Spectral Properties of a Fourth Order Differential Equation

The eigenvalue problem y(4)(λ, x) − (gy′)′(λ, x) = λ2y(λ, x) with boundary conditions y(λ, 0) = 0, y′′(λ, 0) = 0, y(λ, a) = 0, y′′(λ, a) + iαλy′(λ, a) = 0 is considered, where g ∈ C1[0, a] and α > 0. It is shown that the eigenvalues lie in the closed upper half-plane and on the negative imaginary axis. A formula for the asymptotic distribution of the eigenvalues is given and the location of the pure imaginary spectrum is investigated.