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Gamma stability in free product von Neumann algebras. (English) Zbl 1328.46046
Summary: Let $$(M,\varphi)=(M_1,\varphi_1)\ast(M_2,\varphi_2)$$ be a free product of arbitrary von Neumann algebras endowed with faithful normal states. Assume that the centralizer $$M_1^{\varphi_1}$$ is diffuse. We first show that any intermediate subalgebra $$M_1\subset Q\subset M$$ which has nontrivial central sequences in $$M$$ is necessarily equal to $$M_1$$. Then we obtain a general structural result for all the intermediate subalgebras $$M_1\subset Q\subset M$$ with expectation. We deduce that any diffuse amenable von Neumann algebra can be concretely realized as a maximal amenable subalgebra with expectation inside a full nonamenable type $$\mathrm{III}_1$$ factor. This provides the first class of concrete maximal amenable subalgebras in the framework of type III factors. We finally strengthen all these results in the case of tracial free product von Neumann algebras.

##### MSC:
 46L10 General theory of von Neumann algebras 46L09 Free products of $$C^*$$-algebras 46L36 Classification of factors
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