results compare favourably to a state of the art flexible regression and classification method in the statistical literature, the Bayesian additive regression trees (BART) method. User-friendly software packages in Matlab and R implementing the proposed methods are available at https://github.com/VBayesLab.

** Wednesday ** September 19, 2pm in the AGR room

Dr. Lachlan Smith (University of Sydney)

** Title:** Chaos and the flow capture problem: Polluting is easy, cleaning is hard

** Abstract: ** Where do you place pollutant capture units? When objects move through heterogeneous flow environments, such as oceanic micro-plastics, the answer is not obvious. We formulate flow capture problems, involving flows and sinks, and, using dynamical systems techniques, show that blindly positioning capture units carries high risk of failure. Capture efficiency depends on capture rate: long-term efficiency decreases as the number of capture units increases, whereas short-term efficiency increases. Doubling numbers of capture units can more than double the capture rate. The formal description of flow capture problems will impact engineering solutions ranging from atmospheric CO2 capture to oceanic micro-plastic pollution.

** Wednesday ** August 29, 1pm (ONE HOUR EARLIER THAN USUAL!) in the AGR room

Dr. Robyn Araujo (Queensland Univ. of Tech.)

** Title:** Robust Perfect Adaptation in Complex Bionetworks

** Abstract: ** Robustness, and the ability to function and thrive amid changing and unfavourable environments,
is a fundamental requirement for all living systems. Moreover, it has been a long-standing mystery how the
extraordinarily complex communication networks inside living cells, comprising thousands of different interacting
molecules, are able to exhibit such remarkable robustness since complexity is generally associated with fragility.
In this talk I will give an overview of our recent research on robustness in cellular signalling networks, with
an emphasis on the robust functionality known as Robust Perfect Adaptation (RPA). This work is now published
in Nature Communications, and is available here:
https://rdcu.be/M46K. This work has suggested a resolution
to the complexity-robustness paradox through the discovery that robust adaptive signalling networks must be
decomposable into topological basis modules of just two possible types. This newly-discovered modularisation
of complex bionetworks has important implications for evolutionary biology, embryology and development, cancer
research and drug development.

** Wednesday ** September 5, 2pm in the AGR room

Dr. Justin Tzou (Macquarie University)

** Title:** Stability analysis of localised patterns in two and three spatial dimensions

** Abstract: ** We present a matched asymptotics framework for constructing and analysing the stability of localised patterns that arise in singularly perturbed activator-inhibitor reaction-diffusion systems. In two spatial dimensions, by way of analyses of nonlocal eigenvalue problems, we resolve two long-standing problems regarding 1) the stability of spot patterns to oscillatory instabilities, and 2) the stability of stripe patterns to break-up instabilities, the latter motivated by the persistence of striped vegetation patterns on steep hillsides. In three spatial dimensions, we calculate explicit stability thresholds for self-replication and annihilation of spots, and derive a gradient flow that governs their slow dynamics. Joint work with Theodore Kolokolnikov, Michael J. Ward, and Shuangquan Xie.

#### First semester

** Monday (!) ** March 26, 2pm in the AGR room

Prof. Gunther Uhlmann (University of Washington)

** 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. We will also consider the problem of determining the metric from the scattering relation, the so-called lens rigidity problem. The linearization of these problems involve the integration of a tensor along geodesics, similar to the X-ray transform.

We will also describe some recent results, join 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.

Wednesday February 21, 2pm in the AGR room

Prof. Herbert Huppert (University of Cambridge)

** Title:** How to frack into and out of trouble.

** Abstract: ** After a short introduction to the mechanism and politics of fracking, the talk will
concentrate on the fluid mechanics and elastodynamics of driving fluid into cracks and the quite different
response when the pressure is released and the fluid flows back out. Development of the governing equations
will be presented along with their numerical solution and asymptotic analysis in certain useful limits.
Videos of laboratory experiments will be shown and the results compared with the theoretical predictions.

