1. L. V. Ovsiannikov, Group Analysis in Differential Equations (Translation Ed. by W. F. Ames, Academic Press, New York, 1982).
2. P. J. Olver, Applications of the Lie Groups to Differential Equations (Springer, New York, 1986).
3. G. W. Bluman and S. Kumei, Symmetries and Differential Equations (Springer, Berlin, 1989).
4. I. S. Krasil’shchik, V. V. Lychagin and A. M. Vinogradov, Geometry of Jet Spaces and Nonlinear Partial Differential Equations (Gordon and Breach, London, 1986).
5. M. J. Ablowitz and H. Segur, Soliton and the Inverse Scattering Transform (SIAM, Philadelphia, 1981).