Colloquium

Colloquium

Colloquium

The Departmental Colloquia are held on Friday afternoons, from 3:00 to 4:00pm, during the teaching semesters and are followed by refreshments.

2019 Series...session 1

12 July

Date: Friday, 12 July 2019 
Speaker: A/Prof Ad Ridder (Vrije Universiteit)

Venue:  E7B 146 ACE room

Title: TBC

Abstract: TBC

Week 13 - 7  June

Date: Friday, 7 June 2019 
Speaker: Dr Trent Mattner (The University of Adelaide)

Venue:  E7B 146 ACE room

Title: Large-eddy simulations of turbulence using the stretched-vortex subgrid model

Abstract: Turbulent flows are characterised by irregular three-dimensional motion over a wide range of spatial and temporal scales. In a direct numerical simulation of turbulent flow, the entire range of scales must be resolved by the computational grid. In many applications, the range of scales is so enormous that it is not computationally feasible to resolve them all. In a large-eddy simulation, only the large scales are resolved. This reduces computational requirements, but a model is needed to account for the effects of the fine subgrid scales. In this talk, I will discuss the stretched-vortex subgrid model, in which the subgrid scales are modelled as an ensemble of stretched spiral vortices. As with many other subgrid models, simulations using the stretched-vortex model are sensitive to the numerical scheme. High-order schemes with low numerical dissipation are usually preferred, but it is sometimes necessary to relax these requirements. Recent work shows that it is possible to obtain reasonably accurate results using the stretched-vortex model with low-order numerical schemes and artificial damping, provided suitable care is taken.

Week 12 - 31 May

Date: Friday, 31 May 2019 
Speaker: Dr Sophie Hautphenne (The University of Melbourne)

Venue:  E7B 146 ACE room

Title: The Markovian binary tree applied to demography and conservation biology

Abstract: Markovian binary trees form a general and tractable class of continuous-time branching processes, which makes them well-suited for real-world applications. Thanks to their appealing probabilistic and computational features, these processes have proven to be an excellent modelling tool for applications in population biology. Typical performance measures of these models include the extinction probability of a population, the distribution of the population size at a given time, the total progeny size until extinction, and the asymptotic population composition. Besides giving an overview of the main performance measures and the techniques involved to compute them, we discuss recently developed statistical methods to estimate the model parameters, depending on the accuracy of the available data. We illustrate our results in human demography and in conservation biology.

Week 11 - 24 May

Date: Friday, 24 May 2019 
Speaker: Dr Anthony M. Licata (The Australian National University)

Venue:  E7B 146 ACE room

Title: Groups, curves, and categories

Abstract: Abstract: In the 1980s, W. Thurston and others developed tools to understand symmetry groups (mapping class groups) of 2-dimensional manifolds.  In addition to its relevance for topology and geometry, Thurston's theory is important because it has parallels in other parts of group theory, such as the study of arithmetic groups, or the in the study of automorphisms of the free group.  In this talk, I'll explain a bit about Thurston thinks about curves on surfaces, and then try to tell you something about how similar structure arises when one tries to understand the symmetry groups of triangulated categories.

Week 10 - 17 May

Date: Friday, 17 May 2019 
Speaker: A/Prof Sharon Stephen (University of Sydney)

Venue:  E7B 146 ACE room

Title: Hydrodynamic stability of non-Newtonian shear flows

Abstract: I will discuss results from studies to investigate the effect of non-Newtonian viscosity on the transition process from a laminar flow to a turbulent flow at large Reynolds number for two-dimensional and three-dimensional flows. The focus is to determine whether non-Newtonian flows can be used to delay transition to turbulence in practical applications. Numerical and asymptotic methods are used to determine the basic flows and the neutral stability curves, revealing the stabilising or destabilising effect of non-Newtonian viscosity. The original motivation came from experiments in the oil recovery industry revealing spiral corrosion patterns on marble rotating discs in viscoelastic gelled acids. This lead to our first investigations on the stability of non-Newtonian rotating flows, specifically the effect of different viscosity models on the stability of crossflow vortices due to flow over a rotating disc. Subsequently, we considered the stability of the Blasius boundary layer for a non-Newtonian flow. These studies and ongoing work on the effect of non-Newtonian viscosity for the asymptotic suction boundary will be discussed. The results suggest that the choice of viscosity model for shear flows is important. In particular, the power-law viscosity model may not be appropriate. Attention is now focussed on the alternative Carreau viscosity model which is more realistic for small and large shear rates. Recent results relating to nanofluids will also be discussed.

