1. Benettin, G., Cercignani, C., Galgani, L., and Giorgilli, A.: 1980,Lettere Nuovo Cimento 28, 1.
2. Bountis, T.: 1981,Physica 3D, 577.
3. Contopoulos, G.: 1980,Astron. Astrophys. 81, 198.
4. Contopoulos, G.: 1981,Astron. Astrophys. 102, 265.
5. Contopoulos, G.: 1983a,Astron. Astrophys. 117, 89.