Wednesday March 7, 2pm in the AGR room

Prof. Martin Wechselberger (Applied Maths, University of Sydney)

** Title:** Two-stroke relaxation oscillators

** Abstract: ** In classic van der Pol-type oscillator theory, a relaxation cycle consists of two
slow and two fast orbit segments per period (slow-fast-slow-fast). A possible alternative relaxation
oscillator type consists of one slow and one fast segment only. In electrical circuit theory, Le Corbeiller
(published in IEEE 1960) termed this type a two-stroke oscillator (compared to the four-stroke vdP oscillator).
I will provide examples of two-stroke relaxation oscillators and discuss these problems from a geometric
singular perturbation theory point of view "beyond the standard form". It is worth mentioning that
Fenichel's seminal work on geometric singular perturbation theory (published in JDE 1979) discusses this
more general setting, but it has not received much attention in the literature.

Wednesday March 14, 2pm in the AGR room

Prof. Dmitry Pelinovsky (McMaster University, Canada)

** Title: ** Rogue periodic waves in the focusing MKDV and NLS equations

** Abstract:** Rogue periodic waves stand for gigantic waves on a periodic background. The nonlinear
Schrodinger equation in the focusing case admits two families of periodic wave solutions
expressed by the Jacobian elliptic functions dn and cn. Both periodic waves are
modulationally unstable with respect to long-wave perturbations. Exact solutions for the
rogue periodic waves are constructed by using the explicit expressions
for the periodic eigenfunctions of the Zakharov–Shabat spectral problem and
the Darboux transformations. These exact solutions generalize the classical rogue wave
(the so-called Peregrine’s breather). Computations of rogue periodic waves rely on
properties of the nonlinear Schrodinger equation due to its integrability.

### 2017

#### Second semester

Monday December 11, 2pm in the AGR room

Dr. Yulia Peet (Arizona State University)

** Title:** Overlapping and Moving Grid Approaches with Spectral-Element Methods: Concepts and Applications

** Abstract: ** In this talk, we present our recent development of overlapping and moving grid methodology
for high-fidelity computations of fluid flow problems with spectral element methods. Spectral element methods
belong to a class of high-order methods that combine exponential convergence of global spectral methods with
geometrical flexibility of finite-element methods. High-order methods possess low dissipation and low
dispersion errors and are well suited for high-accuracy simulations of turbulent flows. The current development
of overlapping and moving grid approaches enables the application of spectral element methods to a larger
class of problems that involve moving bodies and complex geometries. In this talk, the fundamental concepts
of both the spectral element methodology and the overlapping grid approach will be discussed, followed by a
description and analysis of the methods that we have developed, paying a special attention to the concepts
of stability and accuracy of the proposed methodology. We will proceed by discussing an application of the
developed method to Direct Numerical Simulations of airfoil dynamic stall in the presence of upstream
disturbances. We conclude by showing further potential extensions of the current methodology and list new
applications that can be successfully tackled with this method.

Wednesday July 19

Prof. Boris Khesin (Department of Mathematics, University of Toronto, Canada)

** Title:** Hamiltonian dynamics of vortex membranes

** Abstract: ** We show that an approximation of the hydrodynamical Euler equation
describes the skew-mean-curvature flow on vortex membranes in any
dimension. This generalizes the classical binormal, or vortex filament,
equation in 3D. We present a Hamiltonian framework for dynamics of
higher-dimensional vortex filaments and vortex sheets as singular
2-forms (Green currents) with support of codimensions 2 and 1,
respectively.

Wednesday July 26

Dr. Marianito Rodrigo (School of Mathematics and Applied Statistics University of Wollongong)

** Title:** On a fractional matrix exponential and an explicit method for its calculation

** Abstract: **The matrix exponential arises in many applications, particularly in the solution
of linear systems of ordinary differential equations. The nth derivative of the matrix exponential is equal
to the nth power of the matrix multiplied by the matrix exponential. What is the analogue of this when
the ordinary derivative is replaced by a fractional derivative? In this talk I will define a fractional
matrix exponential and then give an explicit method for calculating the fractional matrixexponential. An
overview of the fractional calculus will be given.

