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

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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