arXiv:2606.30956v1

Discontinuity of Lyapunov exponents vs Entropy for smooth surface diffeomorphisms

Jérôme Buzzi

math.DS37A3537C4037D2537E30

Abstract

Lyapunov exponents are fundamental invariants in smooth ergodic theory describing the asymptotic infinitesimal behavior along typical orbits. This text aims to explain how and why to control Lyapunov exponents using entropy for smooth surface diffeomorphisms. It fits into the framework of our recent joint works with Sylvain CROVISIER and Omri SARIG. We will focus especially on the continuity property of exponents for measures near the maximal entropy measure, by presenting a simplified version of the original argument. Our exposition is geared towards advanced students and researchers in dynamics that are not necessarily familiar with smooth ergodic theory.

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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 lecture notes' decomposition of weak limits with discontinuous Lyapunov exponents, its entropy bounds, and the maximum-entropy consequences for smooth surface diffeomorphisms are correct.

Theorems A, B, and DCorrect

Loss of Lyapunov exponent is isolated in a neutral component

Pages 6–14 and Chapters 2, 6–7 · arXiv:2606.30956v1

Projective lifts of the ergodic measures converge to an invariant lift whose excess over the unstable lift is separated into neutral and hyperbolic parts. The mass β\beta of the hyperbolic part scales both the limiting positive exponent and the entropy contribution. Yomdin reparametrization bounds the entropy carried by the neutral blocks; the refined ergodic-decomposition argument gives the βh(f,μ1)\beta h(f,\mu_1) term in Theorem B and Theorem A follows when entropy converges to the topological value.

Full lecture notes, version 1
Theorems A and BCorrect

Entropy loss is correctly tied to the neutral Lyapunov component

Pages 9–12 and Chapters 5–7 · arXiv:2606.30956v1

The limiting measure is decomposed into hyperbolic and neutral pieces, and upper semicontinuity is applied only after the neutral contribution is isolated. Positive limiting entropy prevents the entire mass from collapsing into zero-exponent components, yielding exactly the weighted conclusions stated.

02Proofs2 reported findingsCorrect

The notes give a complete proof of the principal entropy bound and a correct, explicitly sourced roadmap for the additional refinement to Theorem B.

Chapters 2 and 4–7Correct and complete at the stated lecture-note level

Neutral decomposition and entropy covering estimates close

Pages 33–84 · Chapters 2 and 4–7 · arXiv:2606.30956v1

Pliss selection constructs disjoint neutral blocks with the claimed limiting mass. On complementary blocks, unstable curves admit controlled reparametrizations, and the nonlinear covering counts grow at the stated topological-entropy rate. Chapter 7 openly identifies the extra ergodic-decomposition refinement and points to the complete published proof; it is not presented as a new self-contained derivation in these pedagogical notes.

Chapters 5–7Correct and complete

Lyapunov charts, entropy estimates, and the final decomposition fit together

Pages 55–96 · arXiv:2606.30956v1

Pesin blocks provide uniform local charts on the hyperbolic mass, Yomdin–Newhouse estimates control entropy lost outside them, and the remaining measures are assigned to the zero-exponent component. Diagonal extraction preserves invariance and total mass, and the entropy inequalities pass to the stated limiting decomposition.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.30956v1
Authors listed
Jérôme Buzzi
Audit date
August 18, 2026
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