Uddin, M Alhaz, and M Abdus Sattar. “Second Approximate Solution of Duffing Equation With Strong Nonlinearity by Homotopy Perturbation Method”. GANIT: Journal of Bangladesh Mathematical Society 30 (August 31, 2011): 59–75. Accessed September 25, 2022. https://banglajol.info/index.php/GANIT/article/view/8504.