# Semi-analytic computations of the speed of Arnold diffusion along single resonances in a priori stable Hamiltonian systems

## TL;DR

This paper develops a semi-analytic model to predict the extremely slow evolution of action variables along resonances in a priori stable Hamiltonian systems, aligning well with numerical simulations and advancing understanding of Arnold diffusion.

## Contribution

It introduces a novel semi-analytic approach combining phase analysis and Melnikov integrals to accurately estimate Arnold diffusion speeds in multi-degree-of-freedom Hamiltonian systems.

## Key findings

- Model accurately predicts Arnold diffusion speed over several orders of magnitude.
- Only a small fraction of terms in the Nekhoroshev normal form significantly influence action evolution.
- Good agreement between semi-analytic predictions and numerical simulations.

## Abstract

Cornerstone models of Physics, from the semi-classical mechanics in atomic and molecular physics to planetary systems, are represented by quasi-integrable Hamiltonian systems. Since Arnold's example, the long-term diffusion in Hamiltonian systems with more than two degrees of freedom has been represented as a slow diffusion within the `Arnold web', an intricate web formed by chaotic trajectories. With modern computers it became possible to perform numerical integrations which reveal this phenomenon for moderately small perturbations. Here we provide a semi-analytic model which predicts the extremely slow-time evolution of the action variables along the resonances of multiplicity one. We base our model on two concepts: (i) By considering a (quasi-)stationary phase approach to the analysis of the Nekhoroshev normal form, we demonstrate that only a small fraction of the terms of the associated optimal remainder provide meaningful contributions to the evolution of the action variables. (ii) We provide rigorous analytical approximations to the Melnikov integrals of terms with stationary or quasi-stationary phase. Applying our model to an example of three degrees of freedom steep Hamiltonian provides the speed of Arnold diffusion, as well as a precise representation of the evolution of the action variables, in very good agreement (over several orders of magnitude) with the numerically computed one.