This is a survey of some aspects of the subject of approximation properties for locally compact quantum groups, based on lectures given at the Topological Quantum Groups Graduate School, 28 June -11 July, 2015 in Bed lewo, Poland. We begin with a study of the dual notions of amenability and co-amenability, and then consider weakenings of these properties in the form of the Haagerup property and weak amenability. For discrete quantum groups, the interaction between these properties and various operator algebra approximation properties are investigated. We also study the connection between central approximation properties for discrete quantum groups and monoidal equivalence for their compact duals. We finish by discussing the central weak amenability and central Haagerup property for free quantum groups.Date: May 9, 2016. 1 2 there have been very recent and striking developments on questions related to residual finiteness, (inner)linearity, hyperlinearity, and the Kirchberg factorization property for discrete quantum groups. See [5,6,14,23,9].Acknowledgements. First and foremost, I would like to thank Uwe Franz, Adam Skalski, and Piotr Soltan for organizing such a wonderful graduate school, for inviting me to speak there, and for encouraging me to write this survey. I would also like to thankÉtienne Blanchard and Stefaan Vaes for kindly sharing with me their unpublished notes [11].
PreliminariesIn the following, we write b for the minimal tensor product of C˚-algebras or tensor product of Hilbert spaces, b for the spatial tensor product of von Neumann algebras and d for the algebraic tensor product. All inner products are taken to be conjugate-linear in the second variable. Given vectors ξ, η in a Hilbert space H, we denote by ω ξ,η P BpHq˚the vector state x Þ Ñ xxξ|ηy. If ξ " η, we simply write ω ξ " ω ξ,ξ . 2.1. Locally compact quantum groups. Let us first recall from [55] and [56] that a (von Neumann algebraic) locally compact quantum group (lcqg) is a quadruple G " pM, ∆, ϕ, ψq, where M is a von Neumann algebra, ∆ : M Ñ MbM is a co-associative coproduct, i.e. a unital normal˚-homomorphism such that pι b ∆q˝∆ " p∆ b ιq˝∆, and ϕ and ψ are normal faithful semifinite weights on M such that pι b ϕq∆pxq " ϕpxq1 and pψ b ιq∆pxq " ψpxq1 px P M`q.We call ϕ and ψ the left Haar weight and the right Haar weight of G, respectively, and we write L 8 pGq for the quantum group von Neumann algebra M. Associated with each locally compact quantum group G, there is a reduced quantum group C*-algebra C 0 pGq Ď L 8 pGq with coproduct Let L 1 pGq " L 8 pGq˚be the predual of L 8 pGq. Then the pre-adjoint of ∆ induces on L 1 pGq a completely contractive Banach algebra productwhere we let p b denote the operator space projective tensor product. We will also use the symbol ‹ to denote the induced left/right module actions of L 1 pGq on L 8 pGq:Using the Haar weights, one can construct an antipode S on L 8 pGq satisfying pS b ιqW " W˚. Since, in general, S is unbounded on L 8 pGq, we can not use S to define an involution on L 1 pGq. Howev...