Mathematische Annalen

, Volume 361, Issue 3–4, pp 563–582 | Cite as

\(W^*\)-Superrigidity for arbitrary actions of central quotients of braid groups

  • Ionut Chifan
  • Adrian IoanaEmail author
  • Yoshikata Kida


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 \(\tilde{B}_n\curvearrowright (X,\mu )\) is virtually \(\hbox {W}^*\)-superrigid in the following sense: if \(L^{\infty }(X)\rtimes \tilde{B}_n\cong L^{\infty }(Y)\rtimes \Lambda \), for an arbitrary free ergodic pmp action \(\Lambda \curvearrowright (Y,\nu )\), then the actions \(\tilde{B}_n\curvearrowright X,\Lambda \curvearrowright Y\) are virtually conjugate. Moreover, we prove that the same holds if \(\tilde{B}_n\) is replaced with a finite index subgroup of the direct product \(\tilde{B}_{n_1}\times \cdots \times \tilde{B}_{n_k}\), for some \(n_1,\ldots ,n_k\geqslant 4\). The proof uses the dichotomy theorem for normalizers inside crossed products by free groups from Popa and Vaes (212, 141–198, 2014) in combination with the OE superrigidity theorem for actions of mapping class groups from Kida (131, 99–109, 2008). Similar techniques allow us to prove that if a group \(\Gamma \) is hyperbolic relative to a finite family of proper, finitely generated, residually finite, infinite subgroups, then the \(\hbox {II}_1\) factor \(L^{\infty }(X)\rtimes \Gamma \) has a unique Cartan subalgebra, up to unitary conjugacy, for any free ergodic pmp action \(\Gamma \curvearrowright (X,\mu )\).



We would like to thank Rémi Boutonnet and Cyril Houdayer for useful comments. The first author is especially grateful to Denis Osin for kindly pointing out to him that the main results of this paper also apply to the families of groups described in the Sect. 5.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Department of MathematicsUniversity of IowaIowa CityUSA
  2. 2.Department of MathematicsUniversity of California San DiegoLa JollaUSA
  3. 3.Department of MathematicsKyoto UniversityKyoto Japan

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