Non-selfadjoint singular perturbations and spectral properties of the Orr–Sommerfeld boundary-value problem

A new approach to the analysis of the asymptotic behaviour (and in particular, of the degree of non-orthogonality) of the eigenfunctions and associated functions of non-selfadjoint singularly perturbed operators and boundary-value problems is suggested; the main attention is paid to the case when the spectrum fails to be lower semicontinuous under singular perturbations. As a model case of the transition from a discrete to a continuous spectrum a Sturm–Liouville problem with a small parameter multiplying the second derivative is considered. Spectrum localization is studied and the growth of the degree of non-orthogonality of the system of eigenfunctions and associated functions of the Orr–Sommerfeld problem as the viscosity vanishes is established.