Multiplicity of eigenvalues in certain boundary value problems for the Helmholtz equation
Vasiliy A. Malakhov, Alexey V. Nazarov, А. С. Раевский, С. Б. Раевский · Computational Mathematics and Mathematical Physics · 2013
Boundary value problems for two-layer cylindrical waveguides (namely, for a circular two-layer closed waveguide and a dielectric waveguide in an unbounded medium) are considered. It is shown that the same eigenvalues determining the wave numbers of waves in the waveguides can correspond to different solutions of the boundary value problem.