Martinezamores, P., Mawhin, J., Ortega, R., & Willem, M. (1991). Generic Results for the Existence of Nondegenerate Periodic-solutions of Some Differential-systems With Periodic Nonlinearities. Journal of Differential Equations, 91(1), 138-148. https://doi.org/10.1016/0022-0396(91)90135-V (Original work published 1991)