Wollongong OANCG Seminar
A weekly research seminar hosted at the University of Wollongong on the topics of operator algebras, noncommutative geometry, and related fields. The seminar has been running continuously since 2011.
Key Info
- Regular time: Thursday 3:30 pm Wollongong time (this may vary occasionally and will be noted on the talks below)
- Regular location: 39C.174 - Campus map
- Zoom: Link - Meeting ID: 861 2230 0349 (password provided on request)
- Organisers: Angus Alexander & Alexander Mundey
If you are interested in either giving a talk or being added to the mailing list, please contact the organisers at wollongong.oancg[at]gmail.com.
Talks typically run for around 55 minutes. We welcome speakers ranging from seasoned researchers to graduate students. We hold talks on campus and host remote speakers via Zoom.
Talk Schedule - 2024
Click on "Abstract" to reveal the abstract.
Upcoming talks
- July 25: Francesca Arici (Leiden University) - Zoom Talk
Spheres, Euler classes and the \(K\)-theory of \(C^*\)-algebras of subproduct systemsAbstract
In this talk, we shall consider equivariant subproduct system of Hilbert spaces and their Toeplitz and Cuntz-Pimsner algebras. We will provide results about their topological invariants through \(K(K)\)-theory. More specifically, we will show that the Toeplitz algebra of the subproduct system of an \(SU(2)\)-representation is equivariantly \(KK\)-equivalent to the algebra of complex numbers so that the \((K)K\)- theory groups of the Cuntz-Pimsner algebra can be effectively computed using a Gysin exact sequence involving an analogue of the Euler class of a sphere bundle. Finally, we will discuss why and how \(C^*\)-algebras in this class satisfy \(KK\)-theoretic Poincaré duality. - August 1: Aidan Sims (University of Wollongong)
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TBA - August 8: Torstein Ulsnaes (Leiden University/SISSA) - Zoom Talk
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TBA - August 15: Nigel Higson (Pennsylvania State University)
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Recent talks
- July 18: Yufan Ge (Leiden University) - Zoom Talk
\(SU(2)\)-symmetries of \(C^*\)-algebras: from bricks to buildings.Abstract
In this talk, we will consider subproduct systems coming from \(SU(2)\)-representations and discuss the associated \(C^*\)-algebras. We will first review results concerning irreducible representations from Arici-Kaad, then provide some further results about more general cases. More specifically, we will discuss the structure of the \(SU(2)\)-subproduct systems associated to isotypic representations and multiplicity-free representations. Finally, we will provide results about the \(K\)-theory groups of their Toeplitz algebras. This is joint work in progress with Francesca Arici. - Postponed: David Pask (University of Wollongong) - Zoom Talk
Certain coloured graphs giving higher-rank graphs, and their fundamental groups. Exploring planar higher-rank trees and some other things - Part 3Abstract
Building on our work from the first two talks we show that the \(k\)-graphs \(2 \le k \le 4\) we have built are planar \(k\)-trees. We then talk briefly about gluing coloured graphs. We then show that there is a planar 2-tree which admits a (fixed point) free automorphism of order three, something which does not occur in dimension one. - July 4: Adam Rennie (University of Wollongong)
More about PodleśAbstract
See title. - June 27: David Pask (University of Wollongong) - Zoom Talk
Certain coloured graphs giving higher-rank graphs, and their fundamental groups. Exploring planar higher-rank trees and some other things - Part 2Abstract
Following on from the introduction in the first talk, we introduce a large family of finite planar higher-rank trees inspired by the work of Johnstone and Lambek. They are constructed from ordinary planar graphs, and we show how to demonstrate their properties. - June 20: Ada Masters (University of Wollongong)
Spectral triples on group \(C^*\)-algebras from geometric group theoryAbstract
Since Connes's 1989 paper, the use of length functions to build spectral triples for group \(C^*\)-algebras has become commonplace in noncommutative geometry. This construction, although sound at the level of quantum metric spaces, always gives rise to trivial \(K\)-homology. Using ingredients from geometric group theory, this can be remedied for many CAT(0) groups, including non-discrete groups. In the process, a novel group invariant from (quantum-group-equivariant) \(KK\)-theory is uncovered. Furthermore, the understanding of group extensions in this framework is a microcosm of the more general problem of the constructive unbounded Kasparov product. - June 13: David Pask (University of Wollongong) - Zoom Talk
Certain coloured graphs giving higher-rank graphs, and their fundamental groups. Exploring planar higher-rank trees and some other things - Part 1Abstract
