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2. I. G. Petrovskii, Partial Differential Equations (GITTL, Moscow, 1961; Saunders, Philadelphia, Pa., 1967).
3. V. I. Smirnov, A Course of Higher Mathematics (Gostekhizdat, Moscow, 1953; Addison-Wesley, Reading, Mass., 1964), Vol. 4.
4. O. A. Ladyzhenskaya, Mixed Problems for Hyperbolic Equations (Gostekhizdat, Moscow, 1953) [in Russian].
5. V. A. Il’in, Selected Works (Maks-press, Moscow, 2008), Vol. 1 [in Russian].