# Instability of unidirectional flows for the 2D $\alpha$-Euler equations

## TL;DR

This paper investigates the linear stability of unidirectional flows in the 2D α-Euler equations on a torus, revealing conditions under which these flows are unstable and characterizing their eigenvectors using continued fractions.

## Contribution

It provides a necessary and sufficient instability criterion for unidirectional flows in the 2D α-Euler equations, employing continued fractions techniques.

## Key findings

- Unidirectional flows with a specific geometric condition are linearly unstable.
- Derived a complete characterization of eigenvectors associated with unstable modes.
- Established a criterion for positive eigenvalues based on continued fractions.

## Abstract

We study stability of unidirectional flows for the linearized 2D $\alpha$-Euler equations on the torus. The unidirectional flows are steady states whose vorticity is given by Fourier modes corresponding to a vector $\mathbf p \in \mathbb Z^{2}$. We linearize the $\alpha$-Euler equation and write the linearized operator $L_{B} $ in $\ell^{2}(\mathbb Z^{2})$ as a direct sum of one-dimensional difference operators $L_{B,\mathbf q}$ in $\ell^{2}(\mathbb Z)$ parametrized by some vectors $\mathbf q\in\mathbb Z^2$ such that the set $\{\mathbf q +n \mathbf p:n \in \mathbb Z\}$ covers the entire grid $\mathbb Z^{2}$. The set $\{\mathbf q +n \mathbf p:n \in \mathbb Z\}$ can have zero, one, or two points inside the disk of radius $\|\mathbf p\|$. We consider the case where the set $\{\mathbf q +n \mathbf p:n \in \mathbb Z\}$ has exactly one point in the open disc of radius $\mathbf p$. We show that unidirectional flows that satisfy this condition are linearly unstable. Our main result is an instability theorem that provides a necessary and sufficient condition for the existence of a positive eigenvalue to the operator $L_{B,\mathbf q}$ in terms of equations involving certain continued fractions. Moreover, we are also able to provide a complete characterization of the corresponding eigenvector. The proof is based on the use of continued fractions techniques expanding upon the ideas of Friedlander and Howard.