On Periodic Spectral Problem for a Functional-Differential Equation of Even Order

On Self-Adjoint Boundary Value Problems for a Functional Equation of Even Order

“…Usually such a boundary value problem is called focal [7]. A boundary value problem with periodic boundary conditions is also self-adjoint [8]. These problems are regular, the spectrum of such operators is real and discrete, the eigenfunctions are orthogonal and form a basis in the corresponding spaces.…”