Periodic solutions of quadratic lagrangian systems on p -convex sets Annamaria Canino (1991) Annales de la Faculté des sciences de Toulouse : Mathématiques
Perturbations of Systems Describing the Motion of a Particle in Central Fields Christov, Ognyan, Dragnev, Dragomir (1995) Serdica Mathematical Journal The present paper deals with the KAM-theory conditions for systems describing the motion of a particle in central field.
Presymplectic lagrangian systems. II : the second-order equation problem Mark J. Gotay, James M. Nester (1980) Annales de l'I.H.P. Physique théorique