arXiv:2606.11389v1

Instability of a nonlinear oscillator with small friction and small additive noise

Peter H Baxendale

math.PRmath.DS60H1060J6037H1537A30

Abstract

Let λ=λ(β,σ,a,b)λ= λ(β,σ,a,b) denote the top Lyapunov exponent for the linearization along trajectories of the noisy damped non-linear oscillator x¨+βx˙+ax+bx3=σW˙t\ddot{x}+β\dot{x} + ax+bx^3 = σ\dot{W}_t, where aa, bb and ββ are all positive and σ0σ\neq 0. In 2004 Arnold, Imkeller and Sri Namachchivaya stated without proof that λ(ε2β,εσ,a,b)λε2/3λ(\varepsilon^2 β,\varepsilon σ,a,b) \sim \overlineλ \varepsilon^{2/3} as ε0\varepsilon \to 0 with λ>0\overlineλ > 0. This paper contains a proof of this assertion.

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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 small-friction, small-noise Lyapunov-exponent asymptotic and its explicit positive leading constant are correct. A preliminary scaling identity contains one extraneous factor, but the normalized specialization and all later analysis use the correct scaling.

Theorem 1.1Correct

The top Lyapunov exponent has the stated positive ε2/3\varepsilon^{2/3} asymptotic

Pages 3–4 and Sections 5–8 · Theorem 1.1 · arXiv:2606.11389v1

The Furstenberg–Khasminskii formula is expressed in the Hamiltonian frame, and the corrector estimates isolate the averaged shear/noise contribution. The inner cutoff controls the singular critical point, the outer estimates are integrable under the invariant Gibbs density, and the two error theorems give order ε4/3\varepsilon^{4/3} after restoring the prefactor. The functions defining the leading integral are positive and integrable, so the constant is finite and strictly positive.

Scaling identity in the IntroductionTypo

The transformed linear coefficient should not be divided by the spatial scale

Page 2 · paragraph reducing general a,ba,b to a=b=1a=b=1 · arXiv:2606.11389v1

The parameter transformation printed immediately before the identity is (Aβ,A3/2σ/B,A2a,A2B2b)(A\beta,A^{3/2}\sigma/B,A^2a,A^2B^2b), but the next display writes A2a/BA^2a/B in the third slot. Direct substitution gives A2aA^2a, with no factor B1B^{-1}. The following normalized formula λ(β,σ,a,b)=a1/2λ(a1/2β,a5/4b1/2σ,1,1)\lambda(\beta,\sigma,a,b)=a^{1/2}\lambda(a^{-1/2}\beta,a^{-5/4}b^{1/2}\sigma,1,1) is consistent with the corrected slot, so deleting `/B' is the unique repair and does not affect Theorem 1.1.

02Proofs2 reported findingsCorrect

The ergodic formula, Hamiltonian averaging, singular-region correctors, and error estimates form a complete proof of the asymptotic. The scaling mismatch is a local typographical error outside the normalized proof.

Proposition 2.1 and Appendix ACorrect and complete

Furstenberg–Khasminskii formula and independence of initial data

Page 5 and Appendix A · arXiv:2606.11389v1

The logarithmic derivative of the tangent norm is the displayed observable Q~ε\widetilde Q_\varepsilon. Compactness of the angular variable gives an invariant lift of the Gibbs measure; the Hörmander and controllability lemmas imply smoothness, full support, and uniqueness. The ergodic theorem first gives the integral formula for stationary initial data, and absolute continuity of every positive-time transition law extends the almost-sure limit to every deterministic (x0,y0,v0)(x_0,y_0,v_0) with v00v_0\neq0.

Proposition 5.1, Theorems 6.1 and 7.1, and Proposition 8.1Correct and complete

The four estimates combine at the required error order

Sections 5–8 · arXiv:2606.11389v1

Ergodicity turns every integrable corrector into a vanishing time boundary term, the martingale terms have square-integrable integrands, and the deterministic remainders are split into inner and outer energy regions with matching cutoff powers. The largest remainder after multiplication by the principal ε2/3\varepsilon^{2/3} scale is O(ε4/3)O(\varepsilon^{4/3}), exactly as used in the final triangle inequality.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.11389v1
Authors listed
Peter H Baxendale
Audit date
August 18, 2026
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