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feat(Stability/Floquet): formalize basic Floquet theory and spectral stability - #68

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AdoHaha:feat/floquet-theory
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AdoHaha wants to merge 8 commits into
mcdoll:mainfrom
AdoHaha:feat/floquet-theory

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@AdoHaha

@AdoHaha AdoHaha commented Sep 12, 2026

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This PR formalizes the core results of Floquet theory for linear differential equations with periodic coefficients, following Michael J. Ward, Basic Floquet Theory, Chapter 3.

Main Contributions

  • Periodic Linear Propagators: Transition operators X(t₀, t) satisfying X(t₀, t₀) = id, X(t₁, t₂) ∘ X(t₀, t₁) = X(t₀, t₂), and d/dt X = L(t) ∘ X.
  • Shift Invariance & Fundamental Factorization: HasShiftInvariance X T (X(t₀ + T, t + T) = X(t₀, t)), proving the Floquet factorization into a periodic component and stroboscopic powers.
  • Monodromy Operator & Floquet Multipliers: Defines monodromy and isFloquetMultiplier.
  • Spectral Stability: Asymptotic stability and mode divergence characterizations based on Floquet multipliers (|\rho| < 1) and (|\rho| > 1).

Note: Built on top of #67.

Igor added 8 commits September 8, 2026 00:44
- Add  based on Michael J. Ward's Basic Floquet Theory
- Define , , ,
- Prove Ward Theorem 3.3(i)  and stroboscopic
- Prove Ward Theorem 3.4(i) , , and stroboscopic map
- Prove Ward Theorem 3.4(ii)  x(t) = exp(μ t) • p(t)
- Prove Ward §3.1.2 Goldstone multiplier 1 theorem for autonomous periodic orbits
- Prove Ward §3.2.3 second-order trace stability criteria
- Prove stroboscopic distance contraction and geometric decay
…s to Floquet

- Add : Lyapunov stability of origin under stroboscopic flow via
- Add : asymptotic trajectory convergence to 0 when ‖M‖ < 1
- Add : attractiveness of origin via
- Add : mode convergence to 0 along contracting multiplier |ρ| < 1
- Add : complex roots with ‖z‖ = 1 when |φ| < 1 (Ward §3.2.3 Case I)
- Add : unstable real multiplier ρ > 1 when φ > 1 (Ward §3.2.3 Case II)
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