## Visiting address

Niels Henrik Abels husMoltke Moes vei 35 (map)

0851 OSLO

Norway

Time and place:
Nov. 26, 2018 2:00 PM–3:00 PM,
NHA 723

Time and place:
Oct. 31, 2018 11:00 AM–12:00 PM,
NHA Room 108

Time and place:
Oct. 25, 2018 11:00 AM–5:00 PM,
Blindern

Noncommutative geometry aims to use geometric insight in studying phenomena that are beyond the reach of classical geometry and analysis, using functional analysis and algebra as primary rigorous tools. Over the years it has been applied to a variety of problems, from index theory for foliations, to the Novikov conjecture, quantum Hall effect, standard model, analysis on quantum groups, and many more. The goal of the one-day seminar is to use the opportunity provided by the visit of Adam Rennie from Australia to report on recent advances in the area. Another goal is to strengthen ties between the operator algebra groups in Oslo and Trondheim.

The seminar is part of the project "Pure Mathematics in Norway, 2018-2022", supported by the Bergen Research Foundation and the Tromsø Research Foundation.

Time and place:
Sep. 19, 2018 11:15 AM–12:00 PM,
NHA 108

Prof. Masaki Izumi, Kyoto University, Japan, will give a talk with title:

**The classification of poly-Z group actions on Kirchberg algebras**

**Abstract**: We completely classify outer actions of a poly-Z group G on any Kirchberg algebra A in terms of a principal Aut(A x K)-bundle over the classifying space BG.
This is joint work with Hiroki Matui.

Time and place:
Sep. 19, 2018 10:15 AM–11:00 AM,
NHA 108

Becky Armstrong, University of Sydney, Australia, will give a talk with title:

**Simple graph algebras**

**Abstract**:
Since their introduction twenty years ago, C*-algebras associated to directed graphs have become a popular tool for investigating various classes of C*-algebras, because analytical properties of these C*-algebras depend on much simpler combinatorial properties of the underlying graphs. One such analytical property is simplicity, which plays a fundamental role in the classification program for C*-algebras. In this talk I will first recall the characterisation of simplicity for directed graph C*-algebras. I will then describe the results of my PhD research, in which I characterise simplicity of twisted C*-algebras of topological higher-rank graphs in terms of the underlying graphical and cohomological data. These C*-algebras are constructed using groupoid techniques for the purpose of this simplicity characterisation, but I will also briefly describe two product-system models for twisted C*-algebras of topological higher-rank graphs. (This is joint work with my PhD supervisors, Nathan Brownlowe and Aidan Sims.)

Time and place:
May 16, 2018 12:15 PM–2:00 PM,
Vilhelm Bjerknes' House, Auditorium 2

This is the third and last lecture by **Anders C. Hansen** (Cambridge Univ. and UiO) on this topic. **Vegard Antun** (UiO) will also contribute to the lecture.

**Added June 06**: Slides from the lectures are now available here.

Time and place:
May 15, 2018 10:15 AM–12:00 PM,
Vilhelm Bjerknes' House, Auditorium 4

This is the second in a series of three lectures by **Anders C. Hansen** (Cambridge Univ. and UiO) on this topic. **Vegard Antun** (UiO) will also contribute to the lectures.

**Added June 06**: Slides from the lectures are now available here.

Time and place:
May 14, 2018 12:15 PM–2:00 PM,
Vilhelm Bjerknes' House, Auditorium 2

This is the first in a series of three lectures by **Anders C. Hansen** (Cambridge Univ. and UiO) on this topic. **Vegard Antun** (UiO) will also contribute to the lectures.

**Added June 06**: Slides from the lectures are now available here.

Time and place:
May 3, 2018 1:00 PM–2:00 PM,
Hurricane, Ullevål

Kurusch Ebrahimi-Fard (NTNU) will give a talk with title: Moment-cumulant relations in noncommutative probability and shuffle-exponentials

Abstract: In this talk we consider monotone, free, and boolean moment-cumulant relations from the shuffle algebra viewpoint. Cumulants are described as infinitesimal characters over a particular combinatorial Hopf algebra, which is neither commutative nor cocommutative. As a result the moment-cumulant relations can be encoded in terms of shuffle and half-shuffle exponentials. These shuffle exponentials and the corresponding logarithms permit to express monotone, free, and boolean cumulants in terms of each other using the pre-Lie Magnus expansion. If time permits we will revisit additive convolution in monotone, free and boolean probability and related aspects. Based on joint work with F. Patras (CNRS).