Wednesday August 2

A/Prof Zhi-An Wang (Department of Applied Mathematics, Hong Kong Polytechnic University, Hong Kong)

** Title:** Boundary layers arising from chemotaxis models

** Abstract:** The original well-known Keller-Segel system describing the chemotactic wave propagation
remains poorly understood in many aspects due to the logarithmic singularity. As the chemical assumption rate
is linear, the singular Keller-Segel model can be converted, via a Cole-Hopf type transformation, into a
system of viscous conservation laws without singularity. In this talk, we first consider the dynamics of
the transformed Keller-Segel system in a bounded interval with time-dependent Dirichlet boundary conditions.
By imposing some conditions on the boundary data, we show that boundary layer profiles are present as
chemical diffusion tends to zero and large-time profile of solutions will be determined by the boundary
data (i.e. boundary stabilization). We employ the refined (weighted) energy estimates with the "effective viscous
flux" technique to show the emergence of boundary layer profiles. For asymptotic dynamics of solutions, we
develop a new idea by exploring the convexity of an entropy expansion to get the basic $L^1$-estimate, on
which our results are built up by the method of energy estimates. Finally we gain the results for the original
singular Keller-Segel system by reversing the Cole-Hopf transformation. Numerical simulations are performed
to interpret our analytical results and their implications.

Wednesday August 9

Prof Kenji Kajiwara (Institute of Mathematics for Industry, Kyushu University, Japan)

** Title:** Construction and simulation of discrete integrable model for soil water infiltration problem

** Abstract: ** In this talk, we propose an integrable model and its discretization describing
one-dimensional soil water infiltration problem. The model is formulated as the nonlinear boundary value
problem for a nonlinear diffusion-convection equation, which is transformed
to the Burgers equation by a certain independent variable transformation incorporating the dependent
variable, called the hodograph transformation or the reciprocal transformation. We construct
the discrete model preserving the underlying integrability nature and formulate it as the
self-adaptive moving mesh scheme. If we require the numerical stability and high-precision
coincidence with the special case where the exact solution is obtained, we need some investigation
and modification of the discrete model from the point of view different from integrability.
We discuss this point and show some numerical results.

This talk is based on the paper arXiv:1705.03129 by D. Triadis (Kyushu/La Trobe) , P. Broadbridge (La Trobe), K. Maruno (Waseda) and myself.

Wednesday September 6

Dr. Sophie Calabretto (Department of Mathematics,Macquarie University)

** Title:** Flow external to a rotating torus (or a sphere)

** Abstract: ** The unsteady flow generated due to the impulsive motion of a torus or sphere is a
paradigm for the study of many temporally developing boundary layers. The boundary layer is known to
exhibit a finite-time singularity at the equator. We present results of a study that focuses upon the
behaviour of the flow after the onset of this singularity. Our computational results demonstrate that the
singularity in the boundary layer manifests as the ejection of a radial jet. This radial jet is preceded
by a toroidal starting vortex pair, which detaches and propagates away from the sphere. The radial jet
subsequently develops an absolute instability, which propagates upstream towards the sphere surface.

Wednesday September 13

Dr. Ananta K. Majee (Mathematisches Institut, University of Tuebingen, Germany)

** Title:** On stochastic optimal control in ferromagnetism

** Abstract: ** In this presentation, we study an optimal control problem for the stochastic
Landau-Lifshitz-Gilbert equation on a bounded domain in R^d (d = 1, 2, 3). We first establish existence
of a relaxed optimal control for relaxed version of the problem. As the control acts in the equation
linearly, we then establish existence of an optimal control for the underlying problem. Furthermore,
convergence of a structure presrving finite element approximation for d = 1 and physically relevant
computational studies will be discussed.

