Dinh, Nguyen Van. “On a Variant of the Asymptotic Procedure (II: Weakly Nonlinear Non-Autonomous Systems)”. Vietnam Journal of Mechanics 26, no. 2 (July 1, 2004): 76–82. Accessed April 19, 2024. https://vjs.ac.vn/index.php/vjmech/article/view/5691.