Normal forms, stability and splitting of invariant manifolds I. Gevrey Hamiltonians
Summary (1 min read)
1 Introduction and main results
- Let us briefly recall the setting considered in the first part of this work [Bou12a].
- Such simpler systems are usually called resonant formal forms with a small remainder, and, among other things, they are very important in trying to obtain stability estimates for the evolution of the action variables and “splitting” estimates when the unperturbed invariant torus becomes “hyperbolic” for ε >.
- The method the authors will use in this second part is, in spirit, analogous to the one they used in the first part.
- Now in the Diophantine case, the estimates of Theorem 1.1 can be made more explicit.
- In the first case, that is for perturbations of linear integrable systems, the authors will show on an example that these estimates are very accurate.
2 Applications to stability and splitting estimates
- The authors will give consequences of their normal forms Theorem 1.1 and Theorem 1.3 to the stability of the action variables, and to the splitting of invariant manifolds of a hyperbolic tori.
- Using the Lagrangian character of the invariant manifolds, as in [LMS03] the authors can define a symmetric matrix of size n, called the splitting matrix, whose eigenvalues are called splitting angles, and their main result (Theorem 2.7 below) states that at least d splitting angles are polynomially small.
3 Proof of the main results
- This section is devoted to the proof of Theorem 1.1 (recall that Theorem 1.3 follows from it exactly as in the first part of this work).
- The method is in principle analogous to the one used in [Bou12a], but the technicalities are quite different so the authors need to give complete details.
- Now the authors can finally prove Theorem 1.1, which follows easily from the proposition below.
- I am grateful to Vadim Kaloshin and Marcel Guardia for asking a question related to this work and which was the initial motivation, and to Jean-Pierre Marco for asking already a long time ago to prove upper bounds for the splitting of invariant manifolds for non-analytic systems.
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Frequently Asked Questions (13)
Q2. What is the symplectic map of c, c1, c?
There exist positive constants c, c1, c2, c3 and c4 that depend only on n,R, ω, τ, k and κ such that if ε ≤ cc−(1+τ)1 γ, then there exists a symplectic map Φκ ∈ Ck−κ(DR/2,DR) such thatH ◦
Q3. What is the simplest way to determine the properties of the invariant torus?
The authors will always assume that 1 ≤ d ≤ n, as the case d = 0 is trivial since it corresponds to the zero vector and hence to an invariant torus which consists uniquely of equilibrium solutions.
Q4. What is the simplest way to describe a Hamiltonian system?
For an analytic Hamiltonian system, it is well-known that if the frequency vector is Diophantine, the system can be analytically conjugated to a simpler system where the perturbation has been split into two parts: a resonant part, which captures the important features of the system and whose size is still comparable to ε, and a non-resonant part, whose size can be made exponentially small with respect to ε−a, where the exponent a > 0 depends only on the Diophantine exponent τ .
Q5. What is the inverse of the Hamiltonian flow?
Hλ is still independent of θ1, the level sets of I1 are still invariant, hence the Hamiltonian flow generated by the restriction of Hλ to {I1 = 0} × Dm1 (considered as a flow on Dm1 ) is a λperturbation (in the C1-topology) of the Hamiltonian flow generated by P : as a consequence, it has a hyperbolic fixed point Oλ ∈ Dm1 which is λ-close to O, for λ small enough.
Q6. What is the first assumption regarding the persistence of the torus T,?
their last assumption concerns the existence of orbits which are homoclinic to Tλ,µ: (A.3) For any 0 ≤ λ<· 1 and 0 ≤ µ<·λ, the set W+(Tλ,µ)∩W−(Tλ,µ) \\ Tλ,µ is non-empty.
Q7. What is the simplest way to determine the frequency of the linear flow?
Letting T > 0 be the infimum of the set of such t, the vector v will be called T -periodic, and it is easy to see that the orbits of the linear flow with frequency v are all periodic with minimal period T .Now, for a general vector ω ∈
Q8. What is the effect of the symplectic property of on the stability?
using the fact that Φκ is symplectic and close to identity, the same property remains true for the Hamiltonian H.Now the larger the authors take κ, the smaller is the size of the remainder and hence the longer is the stability time.
Q9. What is the symplectic map of Q?
For Q ≥ 1, assume that Q ·> 1, ε<·∆ω(Q)−1, (13) then, for Rdκ = R− dκδ, there exists a symplectic map Φκ ∈ Ck−κ(DRdκ ,DR) such thatH ◦
Q10. What is the simplest way to define the splitting matrix of T?
Coming back to their original system, the torus Tε = Φ(Tλ,µ) is hyperbolic for H, with stable and unstable manifolds W±(Tε) = Φ(W±(Tλ,µ)), and for γε = Φ(γλ,µ) and pε = Φ(pλ,µ) the authors can define a splitting matrix M(Tε, pε) and splitting angles ai(Tε, pε) for 1 ≤ i ≤ n.
Q11. What is the formula for determining the distance of Xf to the identity?
The authors will need to prove that the Ck−1 norm of the distance of X1f to the identity is bounded (up to constants depending on k and on the domain) by the Ck−1 norm of the vector field Xf (which itself is bounded by the Ck norm of f).
Q12. What is the -perturbation of a Hamiltonian?
For ε ≥ 0, the authors consider an ε-perturbation of an integrable Hamiltonian h in angle-action coordinates, that is a Hamiltonian of the form{
Q13. What are the qualitative and quantitative properties of the invariant torus?
The qualitative and quantitative properties of this invariant torus are then determined by the Diophantine properties of its frequency vector ω = ∇h(0) ∈ Rn.