Wednesday October 4

Dr. Lewis Mitchell (School of Mathematical Sciences, University of Adelaide)

** Title:** Information flows in online social networks

** Abstract: ** The flow of information online is a significant factor in social contagion, rumour
and “fake news” propagation, and protest organisation. Further, online social platforms provide
a unique opportunity for computational social scientists to observe individuals’ spontaneous
interactions over social ties, often through structural or temporal proxies for information. However,
such approaches do not leverage the full extent of information available, namely the time-ordered textual
content of messages. Here we apply information-theoretic techniques to social media data to identify the
extent to which predictive information is encoded in social ties, and that in principle one can profile
an individual from their contacts even if the individual is ``hidden'' within the network. Analysis of
Twitter users shows that 95% of the potential predictive accuracy attainable for an individual is embedded
within their social ties, and numerical simulations on a paradigmatic model of information flow shows that
these techniques are robust.

Wednesday October 11

Dr. David A. Smith (Science (Mathematics), Yale-NUS, Singapore)

** Title:** Nonlocal Problems for Linear Evolution Equations

** Abstract: ** Linear evolution equations, such as the heat and linearized KdV equations, are commonly
studied on finite spatial domains via initial-boundary value problems. Typically, the boundary conditions
specify information about the solution and its derivatives at the edges of the spatial domain. Alternatively,
in place of the boundary conditions, consider "multipoint conditions", where one specifies some linear
combination of the solution and its derivatives evaluated at internal points of the spatial domain. A further
generalization is the "nonlocal" specification of the integral over space of the solution against some
continuous weight. We describe a general framework for studying such problems, and provide solution
representations for 2nd and 3rd order examples.

#### First semester

Wednesday January 25

Dr. Paul Griffiths (Oxford Brookes University, UK)

** Title:** Shear-thinning: A stabilising effect? Yes, no, maybe?

** Abstract: ** In this talk we will investigate how viscosity effects the stability of a fluid flow. By
assuming a shear-thinning viscosity relationship, where an increase in shear-rate results in a decrease in
fluid viscosity, we show that flows can be both stabilised or destabilised, depending on (i) the fluid model
in question and (ii) the ‘amount’ of shear-thinning the fluid exhibits. Using a two-dimensional
boundary-layer flow as our ‘toy model’ we are able to show equivalence between different
shear-thinning models. The effect shear-thinning has on important parameters such as the critical Reynolds
number, and the maximum frequency of the disturbances will be discussed and interpreted in the wider context.

Wednesday February 22

Dr. Maria Vlassiou (Eindhoven University of Technology, Netherlands)

** Title:** Heavy-traffic limits for layered queueing networks

** Abstract: ** Heavy-traffic limits for queueing networks are a topic of continuing interest. Presently,
the class of networks for which these limits have been rigorously derived is restricted. An important
ingredient in such work is the demonstration of state space collapse (SSC), which loosely speaking shows
that in diffusion scale the queuing process for the stochastic model can be approximately recovered as a
continuous lifting of the workload process. This often results in a reduction of the dimensions of the
original system in the limit, leading to improved tractability. In this talk, we discuss diffusion
approximations of layered queuing networks through two examples.

In the first example, we establish a heavy-traffic limit through SSC for a computer network model. For this model, SSC is related to an intriguing separation of time scales in heavy traffic. The main source of randomness occurs at the top layer; the interactions at the other layer are shown to converge to a fixed point at a faster time scale.

The second example focuses on a network of parallel single-server queues, where the speeds of the servers are varying over time and governed by a single continuous-time Markov chain. We obtain heavy-traffic limits for the distributions of the joint workload, waiting-time and queue length processes. We do so by using a functional central limit theorem approach, which requires the interchange of steady-state and heavy-traffic limits. For this model, we show that the SSC property does not hold.

Wednesday March 1

Dr. Daniel Lecoanet (Princeton University, Princeton, USA)

** Title:** Measuring Core Stellar Magnetic Field using Wave Conversion

** Abstract: **
By studying oscillation modes at the surface of stars, astrophysicists are able to infer characteristics of
their deep interior structure. This was first applied to observations of the Sun, but recently space-based
telescopes have measured oscillations in many other stars, leading to many new mysteries in stellar structure
and evolution. Recent work has suggested that low dipole oscillation amplitudes in evolved red giant branch
stars may indicate strong core magnetic fields. Here we present both numerical simulations and analytic
calculations of the interactions of waves with a strong magnetic field. We can solve the problem very accurately
by using the WKB approximation to reduce the 2D PDE into a series of ODEs for different heights. We find that
magnetic fields convert the buoyancy-driven waves observable at the surface of the star to magnetic waves, which
are not present at the surface, in agreement with observations.

