1. J. Hadamard, Le problème de Cauchy et les équations aux dérivées partielles linéaires hyperboliques (Hermann, Paris, 1932; Nauka, Moscow, 1978).
2. V. I. Arnol’d, “Small denominators. I: Mappings of the circumference onto itself,” Izv. Akad. Nauk SSSR, Ser. Mat. 25(1), 21–86 (1961) [Am. Math. Soc. Transl., Ser. 2, 46, 213–284 (1965)].
3. A. V. Bitsadze, Some Classes of Partial Differential Equations (Nauka, Moscow, 1981; Gordon & Breach, New York, 1988).
4. V. P. Burskii, Methods of Analysis of Boundary Value Problems for General Differential Equations (Naukova Dumka, Kiev, 2002) [in Russian].
5. A. D. Ventsel’ and M. I. Freidlin, Fluctuations in Dynamical Systems under Small Random Perturbations (Nauka, Moscow, 1979); Engl. transl.: M. I. Freidlin and A. D. Wentzell, Random Perturbations of Dynamical Systems (Springer, New York, 1984).