1. Baumgarte, J.: 1972, Comp. Methods Appl. Mech. Engirt.
1, 1–16.
2. Baumgarte, J.: 1973, to appear in Celest. Mech.
3. Stiefel, E. L. and Scheifele, G.: 1971, Linear and Regular Celestial Mechanics, Springer-Verlag, Berlin-Heidelberg-New York.
4. Schwarz, H. R.: 1973, to appear in Comp. Methods Appl. Mech. Engin.