We show that if G is a discrete group which does not have the Haagerup property but does have an unbounded cocycle into a C_0 representation and if G acts on a finite von Neumann algebra B such that the inclusion B \subset (B \rtimes G) has the Haagerup property from below then any group-measure space Cartan subalgebra must have a corner which embeds into B inside B \rtimes G.
Taking the action to be trivial we produce examples of II_1 factors N such that N \otimes M is not a group-measure space construction whenever M is a finite factor with the Haagerup property. Taking the action on a probability space with the Haagerup property from below we produce examples of von Neumann algebras which have unique group-measure space Cartan subalgebras. Taking profinite actions of certain products of groups we use the unique Cartan decomposition theorem of N. Ozawa and S. Popa and the cocycle superrigidity theorem of A. Ioana to produce actions which are virtually W*-superrigid.
Let $Γ$ be an irreducible lattice in a product of two locally compact groups and assume that $Γ$ is densely embedded in a profinite group $K$. We give necessary conditions which imply that the left translation action $Γ\curvearrowright K$ is "virtually" cocycle superrigid: any cocycle $w:Γ\times K\rightarrowΔ$ with values in a countable group $Δ$ is cohomologous to a cocycle which factors through the map $Γ\times K\rightarrowΓ\times K_0$, for some finite quotient group $K_0$ of $K$. As a corollary, we deduce that any ergodic profinite action of $Γ=\text{SL}_2(\mathbb Z[S^{-1}])$ is virtually c…
Group measure space decomposition of II_1 factors and W*-superrigidity
本文证明了群测度空间II_1的一个由任意自由遍历概率测度保持（p. m. p.）群Γ在一个相当大的族G中的作用，该族G包含Kazhdan群和一个非平凡群的所有自由积，以及在一个顺从子群上的某些合并的自由积.本文证明了：若T_n表示PSL（n，Z）中的上三角形矩阵群，则T_n上PSL（n，Z）与自身的合并自由积的任意自由混合p. m. p.作用是W*-超刚性的，即L^ \infty（X）\rtimes Γ与任意群测度空间因子L^ \infty（Y）\rtimes Λ之间的同构都来自于作用的共轭性.我们还证明了对于族G中的许多群Γ，Γ的Bernoulli作用是W*-超刚性的.
We show that the class $\mathscr{B}$, of discrete groups which satisfy the conclusion of Popa's Cocycle Superrigidity Theorem for Bernoulli actions, is invariant under measure equivalence. We generalize this to the setting of discrete p.m.p. groupoids, and as a consequence we deduce that any nonamenable lattice in a product of two noncompact, locally compact second countable groups, must belong to $\mathscr{B}$. We also introduce a measure-conjugacy invariant called Weak Pinsker entropy and show that, if G is a group in the class $\mathscr{B}$, then Weak Pinsker entropy is an orbit-equivalence…
Some unique group-measure space decomposition results
利用von Neumann代数上可闭导子理论的方法，我们证明了一类群CR满足以下性质：给定CR中的任意群G_1，G_2，则X上概率空间G_1 x G_2上的任意自由、遍历、保测度作用都产生一个具有唯一群测度空间Cartan子代数的von Neumann代数.将这个结果与Popa的轨道等价超刚性定理相结合，我们得到了W*-超刚性作用的新例子。
We prove that for any two continuous minimal (topologically free) actions of the infinite dihedral group on an infinite compact Hausdorff space, they are continuously orbit equivalent only if they are conjugate. We also show the above fails if we replace the infinite dihedral group with certain other virtually cyclic groups, e.g. the direct product of the integer group with any non-abelian finite simple group.
Let $G_Γ\curvearrowright X$ and $G_Λ\curvearrowright Y$ be two free measure-preserving actions of one-ended right-angled Artin groups with trivial center on standard probability spaces. Assume they are irreducible, i.e. every element from a standard generating set acts ergodically. We prove that if the two actions are stably orbit equivalent (or merely stably $W^*$-equivalent), then they are automatically conjugate through a group isomorphism between $G_Γ$ and $G_Λ$. Through work of Monod and Shalom, we derive a superrigidity statement: if the action $G_Γ\curvearrowright X$ is stably orbit equ…