Wednesday March 22

Sheehan Olver (School of Mathematics and Statistics, University of Sydney)

** Title:** Solving PDEs on triangles using multivariate orthogonal polynomials

** Abstract: ** Univariate orthogonal polynomials have long history in applied and computational mathematics,
playing a fundamental role in quadrature, spectral theory and solving differential equations with spectra
methods. Unfortunately, while numerous theoretical results concerning multivariate orthogonal polynomials
exist, they have an unfair reputation of being unwieldy on non-tensor product domains. In reality, many of
the powerful computational aspects of univariate orthogonal polynomials translate naturally to multivariate
orthogonal polynomials, including the existence of Jacobi operators and the ability to construct sparse partial
differential operators, a la the ultrapsherical spectral method [Olver & Townsend 2012]. We demonstrate
these computational aspects using multivariate orthogonal polynomials on a triangle, including the fast
solution of general partial differential equations.

Wednesday April 5

Professor Shige Peng (Shandong University, Jinan, China)

** Title:** Backward Stochastic Differential Equations Driven by G-Brownian Motion in Finance

** Abstract: ** We present some recent developments in the theory of Backward Stochastic Differential
Equations (BSDEs) driven by a new type of a Brownian motion under a nonlinear expectation space and we
discuss applications of this new class of BSDEs to financial models in which
the uncertainty of volatility is taken into account.

Wednesday April 12

Professor Holger Dullin (School of Mathematics, University of Sydney)

** Title:** A new twisting somersault - 513XD

** Abstract: ** Abstract: Modelling an athlete as a system of coupled rigid body we derive a time-dependent reduced Euler
equation for the dynamics of shape changing bodies. Reconstruction allows to recover the full dynamics
in SO(3), and the number of somersaults is decomposed into a geometric phase and a dynamics phase.
A kick model is used to approximate the dynamics, and using the insight gained from this we propose
a new 10 meter platform twisting somersault dive (FINA code 513XD) that incorporates 5 full twists.

Wednesday April 19 ** Different Location! Carslaw room 535 **

Prof. Nihat Ay (Max-Planck-Institute for the Mathematics in the Sciences, Leipzig, Germany)

** Title:**Information Geometry and its Application to Complexity Theory

** Abstract: ** In the first part of my talk, I will review information-geometric structures and
highlight the important role of divergences. I will present a novel approach to canonical divergences
which extends the classical definition and recovers, in particular, the well-known Kullback-Leibler
divergence and its relation to the Fisher-Rao metric and the Amari-Chentsov tensor.

Divergences also play an important role within a geometric approach to complexity. This approach is based on the general understanding that the complexity of a system can be quantified as the extent to which it is more than the sum of its parts. In the second part of my talk, I will motivate this approach and review corresponding work.

References:

- N. Ay, S.I. Amari. A Novel Approach to Canonical Divergences within Information Geometry. Entropy (2015) 17: 8111-8129.
- N. Ay, J. Jost, H. V. Le, L. Schwachhöfer. Information geometry and sufficient statistics. Probability Theory and Related Fields (2015) 162: 327-364.
- N. Ay, J. Jost, H. V. Le, L. Schwachhöfer. Parametrized measure models. Bernoulli (2016) accepted. arXiv:1510.07305.
- N. Ay, J. Jost, H. V. Le, L. Schwachhöfer. Information geometry. Ergebnisse der Mathematik und Ihrer Grenzgebiete/A Series of Modern Surveys in Mathematics, Springer 2017, forthcoming book.
- N. Ay. Information Geometry on Complexity and Stochastic Interaction. Entropy (2015) 17(4): 2432-2458.

