Events - Page 6

Time: Sep. 17, 2014 2:00 PM4:00 PM

Abstract: We show that the discrete duals of the so called free orthogonal quantum groups have the completely contractive approximation property, analogous to the free groups. The proof relies on the structure of representation categories of these quantum groups, on the C*-algebraic structure of SUq(2), and on the free product techniques of Ricard and Xu. This talk is based on joint work with Kenny De Commer and Amaury Freslon.

Time and place: Sep. 10, 2014 3:15 PM4:15 PM, B71, NHA

Abstract: Independence has been introduced as a regularity property for pairs of commuting injective group endomorphisms of a discrete abelian group with finite cokernel by Joachim Cuntz and Anatoly Vershik. We discuss various characterisations of this regularity property and show how the statements need to be adjusted when removing the restrictions that the group has to be abelian and that the cokernels have to be finite. Somewhat surprisingly, this leads to the concept of *-commutativity. This property is defined for pairs of commuting self-maps of an arbitrary set. As an examples of *-commutativity, we explain a construction related to the Ledrappier shift and indicate how one obtains examples for independent group endomorphisms from this construction. If time permits, we will point out instances where the two notions have been readily used to obtain C*-algebraic results. Roughly speaking, both notions are designed to give rise to pairs of doubly commuting isometries, which significantly simplifies the analysis of the constructed C*-algebras. This is particularly useful when one tries to generalise results from the case of a single transformation to an action generated by finitely many transformations.

Time and place: June 4, 2014 1:00 PM3:00 PM, NHA Hus, B71

Abstract: We talk about independent resolutions for dynamical systems on totally disconnected spaces. Building on earlier work by Cuntz, Echterhoff and Li that allows one to compute the K-theory of totally disconnected systems that admit a so called independent invariant regular basis, we show how any totally disconnected dynamical system admits a resolution of such systems, which in some cases allows for K-theory computations. Based on work by me and X. Li.

Time and place: June 3, 2014 1:15 PM3:00 PM, NHA B71

Marco Matassa (UiO) will give a talk with title: On dimension and integration for spectral triples associated to quantum groups

Abstract: Abstract: I will discuss some aspects of the notions of spectral dimension and non-commutative integral in the context of modular spectral triples. I will focus on two examples: the modular spectral triple for SU_q(2) introduced by Kaad and Senior and the family of spectral triples for quantum projective spaces introduced by D'Andrea and Dąbrowski.

Time and place: May 30, 2014 1:15 PM3:00 PM, NHA Hus, B71

Jens Kaad (Trieste), will give a talk with title "Joint torsion line bundles of commuting operators"


In this talk I’ll associate a holomorphic line bundle to any commuting tuple of bounded operators on a Hilbert space. The transition functions for this bundle are given by the joint torsion which compares determinants of Fredholm complexes. The joint torsion is an invariant of the second algebraic K-group of the Calkin algebra (bounded operators modulo trace class operators). The main step is to prove that the transition functions for the joint torsion line bundle are indeed holomorphic. This is carried out by studying the Quillen-Freed holomorphic determinant line bundle over the space of Fredholm complexes. In particular I will construct a holomorphic section of a certain pull-back of this bundle. The talk is based on joint work with Ryszard Nest.


Time and place: May 28, 2014 11:15 AM12:00 PM, NHA Hus, B1036

Magnus D. Norling will give a talk with title "Universal coefficient theorem in KK-theory". This presentation is part of the final act of the course on "The Baum-Connes conjecture and KK-theory".

Time and place: May 28, 2014 10:15 AM11:00 AM, NHA Hus, B1036

Bas Jordans will give a talk with title  "Higson's characterization of KK-theory". This presentation is  part of the final act of the course on "The Baum-Connes conjecture and Kasparov's KK-theory".

Time and place: May 6, 2014 2:15 PM3:15 PM, NHA Hus, B71

Roberto Conti (Sapienza Università di Roma) will give a talk with title:  Asymptotic morphisms in local quantum physics and study of some models


 We discuss a notion of asymptotic morphisms that is suitable for a description of superselection sectors of a scaling limit theory. In some models, this leads to interesting questions about the explicit form of certain modular operators. (This talk is based on joint work with D. Guido and G. Morsella).


Time and place: Feb. 25, 2014 2:00 PM4:00 PM, NHA B63

Magnus Landstad will give a talk with title: Quantum groups from almost matched pairs of groups - the groupoid approach

Abstract: If G is a locally compact group with two closed subgroups H,K s.t. G=HK, then (H,K) is called a matched pair of subgroups. The construction of a quantum group from such a pair goes back a long time. We shall look at the more general case where the subgroups are almost matched (the complement of HK in G has measure 0), then a groupoid approach to the construction is very useful and many formulas are obtained for free.

I shall start with explaining the concepts needed (quantum groups, groupoids, etc) and then how the groupoid is constructed. Finally we shall look at the special case where G has a compact open subgroup.

This is joint work with A. Van Daele.



