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NCG&T Prague

usually takes place each Tuesday at 16:00 Institute of Mathematics CAS, Blue lecture room,  Zitna  25, Praha 1
Chair: Tristan Bice, Karen Stung

What is noncommutative geometry and topology? The idea stems from the Gelfand theorem which states that the category of compact Hausdorff spaces and commutative C*-algebras are dual. If we drop the condition of commutativity from our C*-algebras, we arrive at the notion of a noncommutative topological space. This can be carried further into the realm of noncommutative of geometry by equipping *-algebras with geometric structures.
Our research focusses on both quantum algebraic and operator algebraic aspects of noncommutative geometry and topology. This includes research in Hopf algebras, quantum groups, and noncommutative complex geometry, while on the operator algebra side, we study C*-algebras, with particular focus on C*-algebras arising from dynamical constructions such as minimal actions, groupoids, and semigroups.
Partially supported by GAČR project 20-17488Y Applications of C*-algebra classification: dynamics, geometry, and their quantum analogues and PRIMUS grant Spectral Noncommutative Geometry of Quantum Flag Manifolds.

Cross Product Exterior Algebras

Ryan Aziz
Université libre de Bruxelles
Tuesday, 6. December 2022 - 16:00 to 17:00
This talk will take place in the blue seminar room, back building, Žitná 25.

It will also be broadcast on Zoom:
https://cesnet.zoom.us/j/91975183920?pwd=SGdlRUhLS21HUnVFTXBKcE1vYXlrQT09
Meeting ID: 919 7518 3920
Passcode: 102707
The first step in noncommutative Riemannian geometry by constructive approach, as develop by Beggs and Majid and many other collaborators, is to determine an exterior algebra of a given algebra in which we can use to build the elements of noncommutative geometry. In this talk, we will see that a strongly bicovariant exterior algebra of cross product Hopf algebras is the cross product of exterior algebras. As an example, we find a strongly bicovariant exterior algebra on \mathbb{C}_q[GL_2]\ltimes \mathbb{C}_q^2, which is a quantum deformation of maximal parabolic P \subset SL_3 and isomorphic to a quotient of \mathbb{C}_q[SL_3]. Moreover, from Manin, we know that the structure of \mathbb{C}_q[GL_2] is largely determined from its coaction on quantum plane \mathbb{C}_q^2. By requiring that this coaction is differentiable, we find that the structure of 4D strongly bicovariant \Omega(\mathbb{C}_q[GL_2]) is largely determined by its coaction on \Omega(\mathbb{C}_q^2). I will also talk a... more

Weak Expectations of Discrete Quantum Group Algebras and Crossed Products

Arnab Bhattacharjee
Charles University
Tuesday, 13. December 2022 - 16:00 to 17:00
This talk will take place in the blue seminar room, back building, Žitná 25.

It will also be broadcast on Zoom:
https://cesnet.zoom.us/j/91975183920?pwd=SGdlRUhLS21HUnVFTXBKcE1vYXlrQT09
Meeting ID: 919 7518 3920
Passcode: 102707
Studying various finite-dimensional approximation properties such as nuclearity, exactness has become one of the central areas of investigation in the theory of C*-algebras. One of the natural questions is whether standard constructions of C*-algebras preserve the approximation properties. In the classical discrete group case, all approximation properties such as nuclearity, weak expectation property of the group C*-algebra can be characterized by the amenability of the group. In this talk, I will present the connection between weak expectations of the discrete quantum group C*-algebras, the crossed product of C*-algebras, and the amenability of the discrete quantum groups. This is joint work with Angshuman Bhattacharya

Self-similarity of substitution tiling semigroups

Mike Whittaker
University of Glasgow
Tuesday, 24. January 2023 - 16:00 to 17:00
This talk will take place in the blue seminar room, back building, Žitná 25.

It will also be broadcast on Zoom:
https://cesnet.zoom.us/j/91975183920?pwd=SGdlRUhLS21HUnVFTXBKcE1vYXlrQT09
Meeting ID: 919 7518 3920
Passcode: 102707
Substitution tilings arise from graph iterated function systems. Adding a contraction constant, the attractor recovers the prototiles. On the other hand, without the contraction one obtains an infinite tiling. In this talk I'll introduce substitution tilings and an associated semigroup defined by Kellendonk. I'll show that this semigroup defines a self-similar action on a topological Markov shift that's conjugate to the punctured tiling space. The limit space of the self-similar action turns out to be the Anderson-Putnam complex of the substitution tiling and the inverse limit recovers the translational hull. This was joint work with Jamie Walton.

Hochschild and twisted Hochschild homology of quantum homogeneous spaces

Fredy Díaz García
TU Dresden
Tuesday, 31. January 2023 - 16:00 to 17:00
This talk will take place in the blue seminar room, back building, Žitná 25.

It will also be broadcast on Zoom:
https://cesnet.zoom.us/j/91975183920?pwd=SGdlRUhLS21HUnVFTXBKcE1vYXlrQT09
Meeting ID: 919 7518 3920
Passcode: 102707
In this talk, we give a brief introduction to the theory of Hochschild and the twisted Hochschild (co)homology of quantum spaces. Some examples of calculations will be given for the quantum SL_q(2) and the Podles quantum spheres showing how to avoid the “dimension drop” phenomenon in Hochschild homology. If time permits I will give some ideas on how to calculate Hochschild homology for quantum projective spaces CP^n_q by using Koszul resolutions.

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