1. P. J. Olver:Applications of Lie Groups to Differential Equations (Springer, Berlin, 1986).
2. L. V. Ovsjannikov:Group Properties of Differential Equations (Siberian Sect. Acad. of Sciences, Novosibirsk, 1962), English translation byG. W. Bluman (Report of the University of Vancouver, Canada, 1967);Group Analysis of Differential Equations (Academic Press, New York, N.Y., 1982).
3. G. W. Bluman andS. Kumei:Symmetries and Differential Equations (Springer, Berlin, 1989).
4. R. Courant andD. Hilbert:Methods of Mathematical Physics (Interscience Publ., New York, N.Y., 1962).
5. V. I. Arnold:Geometrical Methods in the Theory of Differential Equations (Springer, Berlin, 1988).