1. G. B. Whigham, J. Fluid Mech.,22, 273 (1965); Proc. Roy. Soc.,A229, 6 (1967).
2. M. I. Lighthill, J. Inst. Math. Appl.,1, 269 (1965).
3. A Discussion of Nonlinear Theory of Wave Propagation in Dispersive Systems (Organized by M. I. Lighthill), Proc. Roy. Soc.,A229, 1 (1967).
4. C. Lanczos, Variational Principles of Mechanics, University of Toronto Press (1970).
5. G. N. Duboshin, Celestial Mechanics, Fundamental Problems and Methods [in Russian], Fizmatgiz, Moscow (1962).