Abstract

In this note, we investigate coboundaries of interval exchange transformations of three intervals, or 3-IETs. More precisely, we study differentiable functions whose derivative is absolutely continuous, and whose integral is zero and whose derivative also has integral zero. We show that these functions are coboundaries for a typical 3-IET if and only if their values at the endpoints of the domain are zero. We also show the existence of exceptional counterexamples for both possible endpoint behaviors. Our results are obtained by studying the properties of associated skew products.

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Audit summary

Audited against arXiv v4

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 almost-everywhere coboundary result for zero endpoint data, the ergodicity result for equal nonzero endpoint data, and the dense exceptional family are correct.

Theorems 1–2 and Proposition 3Correct

Coboundary and skew-product alternatives are correctly separated

Pages 2–4 and Sections 3–5 · arXiv:2501.06380v4

Inducing a three-interval exchange to the rotation model gives the required continued-fraction control. With vanishing endpoint obstruction, the Roth-type estimates solve the cohomological equation. With a common nonzero endpoint value, Denjoy–Koksma estimates and the essential-value criterion give ergodicity of the real skew product for almost every parameter. The separately constructed exceptional coboundaries do not conflict with the metric statement.

Full paper, version 4
Corollaries 4–5Correct

The trigonometric applications meet the respective endpoint alternatives

Pages 4–5 · Corollaries 4–5 · arXiv:2501.06380v4

The displayed trigonometric observables have the regularity, zero-mean, and endpoint behavior required by the cited main theorem in each case. The almost-everywhere parameter quantifier is preserved, so the corollaries do not strengthen the metric conclusions to every interval exchange.

02Proofs3 reported findingsCorrect

The proofs are correct and complete. A section heading contains a harmless theorem-number typo.

Sections 3–5Correct and complete

Induction, cohomological estimates, and essential values close

Pages 7–18 · Sections 3–5 · arXiv:2501.06380v4

Return-word decompositions preserve the endpoint sums, the exceptional continued-fraction sets are summable, and the transfer functions obtained from the rotation model have the required integrability. In the nonzero case the essential values form a nontrivial closed subgroup and the scaling argument expands it to all of R\mathbb R, proving ergodicity.

Page 11 headingTypo

A proposition is mislabeled as a theorem

Page 11 · heading 'Proof of Theorem 13' · arXiv:2501.06380v4

The heading says 'Proof of Theorem 13', while the nearby result and the argument are Proposition 13. The cross-reference is uniquely recoverable from the numbering and does not change any claim or proof.

Sections 4–5Correct and complete

The obstruction and essential-value proof chains are logically separate and complete

Pages 12–18 · proofs of Theorems 1–2 · arXiv:2501.06380v4

For Theorem 1, balanced induction times yield the special-sum obstruction on a full-measure subsequence. For Theorem 2, Denjoy–Koksma control and recurrence produce a nonzero essential value, after which the scaling argument gives all of R\mathbb R. Neither proof uses the conclusion of the other endpoint regime.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2501.06380v4
Authors listed
Przemysław Berk, Carlos Ospina
Audit date
August 18, 2026
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