English

Compressible subalgebras in II$_1$ factors

Operator Algebras 2025-10-21 v1

Abstract

Given a II1_1 factor MM, a W^*-subalgebra QMQ\subset M is {\it compressible} if for any ε>0\varepsilon>0 there exists a finite set of unitary elements \CalU0\CalU(M)\Cal U_0\subset \Cal U(M) such that 1\CalU0u\CalU0uxuE1MK(C)(x)ε\| \frac{1}{|\Cal U_0|}\sum_{u\in \Cal U_0} uxu^* -E_{1\otimes \Bbb M_K(\Bbb C)}(x)\|\leq \varepsilon, K1\forall K\geq 1, x(QMK(C))1\forall x\in (Q\otimes \Bbb M_K(\Bbb C))_1. Any W^*-subalgebra QQ in a II1_1 factor MM which admits a diffuse W^*-algebra Q0MQ_0\subset M that's free independent to QQ, is compressible in MM. We prove that if QMQ\subset M is compressible, then NL2MQ_NL^2M_Q contains a copy of the coarse NQN-Q bimodule for any AFD subalgebra NMN\subset M. We use this result to provide examples of inclusions of factors M\CalMM\subset \Cal M that are ergodic but not AFD-ergodic, even after stabilizing by \CalB(2N)\Cal B(\ell^2\Bbb N).

Keywords

Cite

@article{arxiv.2510.17076,
  title  = {Compressible subalgebras in II$_1$ factors},
  author = {Sorin Popa},
  journal= {arXiv preprint arXiv:2510.17076},
  year   = {2025}
}

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21 pages