1. A. Ambrosetti, V. Coti Zelati, “Closed orbits of fixed energy for singular hamiltonian systems”, Archive Rat. Mech. and Anal., to appear.
2. A. Ambrosetti, V. Coti Zelati, “Closed orbits of fixed energy for a class of N-body problems”, preprint S.N.S., April 1990
3. A. Ambrosetti, G. Mancini, “On a theorem by Ekeland and Lasry concerning the number of periodic hamiltonian trajectories”, J.Diff. Eq. 43, No 2 (1982) 249 – 256.
4. V.Benci “A geometrical index for the group S1 and some applications to the study of periodic solutions of ordinary differential equations” Comm. Pure and Applied Math., 34 (1981) 393-423
5. I. Ekeland, J.M.Lasry, “On the number of periodic trajectories for a Hamiltonian flow on a convex energy surface”, Ann. of Math. 112 (1980), 283 – 319.