1. M. V. Keldysh, “On completeness of eigen-functions of some classes of not self-adjoint operators,” Usp. Mat. Nauk 26 (4), 15–41 (1971).
2. M. A. Naymark, Linear Differential Operators (Nauka, Moscow, 1969) [in Russian].
3. M. V. Keldysh, “On eigenvalues and eigenfunctions of certain classes of non-selfadjoint equations,” Dokl. Akad. Nauk SSSR 77, 11–14 (1951).
4. A. P. Khromov, “Finite-dimensional perturbations of Volterra operators,” Doctoral (Phys. Math.) Dissertation (Novosibirsk, 1973).
5. A. A. Shkalikov, “On completenes of eigenfunctions and associated function of an ordinary differential operator with separated irregular boundary conditions,” Funkts. Anal. Prilozh. 10 (4), 69–80 (1976).