Periodic solutions to second order differential equations of Liénard type with jumping nonlinearities Alessandro Fonda; Fabio Zanolin — 1987 Commentationes Mathematicae Universitatis Carolinae
A continuous version of Liapunov's convexity theorem Arrigo Cellina; Giovanni Colombo; Alessandro Fonda — 1988 Annales de l'I.H.P. Analyse non linéaire