Determine the exact Borel rank of the strong topological Rokhlin property class

Determine the exact Borel rank of the class of countable groups having the strong topological Rokhlin property.

Background

The paper studies the descriptive complexity of the class STRP\mathsf{STRP} of countable groups whose Cantor-action conjugacy action has a comeager orbit, using the standard Borel space of countable groups. It establishes that STRP\mathsf{STRP} belongs to Π40\boldsymbol{\Pi}^0_4 and is Σ20\boldsymbol{\Sigma}^0_2-hard.

The paper also proves that the locus of finitely presented groups with the strong topological Rokhlin property is Σ30\boldsymbol{\Sigma}^0_3. Consequently, if STRP\mathsf{STRP} is not Σ30\boldsymbol{\Sigma}^0_3, then some non-finitely-presented group has the property. The exact Borel rank of STRP\mathsf{STRP}, however, is left unresolved.

References

We do not determine the exact Borel rank.

Finite-Index Lifting of Strong Topological Rokhlin Property and Descriptive Complexity  (2608.18485 - Luo, 19 Aug 2026) in Remark following Proposition 3.3 (labeled \ref{prop:fp-barrier}), Section 3, Descriptive complexity