1. Danby, J. M. A.: 1962, Fundamentals of Celestial Mechanics, Macmillan, New York.
2. Fédoriouk, M.: 1987, Méthodes Asymptotiques pour les Equations Différentielles Ordinaires Linéaires, MIR, Moscow.
3. Grotta Ragazzo, G., Koiller, J. and Oliva, W. M., Motion of Massive Vortices in Two Dimensions (preprint).
4. Kyner, W. T.: 1968, ?Rigorous and Formal Stability of Orbits around an Oblate Planet?, Mem. AMS 81, 1.
5. Irigoyen, M. and Simó, C.: 1993, ?Non Integrability of the J 2 Problem?, Cel. Mech. and Dynam. Astron. 55, 281.