Feng, X., Y. He, and J. Meng. “Application of Homotopy Perturbation Method to the Bratu-Type Equations”. Topological Methods in Nonlinear Analysis, vol. 31, no. 2, June 2008, pp. 243-52, https://apcz.umk.pl/TMNA/article/view/TMNA.2008.015.