1. L. V. Ovsjannikov:Group Properties of Differential Equations (Novosibirsk, 1962).
2. L. V. Ovsjannikov:Group Analysis of Differential Equations (Academic Press, New York, N.Y., 1982).
3. P.J. Olver:Applications of Lie Groups to Differential Equations (Springer, Berlin, 1986).
4. D. H. Sattinger andO. Weaver:Lie Groups and Algebras, (Springer, New York, N.Y., 1986).
5. G. W. Bluman andS. Kumei:Symmetries and Differential Equations (Springer, Berlin, 1989).