1. M. A. Naimark, "A study of the spectrum and the expansion in eigenfunctions of a non-self-adjoint second-order differential operator on a semiaxis," Trudy Mosk. Matem. Obshch.,3, 181?270 (1954).
2. V. É. Lyantse, "A differential operator with spectral singularities. I," Matem. Sb.,64, 521?561 (1964).
3. V. É. Lyantse, "A differential operator with spectral singularities, II," Matem. Sb.,65, 47?103 (1964).
4. M. G. Gasymov, "Expansion for a non-self-adjoint Schrödinger equation in solutions of the scattering problem," Dokl. Akad. Nauk,179, No. 3, 518?521 (1968).
5. M. G. Gasymov, "Expansion of differential operators with continuous spectrum in eigenfunctions," Izv. Akad. Nauk Azerbaidzhan SSR, Ser. Fiz., Tekhn., i Matem. Nauk, No. 1, 2, 19?39 (1970).