1. A. N. Tikhonov and A. A. Samarskii, Equations of Mathematical Physics (Nauka, Moscow, 1972; Pergamon Press, Oxford, 1964).
2. V. S. Vladimirov, Collection of Problems for Partial Differential Equations (Nauka, Moscow, 1982) [in Russian].
3. E. I. Moiseev and N. Yu. Kapustin, Dokl. Math. 66, 14–18 (2002) [Dokl. Akad. Nauk 385, 20–24 (2002)].
4. N. Yu. Kapustin, Differ. Equations 41, 1490–1492 (2005) [Differ. Uravn. 41, 1413–1415 (2005)].
5. L. S. Pontryagin, Izv. Akad. Nauk, Ser. Mat. 8, 243–280 (1944).