1. G. B. Whitham, Linear and Nonlinear Waves (Wiley, New York, 1974; Mir, Moscow, 1977).
2. E. Kamke, Gewöhnliche Differentialgleichungen (Academie, Leipzig, 1959; Nauka, Moscow, 1966).
3. P. M. Morse and H. Feshbach, Methods of Theoretical Physics (McGraw Hill, New York, 1953; Inostrannaya Literatura, Moscow, 1958, 1959), Vols. 1, 2.
4. S. P. Tsarev, Teor. Mat. Fiz. 122(1), 144 (2000).
5. T. Grava, Teor. Mat. Fiz. 122(1), 59 (2000).