C*-algebra seminar


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"


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: 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.  

Time and place: Feb. 9, 2017 3:15 PM - 5:00 PM, NHA room 735

Elizabeth Gillaspy, p.t. Münster (Germany) will give a talk with title "Wavelets and spectral triples for higher-rank graphs"  

Time and place: Dec. 15, 2016 10:15 AM - 12:00 PM, GHS room 3514

Ulrik Bo Rufus Enstad (Oslo) will give a talk with title: Connections between Gabor frames and Noncommutative Tori

Abstract: A Gabor frame is a special type of frame in the Hilbert space of square-integrable functions on the real line. Gabor frames provide robust, basis-like representations of functions, and have applications in a wide range of areas. They have a duality theory which is deeply linked to Rieffel’s work on imprimitivity bimodules over noncommutative tori. We explore several links between Gabor frames and noncommutative tori, and show how operator algebras can be used to give alternative proofs of theorems from time-frequency analysis.  This talk is based on my Master’s thesis written at NTNU, which reviews Franz Luef’s work on the connections between Gabor frames and modules over noncommutative tori, as well as some joint work with Franz Luef.

Time and place: Dec. 7, 2016 10:15 AM - 12:00 PM, NHA, seminarrom B81

John Quigg, Arizona State University (Tempe), USA, will give a talk with title "The Pedersen rigidity problem".

University of Abstract: If \alpha is an action of a locally compact abelian group G on a C*-algebra A, Takesaki-Takai duality recovers (A,\alpha) up to Morita equivalence from the dual action of \widehat{G} on the crossed product A\rtimes_\alpha G. Given a bit more information, Landstad duality recovers (A,\alpha) up to isomorphism. In between these, by modifying a theorem of Pedersen, (A,\alpha) is recovered up to outer conjugacy from the dual action and the position of A in M(A\rtimes_\alpha G). Our search (still unsuccessful, somehow irritating) for examples showing the necessity of this latter condition has led us to formulate the "Pedersen rigidity problem". We present numerous situations where the condition is redundant, including G discrete or A stable or commutative. The most interesting of these "no-go theorems" is for locally unitary actions on continuous-trace algebras. This is joint work with Steve Kaliszewski and Tron Omland. 

Time and place: Nov. 30, 2016 10:15 AM - 12:00 PM, NHA B81

Abstract: We first discuss C*-simplicity and the unique trace property for discrete groups in light of recent years' development. In particular, we consider amalgamated free products, and give conditions for such to be (and fail to be) C*-simple. Then we define radical and residual classes of groups, and explain that there exists a radical detecting C*-simplicity, in a similar way as the amenable radical detects the unique trace property. The talk is based on joint work with Nikolay A. Ivanov from Sofia University, Bulgaria.

Time and place: Nov. 10, 2016 2:15 PM - 4:00 PM, B735

Michael Whittaker from University of Glasgow will give a talk with title:  New directions in self-similar group theory

Abstract: A self-similar group (G,X) consists of a group G acting faithfully on a homogeneous rooted tree such that the action satisfies a self-similar condition. In this talk I will generalise the above definition to faithful groupoid actions on the path space of more general graphs. This new definition allows us to work out the structure of the KMS state space of associated Toeplitz and Cuntz-Pimsner algebras. This is joint work with Marcelo Laca, Iain Raeburn, and Jacqui Ramagge.  

Time and place: Nov. 9, 2016 10:15 AM - 12:00 PM, B81

Rasmus Bryder (University of Copenhagen) will give a talk with title: Twisted crossed products over C*-simple groups

Abstract: A twisted C*-dynamical system consists of a C*-algebra, a discrete group and a "twisted" action of the group on the C*-algebra, i.e., the group acts by automorphisms on the C*-algebra in a manner determined by a 2-cocycle of the group into the unitary group of the C*-algebra. Whenever the 2-cocycle (or twist) is trivial, the action is given by a group homomorphism of the group into the automorphism group of the C*-algebra. We consider twisted C*-dynamical systems over C*-simple groups (i.e.,groups whose reduced group C*-algebra is simple) and how C*-simplicity affects the ideal structure of reduced crossed products over such dynamical systems.  

Time and place: Oct. 26, 2016 10:15 AM - 12:00 PM, B81
Time and place: Oct. 12, 2016 10:15 AM - 12:00 PM, B81

Andreas Andersson (UiO): An introduction to duality for compact groups in algebraic quantum field theory

Time and place: Sep. 14, 2016 10:15 AM - 12:00 PM, B81

Bartosz Kwasniewski (Odense) will give a talk with title: Paradoxicality and pure infiniteness of  C*-algebras associated to Fell bundles

Abstract: Abstract: In this talk we present conditions implying  pure infiniteness of the reduced cross-sectional $C^*$-algebra $C^*_r(\mathcal{B})$ of a Fell bundle $\mathcal{B}$ over a discrete group $G$. We introduce notions of aperiodicity, $\mathcal{B}$-paradoxicality and  residual $\mathcal{B}$-infiniteness.  We discuss their relationship with similar conditions studied, in the context of crossed products, by the following duos: Laca, Spielberg; Jolissaint, Robertson; Sierakowski, R{\o}rdam;  Giordano, Sierakowski and Kirchberg, Sierakowski.    (based on  joint work with Wojciech Szyma{\'n}ski)

Time and place: Sep. 7, 2016 10:15 AM - 12:00 PM, B801

Abstract: Exploring connections between subfactors and conformal field theories, Vaughan Jones recently observed that planar algebras give rise to unitary representations of the Thompson group F, and more generally,  to unitary representations of the group of fractions of certain categories. Remarkably, this procedure applies to oriented link invariants. In particular, a suitably normalized HOMFLYPT polynomial is a positive definite function on the oriented Thompson group. (Based on joint work with V. Aiello and V. Jones.)

