Qi, Huang, and Dong Yujun. 2005. “Limiting Cases of Asymptotically Positive Linear Conditions and Solvability of Sturm-Liouville Boundary-Value Problems of Duffing Equations”. Topological Methods in Nonlinear Analysis 26 (2):367-84. https://apcz.umk.pl/TMNA/article/view/TMNA.2005.037.