Koshlyakov, V. N. “Method of Averaging in the Problem of Stability of Motion of a Body Suspended from a String”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 4 (April 25, 2007): 467–475. Accessed June 13, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3321.