Wednesday April 26

Professor Robert Dewar (Research School of Physics & Eng., Australian National Univ., Canberra)

** Title:**Variational constructions of almost-invariant tori for 1 1/2-D Hamiltonian systems

** Abstract: ** Action-angle variables are normally defined only for integrable systems, but in order to
describe 3D magnetic field systems a generalization of this concept was proposed recently [1,2] that
unified the concepts of ghost surfaces and quadratic-flux-minimizing (QFMin) surfaces (two strategies for
minimizing action gradient). This was based on a simple canonical transformation generated by a change of
variable, $\theta = \theta(\Theta ,\zeta)$, where $\theta$ and $\zeta$ (a time-like variable) are poloidal
and toroidal angles, respectively, with $\Theta$ a new poloidal angle chosen to give pseudo-orbits that are
(a) straight when plotted in the $\zeta,\Theta$ plane and (b) QFMin pseudo-orbits in the transformed
coordinate. These two requirements ensure that the pseudo-orbits are also (c) ghost pseudo-orbits, but they
do not uniquely specify the transformation owing to a relabelling symmetry. Variational methods of solution
that remove this lack of uniqueness are discussed.

[1] R.L. Dewar and S.R. Hudson and A.M. Gibson, Commun. Nonlinear Sci. Numer. Simulat.

**17**, 2062 (2012) http://dx.doi.org/10.1016/j.cnsns.2011.04.022

[2] R.L. Dewar and S.R. Hudson and A.M. Gibson, Plasma Phys. Control. Fusion

**55**, 014004 (2013) http://dx.doi.org/10.1088/0741-3335/55/1/014004

Wednesday May 3

Prof. Michael Small (The University of Western Australia)

** Title:** Communities Within Networks

** Abstract: ** Many complex systems are naturally represented as networks which lack an underlying geodesic
space. That is, elements of the network are naturally represented by their interconnection and not by their
position in any real space. A favourite problem in complex systems is then how best to infer sensible communities
from the network adjacency matrix. To be able to better frame this question, we first need to more precisely
say something about what we mean by "sensible" communities. The usual way to do this is to define a statistical
measure that quantifies the relative number of inter- to intra- community links - which we call "modularity".
With this in mind, there are several methods one can apply to choose suitable sets of communities which achieve
local optimality of this measure. I will describe some standard methods and some of our own approaches to this
problem. Most recently we have developed methods that embed the network in a suitable geodesic space and then
borrow ideas from computational clustering algorithms to detect communities (joint work with Arif Mahmood,
formerly of UWA now with Qatar University). If I get time, I hope to finish by spending a few minutes talking
about generative algorithms for networks with communities - the problem here is that while we have algorithms
to generate networks with specific "nice" properties (preferential attachment, for example), and we have
algorithms to generate communities, the algorithms to generate "nice" networks with communities are rather clunky.

Wednesday May 17

Dr. Milena Radnovic (The University of Sydney)

** Title:**Geometry, billiards, integrability.

** Abstract: **Starting from the celebrated Poncelet porism, we will present classical and modern results
concerning integrable billiards.

Wednesday May 24

Dr. David Galloway (The University of Sydney)

** Title:**Slow-burning instabilities of Dufort-Frankel finite differencing.

** Abstract: **Du Fort-Frankel is a tactic to stabilise Richardson's unstable 3-level leapfrog time-stepping
scheme. By including the next time level in the right hand side evaluation, it is implicit, but it can be
re-arranged to give an explicit updating formula, thus apparently giving the best of both worlds. Textbooks
prove unconditional stability for the heat equation, and extensive use on a variety of advection-diffusion
equations has produced many useful results. Nonetheless, for some problems the scheme can fail in an interesting
and surprising way, leading to instability at very long times. An analysis for a simple teaching problem
involving a pair of evolution equations that describe the spread of a rabies epidemic gives insight into how
this occurs. An even simpler modified diffusion equation suffers from the same instability. Attempts to fix the
rabies problem by additional averaging are described. One method works for a limited parameter range but beyond
that, instability can take a very long time to appear and its analysis displays interesting subtleties.
This is joint work with David Ivers.