On eigenfunction expansions for a class of irregular quadratic pencils of second-order differential operators
In the twice summable functions space a quadratic pencil of second-order differential operators with constant coefficients and separated boundary conditions, containing the spectral parameter, is considered. The conditions for the coefficients of the pencil and the vector-function \(f\), for which there is a two-fold expansion of \(f\) into biorthogonal Fourier series in terms of pencil eigenfunctions are found.
scientific article; zbMATH DE number 7357849