1. E. Hille, Ordinary Differential Equations in the Complex Domain, A Wiley-Interscience publication, New York-London (1976).
2. J. Hadamar, Le Problème de Cauchy et les Équations aux Derivees Partielles Linéaires Hyperboliques, Hermann, Paris (1932).
3. R. Courant, Partial Differential Equations, New York-London (1962).
4. I. Petrovskii, Partial Differential Equations, Iliffe Books Ltd., London (1967).
5. S. V. Kowalewsky, “Zur Theorie der partiellen Differentialgleichungen,” J. reine angew. Math., 80, 1–32 (1875).