“…In addition to the spectral methods analyzed in the article, it can be the boundary integral method, the finite-difference method, the finite-element method, and etc. The examples considered here have already been investigated previously in [8,9,14]. For completeness, we present the main results for simple cases, writing out the whole series of roots of the eigenvalues , Re( ) 0 k k τ τ ≥ , taking into consideration the degree of degeneracy, which is important for the correct writing of system (12).…”