On diffusion in high-dimensional Hamiltonian systems
The purpose of this paper is to construct examples of diffusion for E-Hamiltonian perturbations
of completely integrable Hamiltonian systems in 2d-dimensional phase space, with d large.
In the first part of the paper, simple and explicit examples are constructed illustrating absence
of ‘long-time’ stability for size E Hamiltonian perturbations of quasi-convex integrable systems
already when the dimension 2d of phase space becomes as large as log 1/E . We first produce
the example in Gevrey class and then a real analytic one, with some additional work.
In the second part, we consider again E-Hamiltonian perturbations of completely integrable
Hamiltonian system in 2d-dimensional space with E-small but not too small, |E| > exp(−d), with
d the number of degrees of freedom assumed large. It is shown that for a class of analytic
time-periodic perturbations, there exist linearly diffusing trajectories. The underlying idea for
both examples is similar and consists in coupling a fixed degree of freedom with a large
number of them. The procedure and analytical details are however significantly different. As
mentioned, the construction in Part I is totally elementary while Part II is more involved, relying
in particular on the theory of normally hyperbolic invariant manifolds, methods of generating
functions, Aubry–Mather theory, and Mather’s variational methods.
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Elsevier