English

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

Operator Algebras 2016-06-02 v2 Dynamical Systems Group Theory

Abstract

We prove that for a weakly mixing algebraic action σ:G(X,ν)\sigma: G\curvearrowright(X,\nu), the nn-cohomology group Hn(GX;T)H^n(G\curvearrowright X; \mathbb{T}), after quotienting out the natural subgroup Hn(G,T)H^n(G,\mathbb{T}), contains Hn(G,X^)H^n(G,\widehat{X}) as a natural subgroup for n=1n=1. If we further assume the diagonal actions σ2,σ4\sigma^2, \sigma^4 are T\mathbb{T}-cocycle superrigid and H2(G,X^)H^2(G, \widehat{X}) is torsion free as an abelian group, then the above also holds true for n=2n=2. Applying it for principal algebraic actions when n=1n=1, we show that H2(G,ZG)H^2(G,\mathbb{Z}G) is torsion free as an abelian group when GG has property (T) as a direct corollary of Sorin Popa's cocycle superrigidity theorem; we also use it (when n=2n=2) to answer, negatively, a question of Sorin Popa on the 2nd cohomology group of Bernoulli shift actions of property (T) groups.

Keywords

Cite

@article{arxiv.1509.08278,
  title  = {A remark on $\mathbb{T}$-valued cohomology groups of algebraic group actions},
  author = {Yongle Jiang},
  journal= {arXiv preprint arXiv:1509.08278},
  year   = {2016}
}

Comments

[v2] minor changes; to appear in the Journal of Functional Analysis