Le Suan Kan. “On Construction of Approximate Solutions for an Autonomous Difference Second Order Equation Describing Oscillating Processes With a Considerable Resistance Force”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 23, no. 6, Oct. 1971, pp. 778-81, https://umj.imath.kiev.ua/index.php/umj/article/view/8626.