Marcelli, C., and F. Papalini. “Comparison Results and Existence of Bounded Solutions to Strongly Nonlinear Second Order Differential Equations”. Topological Methods in Nonlinear Analysis, vol. 36, no. 1, Sept. 2009, pp. 91-110, https://apcz.umk.pl/TMNA/article/view/TMNA.2009.031.