1. Bitsadze, A.V., Differ. Uravn., 1988, vol. 24, no. 5, pp. 825–831.
2. Dezin, A.A., Dokl. Akad. Nauk SSSR, 1954, vol. 96, no. 5, pp. 901–903.
3. Soldatov, A.P., Izv. Ross. Akad. Nauk, 1991, vol. 55, no. 5, pp. 1070–1099.
4. Soldatov, A.P., Differ. Uravn., 1989, vol. 25, no. 1, pp. 136–144.
5. Gakhov, F.D., Kraevye zadachi (Boundary Value Problems), Moscow: Nauka, 1963.