Operator-algebraic superrigidity for $SL_n(\mathbb Z),n\geq 3$

W*-superrigidity for arbitrary actions of central quotients of braid groups

For any $n\geqslant 4$ let $\tilde B_n=B_n/Z(B_n)$ be the quotient of the braid group $B_n$ through its center. We prove that any free ergodic probability measure preserving (pmp) action $\tilde B_n\curvearrowright (X,μ)$ is W$^*$-superrigid in the following sense: if $L^{\infty}(X)\rtimes\tilde B_n\cong L^{\infty}(Y)\rtimesΛ$, for an arbitrary free ergodic pmp action $Λ\curvearrowright (Y,ν)$, then the actions $\tilde B_n\curvearrowright X,Λ\curvearrowright Y$ are stably (or, virtually) conjugate. Moreover, we prove that the same holds if $\tilde B_n$ is replaced with a finite index subgroup …

图积群的冯·诺伊曼代数的刚性 II. 超刚性结果

Rigidity for von Neumann algebras of graph product groups II. Superrigidity results

本文证明了花状图积群Γ可从任意无穷顶点群的非平凡图积群生成的冯·诺伊曼代数范畴中被完全识别，获得了更精确的C*-代数版本，并将W*-超刚性结论推广至性质(T) wreath-like product群的直积。

Character rigidity for lattices and commensurators

We prove an operator algebraic superrigidity statement for homomorphisms of irreducible lattices, and also their commensurators, in certain higher-rank groups into unitary groups of finite factors. This extends the authors' previous work regarding non-free measure-preserving actions, and also answers a question of Connes for such groups.

A remark on $\mathbb{T}$-valued cohomology groups of algebraic group actions

We prove that for a weakly mixing algebraic action $σ: G\curvearrowright(X,ν)$, the $n$-cohomology group $H^n(G\curvearrowright X; \mathbb{T})$, after quotienting out the natural subgroup $H^n(G,\mathbb{T})$, contains $H^n(G,\widehat{X})$ as a natural subgroup for $n=1$. If we further assume the diagonal actions $σ^2, σ^4$ are $\mathbb{T}$-cocycle superrigid and $H^2(G, \widehat{X})$ is torsion free as an abelian group, then the above also holds true for $n=2$. Applying it for principal algebraic actions when $n=1$, we show that $H^2(G,\mathbb{Z}G)$ is torsion free as an abelian group when $G$…

On the structural theory of II_1 factors of negatively curved groups, II: Actions by product groups

This paper includes a series of structural results for von Neumann algebras arising from measure preserving actions by product groups on probability spaces. Expanding upon the methods used earlier by the first two authors \cite{CS}, we obtain new examples of strongly solid factors as well as von Neumann algebras with unique or no Cartan subalgebra. For instance we show that every II$_1$ factor associated with a weakly amenable group in the class $\mathcal S$ of Ozawa is strongly solid, \cite{OzSolid}. There is also the following product version of this result: any maximal abelian $\star$-subal…

Strong Banach Property (T) for Simple Algebraic Groups of Higher Rank

In [Laf08], [Laf09], Vincent Lafforgue proved strong Banach property (T) for $SL_3$ over a non archimedean local field $F.$ In this paper, we extend his results to $Sp_4$ and therefore to any connected almost $F$-simple algebraic group with $F$-split rank $\geq 2.$ As applications, the family of expanders constructed from any lattice of such a group do not admit a uniform embedding into any Banach space of type $>1,$ and any isometric affine action of such a group, or its cocompact lattice, on a Banach space of type $>1$ has a fixed point.