On the partial differential equation u ${}_{x}^{2}$ uxx+2uxuyuxy+u ${}_{y}^{2}$ uyy=0

Aronsson, G., Extension of functions satisfying Lipschitz conditions.Arkiv för Matematik 6, 551–561 (1967).

Bers, L., Mathematical aspects of subsonic and transonic gas dynamics. Wiley, New York. 1958.

Coddington, E. A., andLevinson, N., Theory of ordinary differential equations. McGraw-Hill, New York, 1955.

Courant, R., andHilbert, D., Methods of mathematical physics, vol. II. Interscience, New York, 1962.

Friedman, A., Partial differential equations of parabolic type. Prentice-Hall, N.J., 1964.

Garabedian, P. R., Partial differential equations. Wiley, New York, 1964.

Goursat, E., Cours d’analyse mathématique, T. III (5th ed.), Gauthier-Villars, Paris, 1942.

Petrovsky, I. G., Lectures on partial differential equations. Interscience, New York, 1954.

—, Vorlesungen über die Theorie der gewöhnlichen Differentialgleichungen. Teubner, Leipzig, 1954.

Struik, D. J., Differential geometry (2nd ed.), addison-Wesley, Mass., 1961.