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Cohomology and $L^2$-Betti numbers for subfactors and quasi-regular inclusions

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OASW01 - Structure of operator algebras: subfactors and fusion categories

Co-authors: Sorin Popa (UCLA) and Stefaan Vaes (Leuven)

We introduce L$2$-Betti numbers, as well as a general homology and cohomology theory for the standard invariants of subfactors, through the associated quasi-regular symmetric enveloping inclusion of II$_1$ factors. We actually develop a (co)homology theory for arbitrary quasi-regular inclusions of von Neumann algebras. For crossed products by countable groups Γ, we recover the ordinary (co)homology of Γ. For Cartan subalgebras, we recover Gaboriau's L$2$-Betti numbers for the associated equivalence relation. In this common framework, we prove that the L$2$-Betti numbers vanish for amenable inclusions and we give cohomological characterizations of property (T), the Haagerup property and amenability. We compute the L$2$-Betti numbers for the standard invariants of the Temperley-Lieb-Jones subfactors and of the Fuss-Catalan subfactors, as well as for free products and tensor products.

This talk is part of the Isaac Newton Institute Seminar Series series.

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