arXiv:2607.00781v1

Directional expansion in ergodic actions of countable groups

Michael Björklund, Alexander Fish

math.GRmath.DSmath.RT28D1537A1522E27

Abstract

We study directional expansion for probability-measure-preserving actions of countable groups through a representation-theoretic group property, the cyclic escape property. An infinite countable group has the cyclic escape property if every totally ergodic unitary representation has arbitrarily small fixed-vector projections along infinite cyclic subgroups. This property implies directional expansivity for all totally ergodic actions. We prove that all infinite finitely generated nilpotent groups have the cyclic escape property, and conjecture the same for all infinite finitely generated polycyclic groups. We also prove the cyclic escape property for higher-rank simple lattices whose finite-dimensional unitary representations all have finite image; in particular, for SLn(Z)SL_n(\mathbb Z), PSLn(Z)PSL_n(\mathbb Z), and PGLn(Z)PGL_n(\mathbb Z), n3n\geq 3. By contrast, free groups of rank at least two do not have the cyclic escape property. The proofs exhibit two independent mechanisms: central spectral structure in nilpotent groups and stationary character rigidity in higher-rank lattices.

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Audited against arXiv v1

Not a correctness certificate. A “Correct” result may include yellow typos or minor formal corrections that do not affect substantive soundness. It means this audit found no unresolved substantive error under the stated criteria; it does not replace expert scrutiny or formal verification.

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Detailed mathematical audit

Generated August 18, 2026
01Statements2 reported findingsCorrect

The cyclic-escape results for finitely generated nilpotent groups and higher-rank lattices, and the compact and bounded-torsion obstructions, are correct.

Theorems 1.4, 1.5, 1.7, and 1.9Correct

The group property gives exactly the claimed directional expansion

Pages 3–5 and Sections 5–8 · arXiv:2607.00781v1

Conditional expectation onto cyclic invariants turns a small fixed-vector projection into a set whose cyclic saturation has measure near one. For nilpotent groups, central spectral decomposition separates periodic and aperiodic characters and the induction descends through finite-index central quotients. For higher-rank lattices, stationary character rigidity gives the weakly mixing case, while the finite-image hypothesis removes the finite-dimensional obstruction. Dense translations on SU(2)\mathrm{SU}(2) and Bernoulli actions of bounded-exponent quotients supply the stated counterexamples.

Full paper, version 1
Theorems 1.7 and 1.9Correct

The compact and torsion obstructions delimit the expansion theorems correctly

Introduction and Sections 7–8 · arXiv:2607.00781v1

Finite-dimensional compact factors and torsion characters give precisely the invariant directions excluded by directional expansion. The stated converses preserve the finite-index and ergodicity assumptions, so the obstruction results neither overreach the nilpotent theorem nor silently assume weak mixing.

02Proofs2 reported findingsCorrect

The representation-theoretic induction, character-rigidity, and obstruction proofs are correct and complete.

Sections 2–8Correct and complete

Fixed-vector projections are controlled in every case

Pages 6–30 · Sections 2–8 · arXiv:2607.00781v1

The abelian Fejér argument excludes precisely the torsion atoms forbidden by total ergodicity. The central-extension lemma preserves the required infinite cyclic subgroup and closes the nilpotency induction. In the lattice proof, conjugacy averaging produces a stationary state to which the cited rigidity theorem applies, and the finite-centre reduction uses a verified finite-kernel transfer. The compact-group and torsion-quotient constructions directly violate the defining saturation inequality.

Sections 3–8Correct and complete

Spectral-measure reduction and representation induction close the higher-rank cases

Pages 15–45 · arXiv:2607.00781v1

The cyclic-escape lemma forces spectral mass off periodic characters in the abelian base case. Induction along the central series then transfers this conclusion to nilpotent groups, while finite-dimensional representations are isolated and treated by the compact-image argument rather than absorbed into the mixing case.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2607.00781v1
Authors listed
Michael Björklund, Alexander Fish
Audit date
August 18, 2026
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