ALESSIO, Francesca, CALDIROLI, Paolo and MONTECCHIARI, Piero. On the existence of homoclinic orbits for the asymptotically periodic Duffing equation. Topological Methods in Nonlinear Analysis. Online. 1 December 1998. Vol. 12, no. 2, pp. 275 - 292. [Accessed 22 July 2024].