1. I. M. Gel’fand and G. E. Shilov, Some Problems of the Theory of Differential Equations [in Russian], Fizmatgiz, Moscow (1958).
2. I. G. Petrowsky, “Über das Cauchyche Problem für Systeme von partiellen Differentialgleichungen,” Mat. Sb., 2, No. 5, 815–870 (1937).
3. J. Leray, Hyperbolic Differential Equations, Princeton University Press, Princeton (1952).
4. J. Hadamard, Le Probème de Cauchy et les Équations aux De´rive´es Partielles Line´aires Hyperboliques, Herman, Paris (1932).
5. A. N. Tikhonov and A. A. Samarskii, Equations of Mathematical Physics [in Russian], Nauka, Moscow (1977).