1. Periodic solutions of dissipative dynamical systems;Benci,1990
2. What is Mathematics?: An Elementary Approach to Ideas and Methods;Courant,1941
3. The forced spherical pendulum does have forced oscillations;Furi,1991
4. Über erzwungene Schwingungen bei endlichen Amplituden;Hamel,1922
5. Global results for the forced pendulum equation;Mawhin,2000