Time and place: Feb. 11, 2014 2:15 PM4:00 PM, NHA hus, B63

Antoine Julien (NTNU) will give a talk with title: Tiling spaces, groupoids and K-theory


 In this talk, I will describe how spaces, groupoids and C*-algebras can be associated with aperiodic tilings. In some cases, it is possible to describe the structure of the groupoid combinatorially in terms of augmented Bratteli diagrams. (joint work with Jean Savinien) Time permitting, I will expose a strategy for computing the K-theory of the tiling algebra in terms of the K-theory of AF-algebras (work in progress).


Time and place: Jan. 28, 2014 2:15 PM4:00 PM, NHA B70

Takuya Takeishi, University of Tokyo, will give a talk with title: Bost-Connes system for local fields of characteristic zero

Abstract: The Bost-Connes system, which describes the relation between quantum statistical mechanics and class field theory, was first constructed by Bost and Connes for the rational field, and generalized for arbitrary number fields by the contribution of many researchers. In this talk, we will introduce a generalization of the Bost-Connes sysmtem for local fields of characteristic zero, and introduce some properties.


Time and place: Nov. 22, 2013 11:00 AM12:00 PM, NH Abels hus B71

Judith Packer, University of Colorado (Boulder), will give a talk with title "Noncommutative solenoids and their projective modules"

Abstract: ``Noncommutative solenoids" are certain twisted group $C^*$-algebras, where the groups in question are countably infinitely generated; these algebras can also be generated as direct limits of rotation algebras.  From examining the range of the trace of the $K_0$-groups of the noncommutative solenoids, their finitely generated projective modules can be constructed. We also discuss a way to construct Morita equivalence bimodules between noncommutative solenoids that goes back to work of M. Rieffel, with the new wrinkle of $p$-adic analysis appearing. This work is joint with F. Latr\'emoli\'ere.



Time and place: Oct. 9, 2013 12:15 PM2:00 PM, NHA, seminarrom B71

Bas Jordans (UiO) will give a talk with title: Real dimensional spaces in noncommutative geometry


In noncommutative geometry geometric spaces are given by spectral triples. In this talk we consider a generalisation of these spectral triples to semifinite spectral triples. In analogy to the classical case it is possible to construct the product of two semifinite spectral triples. We will construct this product and derive properties thereof. Also we will describe for each z\in(0,\infty) a semifinite spectral triple which can be considered as having dimension z. As an application these "z-dimensional" semifinite triples will be used for two regularisation methods in physics.


Time and place: Aug. 14, 2013 2:15 PM4:00 PM, NHA, seminarrom B71

Yusuke Isono  from the University of Tokyo will give a talk with title: Strong solidity of II_1 factors of free quantum groups


We generalize Ozawa's bi-exactness to discrete quantum groups and give a new sufficient condition for strong solidity, which implies the absence of Cartan subalgebras. As a corollary, we prove that II_1 factors of free quantum groups are strongly solid. We also consider similar conditions on non-Kac type quantum groups, namely, non finite von Neumann algebras.


Time and place: June 5, 2013 2:15 PM4:00 PM, NHA, seminarrom B71

Erik Bédos will give a talk with title: On equivariant representations of C*-dynamical systems


Let \Sigma=(A, G, \alpha, \sigma) denote a unital discrete twisted C*-dynamical system. In our recent work with Roberto Conti (Rome), it has emerged that the so-called equivariant representations of \Sigma on Hilbert A-modules play an interesting role, complementing the one played by covariant representations. We will discuss some aspects of this notion and illustrate its usefulness in the study of the crossed products associated with \Sigma.


Time and place: May 2, 2013 1:15 PM3:00 PM, NHA, seminarrom B71
Time and place: Apr. 10, 2013 2:15 PM4:00 PM, NHA, seminarrom B71

Adam Skalski (IMPAN) will give a talk with title: Closed quantum subgroups of locally compact quantum groups and some questions of noncommutative harmonic analysis (based on joint work with Matt Daws, Pawel Kasprzak and Piotr Soltan)

Abstract: The notion of a closed subgroup of a locally compact group is a very straightforward concept, often featuring in classical harmonic analysis. I will discuss the possible extensions of this notion to the quantum setting, focusing on the comparison of the two definitions proposed by S. Vaes and S.L.  Woronowicz. I will describe some reformulations of these definitions and explain how they can beshown to be equivalent in many cases; I will also mention certain connections to other problems of quantum harmonic analysis.  

Time and place: Apr. 9, 2013 2:15 PM3:15 PM, NHA, seminarrom B81

Nicolai Stammeier, Westfälische Wilhelms-Universität Münster, will give a talk with title: Product Systems of Finite Type for Certain Algebraic Dynamics and their C*-algebras

Abstract: Let P be a lattice-ordered semigroup with unit acting on a discrete, abelian group G by injective endomorphisms with finite cokernel. Building on the work of Jeong Hee Hong, Nadia S. Larsen and Wojciech Szymanski on product systems of Hilbert bimodules and their KMS-states, one can associate a product system to this dynamical system that turns out to be of finite type. Imposing two additional conditions on the dynamics, namely independence of the endomorphisms for relatively prime elements in P and exactness, we derive presentations of the Nica-Toeplitz algebra and the Cuntz-Nica-Pimsner algebra. Moreover, the latter has lots of nice descriptions and is shown to be a unital UCT Kirchberg algebra.  

