1. Caratheodory, C.: 1965,Calculus of Variations and Partial Differential Equations of the First-Order, Part I, Holden-Day, San Francisco.
2. Deprit, A.: 1981,Celestial Mechanics 24, p. 185.
3. Hough, M.E.: 1985,Celestial Mechanics 36, p. 1.
4. Hough, M.E.: 1986,Celestial Mechanics (to appear).
5. Lefschetz, S.: 1957,Differential Equations: Geometric Theory, Interscience Publishers, N.Y., Chapter VIII.