1. J. Miles, J. Fluid Mech. 106, 131 (1981).
2. Yu. A. Berezin, V. I. Karpman, Zh. Eksp. Teor. Fiz. 46, 1880 (1964) [Sov. Phys. JETP 19, 1265 (1964)].
3. V. V. Kuznetsov, V. E. Nakoryakov, B. G. Pokusaev, and I. R. Shreiber, Pis’ma Zh. Eksp. Teor. Fiz. 23, 194 (1976) [JETP Lett. 23, 172 (1976)].
4. I. L. Bogolyubskii, Pis’ma v Zh. Eksp. Teor. Fiz. 24, 184 (1976) [JETP Lett. 24, 160 (1976)].
5. G. B. Whitham, Linear and Nonlinear Waves (Wiley, New York, 1974; Mir, Moscow, 1977).