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3. Saari, D. G., “Singularities and Collisions of Newtonian Gravitational Systems,” Archiv. for Rational Mechanics, 49, No. 4, pp. 311–320, 1973.
4. 1. Marchal, C., "Qualitative Methods and Results in Celestial Mechanics," (survey paper) 26th International Astronautical Congress (Lisbonne, September 1975)
5. 2. also, ONERA, TP n?? 1975-77, 1975.