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2. G. B. Whitham, Linear and Nonlinear Waves, Wiley-Interscience, New York (1974).
3. L. V. Ovsyannikov, Group Analysis of Differential Equations [in Russian], Nauka, Moscow (1978).
4. N. Kh. Ibragimov, Transformation Groups in Mathematical Physics [in Russian], Nauka, Moscow (1983).
5. V. E. Zakharov, S. V. Manakov, S. P. Novikov, and L. P. Pitaevskii, The Theory of Solitons. The Inverse Scattering Method [in Russian], Nauka, Moscow (1980).