1. A. C. Scott, F. Y. F. Chu, and D. W. McLaughlin, Proc. IEEE,61, 1443 (1973); G. B. Whitham, Linear and Nonlinear Waves, Wiley-Interscience, New York (1974).
2. R. F. Dashen, B. Hasslacher, and A. Neveu, Phys. Rev. D,10, 1414, 4130 (1974);11, 3424 (1975); R. Rajaraman, Phys. Rep.,21C, No. 5 (1975); R. Jackiw, Rev. Mod. Phys.,49, 681 (1977).
3. L. D. Faddeev and L. A. Takhtadzhyan, Usp. Mat. Nauk,29, 249 (1974); Teor. Mat. Fiz.,21, 160 (1974).
4. N. V. Mitskevich, Physical Fields in the General Theory of Relativity [in Russian], Nauka (1969).
5. W. Blaschke, Introduction to Differential Geometry [Russian translation], Gostekhizdat (1957);