The no infinite cocycle crossed-product decomposition conjecture for II1_1 factors

From papers

Let MM be a separable II1_1 factor. Let GG be a countably infinite group, let (B,τ)(B,\tau) be a tracial von Neumann algebra, and let (σ,c)(\sigma,c) be a trace-preserving cocycle action of GG on BB. No infinite cocycle crossed-product decomposition conjecture. There exists a separable II1_1 factor MM such that

M≇Bσ,cGM\not\cong B\rtimes_{\sigma,c}G

for every countably infinite group GG, every tracial von Neumann algebra (B,τ)(B,\tau), and every trace-preserving cocycle action (σ,c)(\sigma,c) of GG on BB.

This would strengthen the known obstruction to scalar or finite-dimensional cocycle crossed-product decompositions. The source states that its methods do not exclude infinite-dimensional 22-cocycles, and more broadly that it remains open whether any II1_1 factor admits no such decomposition with an infinite group.

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Sources & referencesView supporting material

Primary source

Adriana Fernández Quero, Adrian Ioana and Hui Tan, “A class of II_1 factors without non-trivial crossed product decompositions”, arXiv:2606.31929 (2026).

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