1. Über die Integration durch bestimmte Integrale von einer Klasse linearer partieller Differential gleichungen;Lie;Arch. Math.,1881
2. Group Analysis of Differential Equations;Ovsiannikov,1982
3. Transformation Groups Applied to Mathematical Physics;Ibragimov,1985
4. Applications of Lie Groups to Differential Equations;Olver,1986
5. Nonlocal symmetries. Heuristic approach;Akhatov;J. Sov. Math.,1991