1. A. N. Krylov, On Some Differential Equations of Mathematical Physics Having Applications in Engineering (GITTL, Leningrad, 1950) [in Russian].
2. V. A. Chernyatin, Substantiation of the Fourier Method in Mixed Problems for Partial Differential Equations (Mosk. Gos. Univ., Moscow, 1991) [in Russian].
3. M. Sh. Burlutskaya and A. P. Khromov, Izv. Saratov. Univ. Nov. Ser. Mat. 11(4), 3–12 (2011).
4. M. A. Naimark, Linear Differential Operators (Ungar, New York, 1967; Nauka, Moscow, 1969).
5. V. A. Marchenko, Sturm-Liouville Operators and Their Applications (Nauk. Dumka, Kiev, 1977) [in Russian].