Fabry, C., Mawhin, J., & Nkashama, MN. (1986). A Multiplicity Result for Periodic-solutions of Forced Nonlinear 2nd-order Ordinary Differential-equations. Bulletin of the London mathematical society, 18, 173-180. https://doi.org/10.1112/blms/18.2.173 (Original work published 1986)