Chi-Keung Ng
By giving an interesting characterisation of amenable multiplicative unitaries in term of one dimensional representations, we show in a simple way that bicrossproducts of amenable locally compact groups is both amenable and coamenable.
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We classify a certain class of minimal actions of a compact Kac algebra with amenable dual on injective factors of type III. Our main technical tools are the structural analysis of type III factors and the theory of canonical extension of endomorphisms introduced by Izumi.
We show the uniqueness of minimal actions of a compact Kac algebra with amenable dual on the AFD factor of type II$_1$. This particularly implies the uniqueness of minimal actions of a compact group. Our main tools are a Rohlin type theorem, the 2-cohomology vanishing theorem, and the Evans-Kishimoto type intertwining argument.
We prove that the $L^1$-algebra of any non-Kac type compact quantum group does not satisfy operator biflatness. Since operator amenability implies operator biflatness, this result shows that any co-amenable, non-Kac type compact quantum group gives a counter example to the conjecture that $L^1(\Gb)$ is operator amenable if and only if $\Gb$ is amenable and co-amenable for any locally compact quantum group $\Gb$. The result also implies that the $L^1$-algebra of a locally compact quantum group is operator biprojective if and only if $\Gb$ is compact and of Kac type.
We will study two kinds of actions of a discrete amenable Kac algebra. The first one is an action whose modular part is normal. We will construct a new invariant which generalizes a characteristic invariant for a discrete group action, and we will present a complete classification. The second is a centrally free action. By constructing a Rohlin tower in an asymptotic centralizer, we will show that the Connes-Takesaki module is a complete invariant.
We prove that Wang's free orthogonal and free unitary quantum groups are weakly amenable and that their Cowling-Haagerup constant is equal to 1. This is achieved by estimating the completely bounded norm of the projections on the coefficients of irreducible representations of their compact duals. An argument of monoidal equivalence then allows us to extend this result to quantum automorphism groups of finite spaces and even yields some examples of weakly amenable non-unimodular discrete quantum groups with the Haagerup property.
It is shown that the action of ${\rm SL}(3,\mathbb{Z})$ on the Stone-uCech boundary of ${\rm SL}(3,\mathbb{Z}) / {\rm SL}(2,\mathbb{Z}) $ is amenable. This confirms a prediction by Bekka and Kalantar.
In this last article of the series on outer actions of a countable dicrete amenable group on AFD factors, we analyze outer actions of a countable discrete free abelian group on an AFD factor of type $\text{III}_λ$, $0<λ< 1$, and compute outer conjugacy invariants. As a byproduct, we discover the asymmetrization technique for coboundary condition on a $\Bbb T$-valued cocycle of a torsion free abelian group, which might have been known by the group cohomologists. As the asymmetrization technique gives us a very handy criteria for coboundaries, we present it here in detail in the second section.
We prove that weak amenability of a locally compact group imposes a strong condition on its amenable closed normal subgroups. This extends non weak amenability results of Haagerup (1988) and Ozawa--Popa (2010). A von Neumann algebra analogue is also obtained.
We show the equivalence of several amenability type conditions for C*-dynamical systems. As an application, based on the Pimsner--Meyer construction, we give a new method to produce amenable actions on simple C*-algebras.
Residually finite amenable groups that are not Hilbert-Schmidt stable
该研究构造了首批不具备Hilbert-Schmidt稳定性的剩余有限顺从群实例，涵盖多种群结构，还证明这些实例不具备算子HS稳定性，填补了相关领域的研究空白。
Let $F\subseteq H\subseteq G$ be closed subgroups of a locally compact group. In response to a 1972 question by Eymard, we construct an example where the homogeneous factor-space $G/F$ is amenable in the sense of Eymard-Greenleaf, while $H/F$ is not. (In our example, $G$ is discrete.) As a corollary which answers a 1990 question by Bekka, the induced representation $\ind_H^G(ρ)$ can be amenable in the sense of Bekka even if $ρ$ is not amenable. The second example, answering another question by Bekka, shows that $\ind_H^G(ρ)$ need not be amenable even if both the representation $ρ$ and the cose…
Examples of non-amenable, boundary-amenable dynamical systems
研究具有(AP)性质的离散可数群$\Gamma$的作用，当它以特定方式作用于可数集时，在$\partial_\beta \mathfrak{X}$上诱导作用拓扑可和，给出了在$\partial_\beta (\Gamma / \Lambda)$上作用的多种情形及相关应用。
We construct examples of principal groupoids that have weak containment but are not amenable, thus answering questions by Claire Anantharaman-Delaroche and Rufus Willett.
An example of an infinite amenable group with the ISR property
本文证明正整数集上的有限支撑置换群具有不变 von Neumann 子代数刚性（ISR）性质，并完全刻画了其所有不变子代数，首次给出具有 ISR 性质的无限顺从群例子。
A non-uniformly inner amenable group
本文构造了两个初等等价的可数 ICC 群，其中一个顺从而另一个非内顺从，并据此首次给出群冯·诺依曼代数不初等等价的例子，回答了领域内的一个公开问题。
Amenable actions on ill-behaved simple C*-algebras
本文结合Rørdam与作者先前构造，首次给出非顺从群在既非稳定有限也非纯无限的简单可分核C*-代数上的顺从作用，并确保交叉积简单且含有限与无限投影。
We construct a locally compact groupoid with the properties in the title. Our example is based closely on constructions used by Higson, Lafforgue, and Skandalis in their work on counterexamples to the Baum-Connes conjecture. It is a bundle of countable groups over the one point compactification of the natural numbers, and is Hausdorff, second countable and étale.
Every countable group admits amenable actions on stably finite simple C*-algebras
本文证明了每个可数群都能在稳定有限简单C*-代数上构造可数作用，通过正则表示的全福克空间和迹缩放自同构实现。
Simplices of maximally amenable extensions in II$_1$ factors
本文构造了可分 II$_1$ 因子及其极大交换子代数，使极大可扩空间与 $n$ 维实单纯形仿射同构，首次实现恰好具有 $n$ 个极大可扩因子扩张的 masa 实例。
Strongly solid group factors which are not interpolated free group factors
本文构造了具有 Haagerup 性质且弱顺从常数为1的非顺从 ICC 群，证明其生成的 II_1 因子具有强刚性，且不同构于任何插值自由群因子。
We give an example of a non-compact, locally compact group $G$ such that its Fourier-Stieltjes algebra $B(G)$ is operator amenable. Furthermore, we characterize those $G$ for which $A^*(G)$ - the spine of $B(G)$ as introduced by M. Ilie and the second named author - is operator amenable and show that $A^*(G)$ is operator weakly amenable for each $G$.
Non-amenable C$^*$-superrigid groups that are not W$^*$-superrigid
本文构造了非amenable且无法从群von Neumann代数重建的C$^*$-超刚群，并展示了同时满足C$^*$-和W$^*$-超刚性质的自由积群例子。
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