Time and place:
Feb. 6, 2018 11:00 AM–12:00 PM,
Gates of Eden

Zahra Afsar (University of Wollongong, Australia) will give a talk with title: Nica-Toeplitz-algebras of *-commuting local homeomorphisms and equilibrium states

Abstract: Given a family of *-commuting local homeomorphisms on a compact space, we can build a compactly aligned product system of Hilbert bimodules. The product system has a Nica-Toeplitz algebra which carries a gauge action of a higher-dimensional torus, and there are many possible dynamics obtained by composing with different embeddings of the real line in this torus. In this work, which is a joint work with Prof. Astrid an Huef and Prof. Iain Raeburn, I will talk about the equilibrium states of these dynamics. If time allows, I will also provide some examples from higher rank graph theory and reconcile our results with those existing ones.

Time and place:
Jan. 9, 2018 11:00 AM–12:00 PM,
Gates of Eden

Elizabeth Gillaspy from the University of Montana at Missoula, USA, will give a talk with title " Finite decomposition rank and strong quasidiagonality for virtually nilpotent groups "

Abstract: In joint work with Caleb Eckhardt and Paul McKenney, we show that the C*-algebras of discrete, finitely generated, virtually nilpotent groups G are strongly quasidiagonal and have finite decomposition rank. Thus, the only remaining step required to show that primitive quotients of such virtually nilpotent groups G are classified by their Elliott invariant is to check that these C*-algebras satisfy the UCT. Our proof of finite decomposition rank relies on a careful analysis of the relationship between primitive ideals of C*(G) and those of C*(N), where N is a finite-index normal subgroup of G. In the case when N is also nilpotent, we obtain a decomposition of C*(G) as a continuous field of twisted crossed products, which enables us to prove finite decomposition rank of C*(G) by analyzing the decomposition rank of the fibers.

Time and place:
Dec. 19, 2017 2:00 PM–3:00 PM,
Desolation row, Ullevål

Antoine Julien, Universitetet i Nord, will give a talk with title: Rieffel-type projections in higher-dimensional rotation algebras

Abstract: Rieffel first built a non-trivial projection in the rotation algebra by considering a certain C*-module over this algebra, and exploiting the Morita equivalence which it implements. In this talk, I will present how it is possible to extend these ideas to construct explicitly projections in higher-dimensional noncommutative tori. Precisely, our techniques can be applied to the NC tori which are associated with an R^d-flow on a 2d-torus, or equivalently which are given by the crossed product of C(T^d) by Z^d. I will also hint on how this result can be interpreted as constructing Gabor atoms associated with some lattices in the time-frequency space R^{2d}. This is a joint work with Franz Luef (NTNU).

Time and place:
Dec. 13, 2017 11:00 AM–12:00 PM,
Desolation Row, Sognsv. 77B

Abstract: Recently, Steve Kaliszewski, Tron Omland, and I have been investigating the following theorem of Pedersen: two actions of a compact abelian group on C*-algebras A and B are outer conjugate if and only if there is an equivariant isomorphism between the crossed products that respects the positions of A and B. We upgraded this to nonabelian groups (using coactions on the crossed products), and then searched for examples showing that the last condition (on the positions of A and B) is necessary. We failed. This lead us to formulate the "Pedersen Rigidity Problem": if the crossed products of A and B are equivariantly isomorphic, are the actions on A and B outer conjugate? We have been finding numerous "no-go theorems", which give various sufficient conditions for Pedersen Rigidity. Quite recently we have done this for ergodic actions of a compact group, assuming that the actions have "full spectrum". In fact, these actions are (not just outer) conjugate if and only if the dual coactions are. I will summarize our progress on the Pedersen Rigidity Problem and outline the proof of the no-go theorem for these compact ergodic full-spectrum actions.

Time and place:
Dec. 4, 2017 9:00 AM–Dec. 8, 2017 12:00 PM,
Room 108, N.H. Abel's House

Time and place:
Nov. 6, 2017 12:45 PM–1:45 PM,
End of Line, Ullevål

Pawel Kasprzak (Warzaw) will give a talk with title " Quantum actions on discrete quantum spaces"

Abstract:

To any action of a compact quantum group on a von Neumann algebra which is a direct sum of factors we associate an equivalence relation corresponding to the partition of a space into orbits of the action. We show that in case all factors are finite-dimensional (i.e. when the action is on a discrete quantum space) the relation has finite orbits. We then apply this

i) to generalize the classical theory of Clifford, concerning the restrictions of representations to normal subgroups, to the framework of quantum subgroups of discrete quantum groups, itself extending the context of closed normal quantum subgroups of compact quantum groups; ii) to the context of idempotent states showing that the algebra of invariant elements is finite dimensional if and only if the corresponding state is normal. Joint work with K. De Commer, A. Skalski and P. Sołtan.

