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Our seminar runs on Tuesdays from 13:00 to 14:00. Contact:
09/09/2025: Robert-Mihai Neagu (KU Leuven), at 13:00 in the CP3 meeting room.
Title: Noncommutative coloured entropy
Abstract: Building on the classical noncommutative entropy for automorphisms of nuclear C*-algebras, I will introduce a formally different notion of entropy which uses the more refined cpc approximations given by finite nuclear dimension or finite decomposition rank. In the sequel, I will explore the typical values of this entropy. This is joint work with Bhishan Jacelon.
16/09/2025: Villads Ussing Bojesen (SDU), at 13:00 in the IMADA seminar room.
Title: The (Local) Lifting Problem for C*-Algebras
Abstract: A C*-algebra is said to have the Lifting Property (LP) if every completely positive map into a quotient C*-algebra has a completely positive lift. Through the years, the LP and its local version, the LLP, have been getting a lot of traction both because of their usefulness, but also their relevance in connecting various major problems such as Kirchberg's QWEP-conjecture, Connes' Embedding Problem and Tsirelson's Problem. I will introduce these properties and study the problem of finding groups whose full group C*-algebras fail the LLP. I will present a class of examples due to Ioana, Spaas and Wiersma which in particular include SL3(Z).
The seminar is based on my master's thesis: “Lifting Properties of Group C*-Algebras and Operator Spaces.”
30/09/2025: David Jekel (Københavns Universitet), at 13:00 in U177.
Title: The unitary group of a II_1 factor is SOT-contractible
Abstract: I show that the unitary group of any SOT-separable II_1 factor M, with the strong operator topology, is contractible. Combined with several old results, this implies that the same is true for any SOT-separable von Neumann algebra with no type I_n direct summands (n < infinity). The proof for the II_1-factor case uses regularization via free convolution and Popa's theorem on the existence of approximately free Haar unitaries in II_1 factors. I will also explain some of the bigger picture of the free probability ingredients.
07/10/2025: Jamie Bell (University of Münster), at 13:00 in U166.
Title: Stable rank one in nonnuclear crossed products
Abstract: I’ll give an overview of stable rank one and the C*-algebras known to have it, then describe a new approach to studying stable rank one in possibly nonnuclear crossed products. As an application, we show that stable rank one is generic for natural classes of minimal actions of free groups on the Cantor set. This is joint work with Shirly Geffen and David Kerr.
21/10/2025: TBA, at 13:00 in U163.
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28/10/2025: TBA, at 13:00 in U183.
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04/11/2025: TBA, at 13:00 in U166.
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11/11/2025: TBA, at 13:00 in U163.
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18/11/2025: Rufus Willett (University of Hawai'i at Mānoa), at 13:00 in U168.
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25/11/2025: TBA, at 13:00 in U168.
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2/12/2025: TBA, at 13:00 in U168.
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9/12/2025: TBA, at 13:00 in U164.
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16/12/2025: TBA, at 13:00 in U164.
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