1. V. A. Belinskii and V. E. Zakharov, Zh. Eksp. Teor. Fiz.,75, 1953 (1978).
2. M. Born and E. Wolf, Principles of Optics, Pergamon Press (1970).
3. G. B. Whitham, Linear and Nonlinear Waves, Wiley-Interscience, New York (1974).
4. V. A. Fock, The Theory of Space, Time and Gravitation, Oxford (1964).
5. S. W. Hawking and G. F. R. Ellis, The Large Scale Structure of Space-Time, CUP (1973).