H. Langer et C. Tretter, SPECTRAL PROPERTIES OF THE ORR-SOMMERFELD PROBLEM, Proceedings of the Royal Society of Edinburgh. Section A. Mathematics, 127, 1997, pp. 1245-1261
In this paper, we study the Orr-Sommerfeld problem on a finite interva
l. It is shown that the eigenfunctions and associated functions form a
Bari basis in a suitable Hilbert space if the unperturbed velocity pr
ofile u is sufficiently smooth. To this end, the Orr-Sommerfeld proble
m is considered as a bounded perturbation of a certain self-adjoint sp
ectral problem.