Singular points of the continuous spectrum of a two-dimensional periodic waveguide

One investigates the eigenfunctions of the continuous spectrum of a waveguide in the neighborhood of a special singular point of the continuous spectrum. One proves the existence of a complete collection of eigenfunctions of the continuous spectrum in the neighborhood of such a point.Translated from Problemy Matematicheskogo Analiza, No. 10, pp. 116–123, 1986.