Computer-assisted estimates for Birkhoff normal forms

TL;DR

This paper develops a rigorous, computer-assisted method to estimate stability times near elliptic equilibria in Hamiltonian systems using Birkhoff normal forms, with applications to celestial mechanics models.

Contribution

It extends classical estimate schemes for Birkhoff normal forms to provide explicit, validated lower bounds on stability times through computer assistance, including resonant cases.

Findings

Explicit stability time bounds for Hénon-Heiles model.

Application to Circular Planar Restricted Three-Body Problem.

Resonant normal forms can be more effective for Trojan asteroid models.

Abstract

Birkhoff normal forms are commonly used in order to ensure the so called "effective stability" in the neighborhood of elliptic equilibrium points for Hamiltonian systems. From a theoretical point of view, this means that the eventual diffusion can be bounded for time intervals that are exponentially large with respect to the inverse of the distance of the initial conditions from such equilibrium points. Here, we focus on an approach that is suitable for practical applications: we extend a rather classical scheme of estimates for both the Birkhoff normal forms to any finite order and their remainders. This is made for providing explicit lower bounds of the stability time (that are valid for initial conditions in a fixed open ball), by using a fully rigorous computer-assisted procedure. We apply our approach in two simple contexts that are widely studied in Celestial Mechanics: the H\'enon-Heiles model and the Circular Planar Restricted Three-Body Problem. In the latter case, we adapt our scheme of estimates for covering also the case of resonant Birkhoff normal forms and, in some concrete models about the motion of the Trojan asteroids, we show that it can be more advantageous with respect to the usual non-resonant ones.