1. D J Struik,Lectures on classical differential geometry, 2nd Edition (Dover, New York, 1961)
2. M Spivak,A comprehensive introduction to differential geometry, (Publish or Perish, Boston, 1970) Vols. 1–4
3. C S Gardner, J M Greene, M D Kruskal and R M Miura,Phys. Rev. Lett. 19, 1095 (1967)
4. F Lund and T Regge,Phys. Rev. D14, 1524 (1976)
5. G L Lamb,Phys. Rev. Lett. 37, 235 (1976)