Time and place: Feb. 13, 2013 2:13 PM4:00 PM, NHAbels hus, B71

Roberto Conti, Universita La Sapienza (Rome), Italy, will give a talk with title: The dark side of the Cuntz algebras

Abstract: We provide an overview of our recent work on the automorphism group of Cuntz algebras and discuss a number of open problems. (Joint work with J.H. Hong and W. Szymanski).




Time and place: Feb. 7, 2013 2:15 PM4:00 PM, NHA, seminarrom B71

Gunther Cornelissen, Utrecht University (The Netherlands) will give a talk with title "Dynamical systems and point counting"

Abstract: Counting the points of a curve over finite fields doesn't determine the curve up to isomorphism; this is analogous to the famous fact that you cannot "hear the shape of a drum". I will show how to remedy this by counting points in a weighted way (namely, using abelian L-series). The method of proof is through the theory of dynamical systems, and - curiously - arose from a study of a quantum statistical mechanical system.  

Time and place: Feb. 5, 2013 2:15 PM4:00 PM, NHA, seminarrom B62

Makoto Yamashita, p.t. University of Copenhagen, will give a talk with title "Tannaka-Krein duality of quantum homogeneous spaces"

Abstract: We study ergodic actions of compact quantum groups on operator algebras  (quantum homogeneous spaces) from the categorical point of view. The language of Rep(G)-module C*-categories gives a reconstruction theorem of quantum homogeneous spaces, which also captures the equivariant Morita equivalence and the equivariant homomorphisms.  For the case of quantum SU(2), the universality of Rep(SUq(2)) allows us to obtain a combinatorial description of quantum homogeneous spaces and their properties. Based on joint work with Kenny De Commer (arXiv:1211.6552, arXiv:1212.3413).  

Time and place: Jan. 30, 2013 2:13 PM4:00 PM, NHA, seminarrom B71

Magnus Dahler Norling, UiO, will give a talk with title: The K-theory of some reduced inverse semigroup C*-algebras

Abstract: We use a recent result by Cuntz, Echterhoff and Li about the K-theory of certain reduced C*-crossed products to describe the K-theory of C*_r(S) when S is an inverse semigroup satisfying certain requirements. A result of Milan and Steinberg allows us to show that C*_r(S) is Morita equivalent to a crossed product of the type handled by Cuntz, Echterhoff and Li. We apply the results to graph inverse semigroups and the inverse semigroups of one-dimensional tilings.

Time and place: Jan. 15, 2013 2:15 PM3:30 PM, NHA, seminarrom B62

Fred Shultz, Wellesley College, USA will give a talk with title "Decomposing separable states".

This is the first in a joint seminar series organised by the Operator Algebra group (UiO), Several Complex Variable group (UiO) and the CAS group.  The plan is to have seminars every other week.  

Abstract: This talk will begin with a brief introduction to entanglement and its applications, since that motivates the mathematics to be discussed.  In the title of this talk, a state is a positive linear functional on the tensor product of the algebras of m x m and n x n complex matrices.  Such a state is separable if it is a convex combination of product states. An interesting open problem is to give a useful criterion for a state to be separable. A related problem is to give a systematic way to find a decomposition of a separable state into a convex combination of product states.  This talk will describe such a decomposition for a class of separable states that is of both physical and mathematical interest.  This decomposition is also applicable to a class of completely positive maps (which correspond to certain quantum channels). This is joint work with Erik Alfsen.  

Time and place: Jan. 14, 2013 1:15 PM2:00 PM, NHAbels hus, B71

Magnus Landstad (NTNU) will give a talk with title: Exotic group C*-algebras and noncommutative duality.

Abstract: It  has long been known that for a (non-amenable) locally compact group G there are many C*-algebras between the full and reduced group C*-algebra. First I will discuss to what extent these intermediate algebras can be called group C*-algebras. Then I will look at algebras between the full and reduced crossed product, and the various types of coactions (full, maximal, normal) a group can have. To make arguments a little simpler, we shall assume G to be discrete.

Time and place: Dec. 6, 2012 12:15 PM2:00 PM, NHA, seminarrom B71

Yoshiko Ogata, University of Tokyo, will give a talk with title: Approximating macroscopic observables in quantum spin systems with commuting matrices

Abstract: Macroscopic observables in a quantum spin system are spatial means of local observables in a UHF algebra. One of their properties is that they commute asymptotically as the system size goes to infinity. It is not true that any given set of asymptotically commuting matrices can be approximated by commuting ones in the norm topology. The main statement of this talk is that this is true for macroscopic observables.