Time and place: June 16, 2016 10:15 AM - 12:00 PM, NHA B735

In this talk I will present a paper by D. Bisch, R. Nicoara and S. Popa where continuous families of irreducible subfactors of the hyperfinite II_1 factor which are non-isomorphic, but have all the same standard invariant are constructed. In particular, they obtain 1-parameter families of irreducible, non-isomorphic subfactors of the hyperfinite II_1 factor with Jones index 6, which have all the same standard invariant with property (T).

Time and place: May 19, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract: This talk addresses some of the fundamental barriers in the theory of computations. Many computational problems can be solved as follows: a sequence of approximations is created by an algorithm, and the solution to the problem is the limit of this sequence (think about computing eigenvalues of a matrix for example). However, as we demonstrate, for several basic problems in computations such as computing spectra of operators, solutions to inverse problems, roots of polynomials using rational maps, solutions to convex optimization problems, imaging problems etc. such a procedure based on one limit is impossible. Yet, one can compute solutions to these problems, but only by using several limits. This may come as a surprise, however, this touches onto the boundaries of computational mathematics. To analyze this phenomenon we use the Solvability Complexity Index (SCI). The SCI is the smallest number of limits needed in order to compute a desired quantity. The SCI phenomenon is independent of the axiomatic setup and hence any theory aiming at establishing the foundations of computational mathematics will have to include the so called SCI Hierarchy. We will specifically discuss the vast amount of classification problems in this non-collapsing complexity/computability hierarchy that occur in inverse problems, compressed sensing problems, l1 and TV optimization problems, spectral problems, PDEs and computational mathematics in general.

Time and place: May 4, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract:  In a recent work with R. Conti (La Sapienza Univ., Rome), we have introduced a notion of positive definiteness for certain functions associated to a (unital, discrete) C*-dynamical system. We will sketch the proof of a Gelfand-Raikov type theorem for such functions and use it to construct complete positive maps on the full and the reduced C*-crossed products of the system. We will also explain how a natural definition of amenability for C*-dynamical systems emerges from our work. 

Time and place: Apr. 20, 2016 2:15 PM - 3:15 PM, NHA B735

Abstract: The talk will be on positive linear maps of the n x n matrices into itself, a topic which has become quite popular in quantum information theory.  The maps closest to physics are the completely positive ones. I´ll discuss an approximation by a completely positive map to a positive map via the trace , called the “structural physical approximation”, the SPA of the map. Much of the talk will circle around a counter example  to a conjecture on the SPA.

Time and place: Apr. 13, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract: In the classification program for C*-algebras some of the usual assumptions put on the algebras are that they are simple or have at most have finitely many ideals. We often also want algebras that have real rank 0. In this talk we will discuss how to classify certain graph algebras with uncountably many ideals and without real rank 0. There will be examples and applications. Joint work with S. Eilers, G. Restorff, and E. Ruiz

Time and place: Apr. 6, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract: There are many interesting examples of groups acting on trees, arising in various fields (e.g. combinatorial group theory, number theory, geometry). When a group acts on a tree, it necessarily also acts on the boundary of the tree, a (totally disconnected) compact Hausdorff space.  The C*-algebras obtained from the crossed product construction include many fundamental examples.  I will describe methods for analyzing such crossed products, developed in joint work with Nathan Brownlowe, Alex Mundey, David Pask and Anne Thomas.

Time and place: Mar. 16, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract: In this follow-up talk, I shall outline how the boundary quotient diagram may be useful for K-theoretic considerations. We start with the diagram within the context of integral dynamics, and then speculate about potentially promising directions of generalizations.

Time and place: Mar. 9, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract: In this follow-up talk, we shall review the results on the structure of KMS states from the case studies of - the ax+b semigroup over the natural numbers (Laca-Raeburn and Brownlowe-an Huef-Laca-Raeburn), - integer dilation matrices (Laca-Raeburn-Ramagge), - self-similar actions (Laca-Raeburn-Ramagge-Whittaker), and - Baumslag-Solitair monoids (Clark-an Huef-Raeburn) from the perspective of the boundary quotient diagram for the respective right LCM semigroups. We will also discuss (to some extent) similarities and differences of the proofs among these cases.

Time and place: Mar. 2, 2016 2:15 PM - 4:00 PM, NHA B735

Abstract: For the ax+b semigroup over the natural numbers, which is known to be part of a quasi-lattice ordered group, Laca and Raeburn considered its Nica-Toeplitz algebra and its Cuntz-Nica-Pimsner algebra, with a special appeal to nice presentations by generators and relations as well as the structure of KMS states for a natural dynamics. Shortly thereafter, Brownlowe-an Huef-Laca-Raeburn showed that there are two intermediate quotients between the Nica-Toeplitz algebra and the Cuntz-Nica-Pimsner algebra that exhibit interesting structural properties, especially with regards to KMS states. Since then, analogous quotients have been considered (partly in disguise) in a growing list of case studies on the KMS state structure, e.g. for dilation matrices, self-similar actions, and Baumslag-Solitair monoids. Somewhat surprisingly, all these case studies can be viewed from the perspective of semigroup C*-algebras of right LCM semigroups, and in this talk, I shall describe a unifying perspective on such boundary quotient diagrams. Thereby several questions concerning the general structure of right LCM semigroups are raised.