Week 9 - 10 May

Date: Friday, 10 May 2019 
Speaker: Prof Jean Yang (University of Sydney)

Venue:  E7B 146 ACE room

Title: Integration and cell type identification of multiple single-cell transcriptomics datasets

Abstract:

Technological advances such as large-scale single cell transcriptome profiling has exploded in recent years and enabled unprecedented insight into the behaviour of individual cells. In particular, Single cell RNA-Sequencing (scRNA-Seq) technology allows for cell-type specific characterization of gene expression values, towards understanding underlying biological processes.  Concerted examination of multiple collections of scRNA-Seq data promises further biological insights that cannot be uncovered with individual datasets. However, such integrative analyses and cell type identification are challenging and require sophisticated methodologies. To enable effective interrogation of multiple scRNA-Seq datasets, we have developed a novel algorithm, named scMerge, that removes unwanted variation by identifying stably expressed genes and utilizing pseudo-replicates across datasets. Biological knowledge such as cell type information can be easily incorporated into scMerge to further improve performance. We evaluated scMerge using seven publicly available scRNA-Seq data collections and found our method effectively removed batch and dataset-specific effects across a wide range of biological systems. To identify cell types, we further employed statistical machine learning methodologies together with cell type information from an external dataset to identify cells in a hierarchically setting. We evaluated our novel approach scClassify and found that it consistently achieves low classification error rates as well as enables finer cell type identification.

Week 8 - 03 May

Date: Friday, 03 May 2019 
Speaker: Dr Nora Ganter (The University of Melbourne)

Venue:  E7B 146 ACE room

Title: Codes, periodicity and triality and all that

Abstract:

I will explain how the Hamming code H_8 is at the heart of some famous exceptional phenomena in dimension eight.

Week 7 - 12 April

Date: Friday, 12 April 2019 
Speaker: Professor Gareth Peters (Heriot-Watt University)

Venue:  E7B 146 ACE room

Title: Some Properties of Symmetric Non-Independent Increment Alpha-Stable Processes

Abstract:

In this presentation, I will discuss new approaches to characterize alpha-Stable stochastic processes which don't require independent increment requirements in their constructions. This will be obtained via a new class of sufficient conditions for right additivity of the covariation to be achieved with regard to restrictions on the spectral measure characterizing the process.

Week 6 - 05 April

Date: Friday, 05 April 2019 
Speaker: Professor Aidan Sims (University of Wollongong)

Venue:  E7B 146 ACE room

Title: Graphs, algebras and C*-algebras

Abstract:

I will tell the tale of a pair of long-lost siblings: graph C*-algebras, who grew up in the Analysis household, and Leavitt-path algebras, who were raised by the Algebraists. Starting from a brief and non-technical sketch of what each of these things is and why we might care about them, I’ll describe how a decade of work showed that what at first seemed like a collection of remarkable but isolated coincidences were actually the markers of a deep connection that has ended up linking much larger classes of C*-algebras and abstract algebras than anyone could originally have guessed. I won’t be assuming any background in either C*-algebras or abstract algebra – this talk should be accessible to everyone.

Week 5 - 29 March

Date: Friday, 29 March 2019 
Speaker: Dr Douglas Brumley (University of Melbourne)

Venue:  E7B 146 ACE room

Title: Bacteria push the limits of chemotactic precision to navigate dynamic chemical gradients

Abstract:

The limited precision of sensory organs places fundamental constraints on organismal performance. An open question, however, is whether organisms are routinely pushed to these limits, and how limits might influence interactions between populations of organisms and their environment. By combining a method to generate dynamic, replicable resource landscapes, high-speed tracking of freely moving bacteria, a new mathematical theory, and agent-based simulations, we show that sensory noise ultimately limits when and where bacteria can detect and climb chemical gradients. Our results suggest the typical chemical landscapes bacteria inhabit are dominated by noise that masks shallow gradients, and that the spatiotemporal dynamics of bacterial aggregations can be predicted by mapping the region where gradient signal rises above noise.