In this first of three talks I shall run through some background on certain coloured directed graphs which give rise to higher rank-graphs. There will also be some examples and motivation for my investigation. The examples given have the property that there is no path of length \(3\), which makes their fundamental group quite easy to compute and, for the examples provided is always trivial. To explain where these examples come from we must first indulge in a little more background on planarity, duality and colourability of graphs before moving on next week. - June 6: Adam Rennie (University of Wollongong)
The Levi-Civita connection on noncommutative differential formsAbstract
By combining Hilbert module and algebraic techniques, we give necessary and sufficient conditions for the existence of Hermitian and torsion-free connections on noncommutative one-forms, such as those arising from spectral triples. With additional structure we give a sufficient condition for uniqueness. Our methods are constructive, use standard definitions, and allow computation of curvature with comparable difficulty to the differential geometry case. Joint work with Bram Mesland. arXiv:2403.13735 and 2404.07957 - May 30: Adam Rennie (University of Wollongong)
Differential calculi for algebrasAbstract
As a warm-up to next week's talk I will run over the basic elements of calculus for algebras. - May 23: Dimitris Gerontogiannis (Leiden University) - Zoom Talk
The log-Laplacian on Ahlfors regular spaces and noncommutative boundariesAbstract
The Laplace-Beltrami operator is a fundamental tool in the study of compact Riemannian manifolds. In this talk, I will introduce the logarithmic analogue of this operator on Ahlfors regular spaces. These are metric-measure spaces that might lack any differential or algebraic structure. Examples are compact Riemannian manifolds, several fractals, self-similar Smale spaces and limit sets of hyperbolic isometry groups. Further, this new operator is intrinsically defined, its spectral properties are analogous to those of elliptic pseudo-differential operators on manifolds and exhibits compatibility with non-isometric actions in the sense of noncommutative geometry. This is joint work with Bram Mesland (Leiden). If time allows, I will also discuss a recent joint work with Magnus Goffeng (Lund) and Bram Mesland on the spectral geometry of Cuntz-Krieger algebras with respect to the log-Laplacian - May 16: Aidan Sims (University of Wollongong)
Hilbert modules and Morita EquivalenceAbstract
One of the most ubiquitous tools in the theory of C*-algebras is the theory of Hilbert modules, induced representations, and Morita equivalence. Since a lot of the usual crowd are away at the moment, and since some of our honours students have just finished learning about GNS representations, I will try to give an overview of what Hilbert modules are, how they are used to induce representations, what a Morita equivalence is, and what it has to do with full corners and with stable isomorphism. - May 9: Alexander Mundey (University of Wollongong)
Zappa-Szép products of categories and their homologyAbstract
The Zappa-Szép product, a generalisation of the semidirect product, traditionally arises in group theory. Rather than starting with one group acting on another by automorphisms, it features two groups acting on each other in a mutually compatible way. In this talk, I will discuss the extension of Zappa-Szép products to categories and demonstrate how this framework encompasses a variety of dynamical systems of interest to C*-algebraists, such as self-similar actions and k-graphs. I will also outline how the homology of a Zappa-Szép product of categories can be derived from the homological data of its constituent categories. This talk is based on joint work with Aidan Sims. - May 2: Eva-Maria Hekkelman (University of New South Wales)
The Joy of MOIsAbstract
Operator integrals appear in various contexts of noncommutative geometry. Proofs of the local index formula, expansions of the spectral action and asymptotics of the heat trace are just a few examples. Thankfully, back in the day, the Soviet school of mathematics cooked up a theory of Multiple Operator Integrals (MOIs) which provides a big toolbox for handling complicated integrals of bounded operators. This is still an active field of research. Sadly, it currently isn't applicable to NCG since our integrals have unbounded operators all over the place. In joint work with Ed McDonald and Teun van Nuland we adapted the theory of MOIs to the framework of abstract pseudodifferential operators Connes–Moscovici/Higson style, which makes MOI theory usable in NCG contexts. We hope this greatly reduces the time noncommutative geometers have to spend fiddling with nasty operator integral analysis. - April 18: Becky Armstrong (Victoria University of Wellington)
Representing topological full groups in Steinberg algebras and C*-algebrasAbstract
Topological full groups are a useful groupoid invariant that have been used to solve important open problems in group theory. Steinberg algebras are a purely algebraic analogue of groupoid C*-algebras that generalise both Leavitt path algebras and Kumjian–Pask algebras. The Steinberg algebra of an ample Hausdorff groupoid is a quotient of the algebra generated by the inverse semigroup of compact open bisections of the groupoid. Since the topological full group of an ample Hausdorff groupoid sits inside this inverse semigroup, it is natural to ask what the relationship is between the algebra of the topological full group and the Steinberg algebra of the groupoid. In this talk I will present recent results answering this question. (This is joint work with Lisa Orloff Clark, Mahya Ghandehari, Eun Ji Kang, and Dilian Yang.) - April 11: Isaac Bankier (University of Wollongong)
