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Some Rigidity Properties of von Neumann Algebras

Abstract : In this dissertation, I study several rigidity properties of von Neumann algebras. In Chapter 1, we prove the relative solidity of Bernoulli crossed products of arbitrary type. This result is based on Popa's deformation/rigidity and generalizes a theorem of Chifan and Ioana in the tracial case. As a consequence, when the acting group is non-amenable, the crossed product is prime (cannot be decomposed nontrivially as a tensor product of two factors) and the associated equivalence relation is solid.In Chapter 2, we study full factors in relation with the spectral gap property. The main result is a spectral gap characterization of full type III factors which is similar to Connes' characterization in the tracial case. This allows us to better understand the structure of these factors and their automorphism group. We generalize a theorem of Jones by giving a sufficient condition for a crossed product to be full. This condition is necessary when the group is abelian. In particular, we obtain a complete characterization of the type III_1 whose core is full. In a joint work with C. Houdayer and P. Verraedt, we show that a tensor product of two full factors is also full and we compute its Connes invariants. We also prove a unique McDuff decomposition theorem that generalizes a result of Popa in the II_1 case. In Chapter 3, we study McDuff factors, i.e. those factors that can absorb tensorially the hyperfinite factor, as well as their counterpart in ergodic theory, the so-called stable equivalence relations. We obtain a new "spectral gap like" characterization of these properties, based on a maximality argument. With this refined characterization, we are able to prove the following rigidity result: a direct product of two stable equivalence relations is stable if and only if one of them is already stable. The analoguous problem on McDuff factors remains open, but we do give some partial results.
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Submitted on : Tuesday, August 28, 2018 - 11:59:05 AM
Last modification on : Sunday, October 18, 2020 - 4:21:26 PM
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Amine Marrakchi. Some Rigidity Properties of von Neumann Algebras. Dynamical Systems [math.DS]. Université Paris Saclay (COmUE), 2018. English. ⟨NNT : 2018SACLS132⟩. ⟨tel-01863294⟩



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