# The $\beta$ Fermi-Pasta-Ulam-Tsingou Recurrence Problem

## TL;DR

This paper investigates the FPUT recurrence in the $eta$-FPUT chain, revealing how recurrence times depend on energy, nonlinearity, and soliton interactions, with distinctions between positive and negative $eta$ cases.

## Contribution

It provides a comprehensive numerical and analytical study of the $eta$-FPUT recurrence, including the continuum limit and soliton interactions, highlighting differences between positive and negative $eta$.

## Key findings

- Recurrence time $T_r$ depends on the parameter $S$ in a predictable way.
- In the continuum limit, $T_r$ scales as $|S|^{-1/2}$ for large $|S|$.
- Differences in energy mixing and recurrences between positive and negative $eta$ are explained by soliton-kink interactions.

## Abstract

We perform a thorough investigation of the first FPUT recurrence in the $\beta$-FPUT chain for both positive and negative $\beta$. We show numerically that the rescaled FPUT recurrence time $T_{r}=t_{r}/(N+1)^{3}$ depends, for large $N$, only on the parameter $S\equiv E\beta(N+1)$. Our numerics also reveal that for small $\left|S\right|$, $T_{r}$ is linear in $S$ with positive slope for both positive and negative $\beta$. For large $\left|S\right|$, $T_{r}$ is proportional to $\left|S\right|^{-1/2}$ for both positive and negative $\beta$ but with different multiplicative constants. In the continuum limit, the $\beta$-FPUT chain approaches the modified Korteweg-de Vries (mKdV) equation, which we investigate numerically to better understand the FPUT recurrences on the lattice. In the continuum, the recurrence time closely follows the $|S|^{-1/2}$ scaling and can be interpreted in terms of solitons, as in the case of the KdV equation for the $\alpha$ chain. The difference in the multiplicative factors between positive and negative $\beta$ arises from soliton-kink interactions which exist only in the negative $\beta$ case. We complement our numerical results with analytical considerations in the nearly linear regime (small $\left|S\right|$) and in the highly nonlinear regime (large $\left|S\right|$). For the former, we extend previous results using a shifted-frequency perturbation theory and find a closed form for $T_{r}$ which depends only on $S$. In the latter regime, we show that $T_{r}\propto\left| S\right|^{-1/2}$ is predicted by the soliton theory in the continuum limit. We end by discussing the striking differences in the amount of energy mixing as well as the existence of the FPUT recurrences between positive and negative $\beta$ and offer some remarks on the thermodynamic limit.