The Groupoid Interpretation of EqualityAbstract
Primary doctrines are a construction presenting a system of first order logic over a base category. Primary doctrines need not have internal notions of equality, however this can be added cofreely, by taking "quotients" by all possible internal equivalence relations. In the proof relevant generalisation of primary doctrines, called primary fibrations, we will see that internal groupoids play the role of equivalence relations. - April 4: Nate Brown (Pennsylvania State University) - Colloquium Talk
Inclusive teaching: a mathematician's viewAbstract
After 20 years of research in theoretical maths, I switched to educational research. Specifically, inequity in STEM education at the university level. In this talk I'll present a representative sample of the body of research demonstrating inequities and their impacts. Correcting course requires a critical examination of higher education from all angles, but in the second half of my talk I'll focus on inclusive teaching as a lever for change. I'll discuss my own journey to becoming a more inclusive teacher and some cornerstones of my approach. - March 28: Arnaud Brothier (University of New South Wales)
Forest-skein categories and groupsAbstract
Vaughan Jones found unexpected connections between subfactor theory and Richard Thompson's group while attempting to construct conformal field theories (CFT). This led to numerous fruitful applications and among others provided a novel way to construct group actions using categories. I am initiating a program strengthening Jones' visionary work where the Thompson group is replaced by a family of groups that I name "forest-skein groups". These groups are interesting on their own, satisfying exceptional properties and having powerful extra-structures for studying them. They are isotropy subgroup of forest-skein categories: categories of planar diagrams mod out by certain skein relations (just like planar algebras are in Jones' subfactor framework). I will briefly say a word about the story of Jones's discovery and then present forest-skein categories and groups using explicit examples. - March 21: Elizabeth Mabbutt (University of Wollongong)
What is a Sobolev Space and Why Do We Care?Abstract
In simple terms, a Sobolev space is a normed vector space where two functions are “close together” with respect to the Sobolev norm if and only if their derivatives up to order \(m\) are “close together” with respect to the \(L^p\) norm. To understand what this means, a lot of background in functional analysis is required. In this talk, I will explain this background, eventually coming up with a rigorous definition of what a Sobolev space is, and will then show why we care about these spaces. - March 14: Nicholas Seaton (University of Wollongong)
Turning nasty groups into nice onesAbstract
In 2011, an Heuf, Kumjian and Sims provided a classification for Fell algebras. They showed that two such algebras are Morita equivalent over their spectrum \(X\) if and only if they have the same class in the equivariant sheaf cohomology group \(H^2(X)\). This group is nasty, it is very difficult to work with as it is defined using abstract nonsense. In this talk, I will show that this group is isomorphic to a nice group, that is, a direct limit of twists groups. - March 7: Alex Paviour (University of Wollongong)
A glimpse at Axiomatic QFTAbstract
I will discuss the motivation for studying quantum field theory by describing the physical problems it tries to address. I will then describe an influential attempt at a mathematically rigorous basis for QFT, the Wightman axioms, and their physical interpretation. I'll give some physically uninteresting but mathematically challenging examples of quantum fields satisfying these axioms, discuss the benefits and shortcomings of this approach to QFT, and point to some important theorems. - February 29: Angus Alexander (University of Wollongong)
Stationary scattering theoryAbstract
In this talk I will discuss some basic quantities in scattering theory and their physical motivations, as well as how to use the stationary approach to obtain scattering information. - February 22: Aidan Sims (University of Wollongong)
When do \(k\)-graphs embed in their fundamental groupoids, and why does it matter?Abstract
Every graph \(C^*\)-algebra is Morita equivalent to a crossed product by its fundamental group of a \(C^*\)-algebra that is itself Morita equivalent to a commutative AF algebra. It turns out that this is because graphs always embed in their fundamental groupoids. Higher-rank graphs (aka \(k\)-graphs) are a different story. I’ll explain why, starting with what a \(k\)-graph and its fundamental groupoid are. This is joint work with Nathan Brownlowe, Alex Kumjian and David Pask. - Talks from previous years