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.