Week 4 - 22 March

Date: Friday, 22 March 2019 
Speaker: Prof Graham Farr (Monash University)

Venue:  E7B 146 ACE room

Title: Powerful sets: a generalisation of binary linear spaces

Abstract:

A set  S  of binary vectors, with positions indexed by  E, is said to be a powerful code if, for all subsets  X  of  E, the number of vectors in  S  that are zero in the positions indexed by X is a power of 2. By treating binary vectors as characteristic vectors of subsets of  E, we say that a set  S  of subsets of  E  is a powerful set if the set of characteristic vectors of sets in  S  is a powerful code. Powerful sets (codes) include binary linear codes (equivalently, cocircuit spaces of binary matroids), but much more besides.
In this talk, we investigate the combinatorial properties of powerful sets. We prove fundamental results on special elements (loops, coloops, frames, near-frames, and stars), their associated types of single-element extensions, various ways of combining powerful sets to get new ones, and constructions of nonlinear powerful sets. We show that every powerful set is determined by its collection of minimal nonzero members. Finally, we show that the number of powerful sets is doubly exponential, and hence that almost all-powerful sets are nonlinear.

(Joint work with Yezhou Wang, University of Electronic Science and Technology of China (UESTC).)

Week 3 - 15 March

Date: Friday, 15 March 2019 
Speaker: Dr Vera Roshchina (University of New South Wales)

Venue:  E7B 146 ACE room

Title: Facial Structure of Convex Sets

Abstract:

Convex geometry is a foundation for several applied research areas, and many problems in this field originate from the challenges encountered in modelling and in the design of optimisation algorithms. This includes such well-known open problems as the generalised Lax conjecture and the 9th problem of Stephen Smale. I will discuss some subtleties pertaining to the facial structure of convex sets, and introduce new ideas for capturing irregularities in the facial arrangements based on lexicographic tangents and subtransversality. I will also demonstrate how some specific features of the facial structure affect the behaviour of optimisation algorithms, using examples from conic programming and projection methods.

Week 2 - 08 March

Date: Friday, 08 March 2019 @ 3:30pm
Speaker: Prof Nadja Klein (Humboldt-University, Berlin)

Venue:  E7B 146 ACE room

Title: Bayesian Effect Selection in Structured Additive Distributional Regression Models

Abstract: 

We propose a novel spike and slab prior specification with scaled beta prime marginals for the importance parameters of regression coefficients to allow for general effect selection within the class of structured additive distributional regression. This enables us to model effects on all distributional parameters for arbitrary parametric distributions, and to consider various effect types such as non-linear or spatial effects as well as hierarchical regression structures. Our spike and slab prior relies on a parameter expansion that separates blocks of regression coefficients into overall scalar importance parameters and vectors of standardised coefficients. Hence, we can work with a scalar quantity for effect selection instead of a possibly high-dimensional effect vector, which yields improved shrinkage and sampling performance compared to the classical normal-inverse-gamma prior. We investigate the propriety of the posterior, show that the prior yields desirable shrinkage properties, propose a way of eliciting prior parameters and provide efficient Markov Chain Monte Carlo sampling. Using both simulated and three large-scale data sets, we show that our approach is applicable for data with a potentially large number of covariates, multilevel predictors accounting for hierarchically nested data and non-standard response distributions, such as bivariate normal or zero-inflated Poisson.

Week  1 - 01 March

Date: Friday, 01 March 2019
Speaker: Zsuzsanna Dancso (University of Sydney)

Venue:  E7B 146 ACE room

Title: Loops on a punctured disk, and knotted tubes in R^4

Abstract: 

Classically, knots are one-dimensional strings knotted in three-dimesional space. In this talk we'll discuss two very different flavours ganeralised knot theory: one higher dimensional, one "very low dimensional". We'll end with a mystery: there is a correspondence between certain invariants of these two different knot theories, which must be rooted in a relationship between their topologies - and we don't understand how.

For past seminars, please go to our archive.

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