Rigorous Derivation of the Wave Kinetic Equation for full $\beta$-FPUT System

Abstract

The Fermi--Pasta--Ulam--Tsingou (FPUT) system, describing the evolution of $N$ coupled harmonic oscillators, has been the subject of much attention since the 1950's when experiments which contradicted predictions of thermalization of the system. A full explanation of this behavior is still not fully known. Here, we rigorously derive the corresponding wave kinetic equation, which provides a precise evolution of the statistics for the FPUT system and demonstrates thermalization in an appropriate regime. In particular, we justify the kinetic equation for the 4-wave $\beta$-FPUT system in the kinetic limit $N \to \infty$ and $\beta \to 0$ for weakly nonlinear scaling laws $\beta \sim N^{-\gamma}$, reaching times up to $T_{\mathrm{kin}}^{2/3}$, where $T_{\mathrm{kin}}$ represents the kinetic (thermalization) timescale. While we use a typical diagrammatic expansion to derive the kinetic equations, few works have dealt with nonlinearities with non-resonant terms, which are not part of the kinetic equation, which is the major novelty of this work. The only other such work \cite{DIP25} made use of a normal form method to push the non-resonant terms to higher order nonlinearities. Here, we directly incorporate the non-resonant terms into the diagrammatic expansion and demonstrate corresponding gains. This method can be adapted to other 4-wave non-resonant nonlinearities.