Alessio, Francesca, Paolo Caldiroli, and Piero Montecchiari. 1998. “On the Existence of Homoclinic Orbits for the Asymptotically Periodic Duffing Equation”. Topological Methods in Nonlinear Analysis 12 (2):275-92. https://apcz.umk.pl/TMNA/article/view/TMNA.1998.042.