1. A. Ambrosetti and P. H. Rabinowitz,Dual variational methods in critical point theory and applications, J. Funct. Anal14, 349?381 (1973).
2. G. Arioli and F. Gazzola,Periodic motions of an infinite lattice of particles with nearest neighbor interaction, to appear in Nonlinear Analysis TMA.
3. G. Arioli, F. Gazzola and S. Terracini,Multibump periodic motions of an infinite lattice of particles, to appear in Mathematische Zeitschrift.
4. V. Benci, G. F. Dell'Antonio and B. D'Onofrio,Index theory and stability of periodic solutions of Lagrangian systems, C. R. Acad. Sci. Paris Ser. I-Math315 (5), 583?588 (1992).
5. V. Benci and P. H. Rabinowitz,Critical points theorems for indefinite functional, Inv. Math.52, 241?273 (1979).