Time and place:
Oct. 23, 2017 12:45 PM–1:45 PM,
Gates of Eden

Erik Bedos (UiO) will give a talk with title "On Exel-Pardo algebras as Cuntz-Pimsner algebras"

Abstract:

In a joint work with S. Kaliszweski and J. Quigg (https://arxiv.org/pdf/1512.07302.pdf, to appear in JOT), we consider a continuous action of a locally compact group G on a topological graph E, equipped with a G-valued continuous cocycle, and show how to construct a C*-correspondence from these data, giving rise to a Toeplitz algebra and a Cuntz-Pimsner algebra. In the talk we will sketch this construction, but restrict ourselves to the discrete case. We will also describe these algebras in terms of generators and relations when E is row-finite. As a corollary we get that the associated Cuntz-Pimsner algebra coincides with the C*-algebra recently introduced by Exel and Pardo when E is finite and sourceless.

Time and place:
Oct. 16, 2017 12:45 PM–1:45 PM,
Gates of Eden, Ullevål

Nicolai Stammeier (Oslo) will give a talk with title: The inner structure of boundary quotients of right LCM semigroups

Abstract: In joint work with Roberto Conti, Stefano Rossi, and Valeriano Aiello, we use semidirect products built from algebraic dynamical systems to model right LCM semigroups to study various structural aspects in connection with a selection of distinguished subalgebras of the associated boundary quotients. Our two guiding examples are integral dynamics as considered in work of Barlak - Omland - Stammeier, and exact injective group endomorphisms of discrete abelian groups with finite cokernel as studied by Cuntz and Vershik.

Time and place:
Sep. 19, 2017 12:45 PM–1:45 PM,
Gates of Eden

Adam Sørensen (UiO) will give a talk with title: C*-stable groups.

Abstract: In this talk we will look at when group C*-algebra have stable relations, which loosely speaking means that any almost representation of the group in a C*-algebra will be close to an exact representation. A particularly interesting case is if we assume the C*-algebra is finite dimensional. I will mostly discuss a collection of examples. The talk is based on ongoing joint work with Søren Eilers and Tatiana Shulman.

Time and place:
Aug. 11, 2017 11:00 AM–12:00 PM,
room 108, Niels Henrik Abels hus

Please access the abstract for detailed information on the contents of this talk.

Time and place:
June 27, 2017 11:00 AM–12:00 PM,
"Desolation Row", 3rd floor, Sognsv. 77

Prof.** Lia Vas** (University of the Sciences, Philadelphia, USA) will give a talk with title:

* Algebraization of Operator Theory*

Time and place:
May 12, 2017 12:15 PM–2:00 PM,
NH Abel's hus, room U26

This is the second of two lectures by Anders Hansen (Cambridge Univ. and UiO) on Foundational Computational Problems in l^1 and Total Variation Regularisation.

Time and place:
May 11, 2017 10:15 AM–12:00 PM,
VB, Auditorium 4

This is the first of two lectures by Anders Hansen (Cambridge Univ. and UiO) on Foundational Computational Problems in l^1 and Total Variation Regularisation.

Time and place:
May 9, 2017 2:15 PM–4:00 PM,
VB, Auditorium 4

This is the second of two lectures by Anders Hansen (Cambridge Univ. and UiO) on Compressed sensing - Structure and Imaging.

Time and place:
May 8, 2017 12:15 PM–2:00 PM,
VB, Auditorium 2

This is the first of two lectures by Anders Hansen (Cambridge Univ. and UiO) on Compressed sensing - Structure and Imaging.

Time and place:
Mar. 15, 2017 10:15 AM–12:00 PM,
NHA 738

Adam Sørensen (Oslo) will give a talk with title: Overlapping qubits

Abstract: I will discuss the paper "Overlapping Qubits" by Chao, Reichardt, Sutherland, and Vidick (arXiv:1701.01062 - category: Quantum Physics!). Qubits are the bits of quantum computing. In the paper the authors take the point of view that a qubit mathematically is described by a pair of anticommuting reflections on a finite dimensional Hilbert space. Two qubits are independent if their defining operators commute. The central point of the paper is that when performing observations we should not expect two qubits to be exactly independent, rather we should expect them to be almost independent, i.e. the norms of the commutators should be small. This naturally leads to questions about almost commuting matrices, which is why I care. I will attempt to explain how questions of almost commuting matrices come up, and how the physicists answer them.