Existence of periodic orbits for vector fields via Fuller index and the averaging method. Benevieri, Pierluigi; Gavioli, Andrea; Villarini, Massimo — 2004 Electronic Journal of Differential Equations (EJDE) [electronic only]
Smooth linearization of centres Massimo Villarini — 2000 Annales de la Faculté des sciences de Toulouse : Mathématiques