1. Benevieri, P., Calamai, A., Furi, M., Pera, M.P.: A continuation result for forced oscillations of constrained motion problems with infinite delay, to appear in Adv. Nonlinear Stud.
2. Furi M., Pera M.P.: On the existence of forced oscillations for the spherical pendulum. Boll. Un. Mat. Ital. 4(B(7)), 381–390 (1990)
3. Furi, M., Pera, M.P.: The forced spherical pendulum does have forced oscillations. In: Delay Differential Equations and Dynamical Systems, Claremont, 1990, Lecture Notes in Mathematics, vol. 1475, pp. 176–182. Springer, Berlin, (1991)
4. Furi M., Pera M.P.: On the notion of winding number for closed curves and applications to forced oscillations on even-dimensional spheres. Boll. Un. Mat. Ital. 7-A(7), 397–407 (1993)
5. Gaines R., Mawhin J.: Coincidence Degree and Nonlinear. Differential Equations Lecture Notes in Mathematics, vol. 568. Springer, Berlin (1977)