Characteristic-invariant conjecture for almost periodic actions on the hyperfinite type II_1 factor

From papers

Let GG be the acting group and let RR be the hyperfinite type II1\mathrm{II}_1 factor. Let α:GR\alpha:G\curvearrowright R be an almost periodic action such that RαGR\rtimes_\alpha G is a factor. Define

H={hGαhInn(R)}H=\{h\in G\mid \alpha_h\in\operatorname{Inn}(R)\}

and, for hHh\in H, choose vU(R)v\in\mathcal{U}(R) with Ad(v)=αh\operatorname{Ad}(v)=\alpha_h. The associated bicharacter is χ:G×HT\chi:G\times H\to\mathbb{T}, defined by χ(g,h)=αg(v)v\chi(g,h)=\alpha_g(v)v^*.

Characteristic-invariant conjecture. The action α\alpha is classified up to cocycle conjugacy by the characteristic invariant (H,χ)(H,\chi).

The source identifies this as the remaining case in the classification of almost periodic actions on the hyperfinite type II1\mathrm{II}_1 factor when the crossed product is a factor but the action is not outer. The status of this classification problem is not resolved in the supplied text.

Progress summary

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Sources & referencesView supporting material

Primary source

Cyril Houdayer and Amine Marrakchi, “Uniqueness of almost periodic outer flows on the hyperfinite type II_1 factor”, arXiv:2605.02781 (2026).

Additional references

2 papers in this index state this conjecture (2018–2026). The statement above is taken from the most recent of them; the others are arXiv:1803.06870.

Solutions 0

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