On 2D Euler equations. I. On the energy–Casimir stabilities and the spectra for linearized 2D Euler equations

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Abstract
In this paper, we study a linearized two-dimensional Euler equation. This equation decouples into infinitely many invariant subsystems. Each invariant subsystem is shown to be a linear Hamiltonian system of infinite dimensions. Another important invariant besides the Hamiltonian for each invariant subsystem is found, and is utilized to prove an ``unstable disk theorem'' through a simple Energy-Casimir argument. The eigenvalues of the linear Hamiltonian system are of four types: real pairs ($c,-c$), purely imaginary pairs ($id,-id$), quadruples ($\pm c\pm id$), and zero eigenvalues. The eigenvalues are computed through continued fractions. The spectral equation for each invariant subsystem is a Poincar\'{e}-type difference equation, i.e. it can be represented as the spectral equation of an infinite matrix operator, and the infinite matrix operator is a sum of a constant-coefficient infinite matrix operator and a compact infinite matrix operator. We have obtained a complete spectral theory