Positive measure of effective quasi-periodic motion near a Diophantine torus

TL;DR

This paper proves that near a Diophantine torus in a Hamiltonian system, there exists a large measure set where motion is effectively quasi-periodic for extremely long times, supporting a weaker version of Herman's conjecture.

Contribution

It establishes the existence of a positive measure neighborhood around a Diophantine torus where trajectories are effectively quasi-periodic for doubly exponentially long times, despite the conjecture remaining open.

Findings

Existence of a positive measure set with effectively quasi-periodic motion.

The measure of the complement is exponentially small.

Long-time stability of trajectories near the Diophantine torus.

Abstract

It was conjectured by Herman that an analytic Lagrangian Diophantine quasi-periodic torus $\mathcal{T}_0$, invariant by a real-analytic Hamiltonian system, is always accumulated by a set of positive Lebesgue measure of other Lagrangian Diophantine quasi-periodic invariant tori. While the conjecture is still open, we will prove the following weaker statement: there exists an open set of positive measure (in fact, the relative measure of the complement is exponentially small) around $\mathcal{T}_0$ such that the motion of all initial conditions in this set is "effectively" quasi-periodic in the sense that they are close to being quasi-periodic for an interval of time which is doubly exponentially long with respect to the inverse of the distance to $\mathcal{T}_0$. This open set can be thought as a neighborhood of a hypothetical invariant set of Lagrangian Diophantine quasi-periodic tori, which may or may not exist.