Furi, Massimo, and Marco Spadini. “Multiplicity of Forced Oscillations for the Spherical Pendulum”. Topological Methods in Nonlinear Analysis 11, no. 1 (March 1, 1998): 147–157. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.1998.009.