Eigenvalues of the linearized 2D Euler equations via Birman-Schwinger and Lin's operators

TL;DR

This paper investigates spectral instability of steady states in the 2D Euler equations using Birman-Schwinger operators and Fredholm determinants, providing new characterizations of unstable eigenvalues and an alternative proof of Lin's instability theorem.

Contribution

It introduces a novel spectral analysis framework for the linearized 2D Euler equations using Birman-Schwinger and 2-modified determinants, offering new insights into eigenvalue instability criteria.

Findings

Characterizes unstable eigenvalues via zeros of a Fredholm determinant.

Provides an alternative proof of Lin's instability theorem.

Links eigenvalues to the spectrum of a limiting elliptic operator.

Abstract

We study spectral instability of steady states to the linearized 2D Euler equations on the torus written in vorticity form via certain Birman-Schwinger type operators $K_{\lambda}(\mu)$ and their associated 2-modified perturbation determinants $\mathcal D(\lambda,\mu)$. Our main result characterizes the existence of an unstable eigenvalue to the linearized vorticity operator $L_{\rm vor}$ in terms of zeros of the 2-modified Fredholm determinant $\mathcal D(\lambda,0)=\det_{2}(I-K_{\lambda}(0))$ associated with the Hilbert Schmidt operator $K_{\lambda}(\mu)$ for $\mu=0$. As a consequence, we are also able to provide an alternative proof to an instability theorem first proved by Zhiwu Lin which relates existence of an unstable eigenvalue for $L_{\rm vor}$ to the number of negative eigenvalues of a limiting elliptic dispersion operator $A_{0}$.