Amenability of Hopf von Neumann algebras and Kac algebras

Zhong Jin Ruan

Research output: Contribution to journalArticlepeer-review


Let (Μ, Γ) be a Hopf von Neumann algebra. The operator predual Μ * of Μ is a completely contractive Banach algebra with multiplication m = Γ *: Μ * ⊗̂ Μ * → Μ *. We call (Μ, Γ) operator amenable if the completely contractive Banach algebra Μ * is operator amenable, i.e., for every operator Μ *-bimodule V, every completely bounded derivation from Μ * into the dual Μ *-bimodule V* is inner. There is a weaker notion of amenability introduced by D. Voiculescu. We say that a Hopf von Neumann algebra (Μ, Γ) is Voiculescu amenable if there exists a left invariant mean on Μ. We show that if a Hopf von Neumann algebra (Μ, Γ) is operator amenable, then it is Voiculescu amenable. For Kac algebras, there is a strong Voiculescu amenability. We show that for discrete Kac algebras, these amenabilities are all equivalent. In fact, if we let K = (Μ, Γ, K, φ) be a discrete Kac algebra and let K̂ = (Μ̂, Γ̂, K̂, φ̂) be its (compact) dual Kac algebra, then the following are equivalent: (1) K is operator amenable; (2) K is Voiculescu amenable; (3) The von Neumann algebra Μ̂ is hyperfinite; (4) K is strong Voiculescu amenable; (5) K̂ is operator amenable; (6) Μ̂ * has a bounded approximate identity.

Original languageEnglish (US)
Pages (from-to)466-499
Number of pages34
JournalJournal of Functional Analysis
Issue number2
StatePublished - Aug 1 1996
Externally publishedYes

ASJC Scopus subject areas

  • Analysis


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