1. Whitham, G. B.: Studies in Applied Mathematics (ed. A. H. Taub), vol. 7. Mathematical Association of America Studies in Mathematics. Englewood Cliffs, N.J.: Prentice-Hall 1971.
2. Choquet-Bruhat, Y.: Commun. math. Phys.12, 16 (1969).
3. Madore, J.: Commun. math. Phys.27, 291 (1972).
4. Bretherton, F. P.: Proc. Roy. Soc. (London) A302, 555 (1968).
5. Dougherty, J. P.: J. Plasma Phys.4, 761 (1970).