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3. Ovsiannikov, L.V.: Group Analysis of Differential Equations. Academic Press, New York (1982)
4. Ibragimov, N.H.: Transformation Groups Applied to Mathematical Physics. D. Reidel Publishing Company, Dordrecht (1985)
5. Olver, P.J.: Applications of Lie Groups to Differential Equations. Springer, New York (1986)