1. Leehey, P.: On the existence of not necessarily unique solutions of a classical hyperbolic boundary value problem for non-linear second order partial differential equations in two independent variables. Ph. D. thesis, Brown University, June 1950.
2. Hartman, P., & A. Wintner: On hyperbolic partial differential equations. Amer. J. Math. 74, 834–864 (1952).
3. Diaz, J. B.: On an analogue of the Euler-Cauchy polygon method for the numerical solution of u
xy = F(x, y, u, ux, uy). Arch. Rational Mech. Anal. 1, No. 4, 357–390 (1958).
4. Jordan, C.: Cours d'Analyse, 3rd edit., vol. 3, pp. 369–371. 1915.
5. Picard, E.: Leçons sur quelques types simples d'équations aux dérivées partielles avec des applications à la physique mathematique.