1. Diaz, J. B.: On an analogue of the Euler-Cauchy polygon method for the numerical solution of u
xyz
=f(x, y, u, u
x
, u
y
). Archive for Rational Mechanics and Analysis 1, No. 4, 357–390 (1958).
2. Hartman, P., & A. Wintner: On hyperbolic partial differential equations. Amer. J. Mathematics 74, 834–864 (1952).
3. 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.
4. Moore, R. H.: On approximate solutions of non-linear hyperbolic partial differential equations. Archive for Rational Mechanics and Analysis 6, No. 1, 75–88 (1960).