# Dynamics of breathers in the Gardner hierarchy: universality of the variational characterization

## TL;DR

This paper introduces a universal variational characterization of breather solutions across the Gardner hierarchy, demonstrating their existence, common elliptic ODE, and ill-posedness results for the hierarchy.

## Contribution

It establishes a universal variational framework for breathers in the Gardner hierarchy, showing they satisfy the same elliptic ODE regardless of hierarchy order.

## Key findings

- Breather solutions exist for all hierarchy levels.
- All breathers satisfy the same fourth order elliptic ODE.
- Ill-posedness results are proved for the entire hierarchy.

## Abstract

We present a new variational characterization of breather solutions of any equation of the \emph{focusing} Gardner hierarchy. This hierarchy is characterized by a nonnegative index $n$, and $2n+1$ represents the order of the corresponding PDE member. In this paper, we first show the existence of such breathers, and that they are solutions of the (2n+1)th-order Gardner equation. Then we prove a \emph{variational universality property}, in the sense that all these breather solutions satisfy the \emph{same} fourth order stationary elliptic ODE, regardless the order of the hierarchy member. This fact also characterizes them as critical points of the same Lyapunov functional, that we also construct here. As by product of our approach, we find breather solutions of the hierarchy of (2n+1)th-order mKdV equations, as well as a respective characterization of them as solutions of a fourth order stationary elliptic ODE. We also extend part of these results to the periodic setting, presenting new breather solutions for the 5th and 7th mKdV members of the hierarchy. Finally, we prove ill-posedness results for the whole Gardner hierarchy, by using appropiately their breather solutions.