Bisconti, Luca, and Marco Spadini. “Harmonic Perturbations With Delay of Periodic Separated Variables Differential Equations”. Topological Methods in Nonlinear Analysis 46, no. 1 (September 1, 2015): 261–281. Accessed July 3, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2015.046.