MartynyukA. A., and Martynyuk-ChernienkoY. A. “Existence, Uniqueness, and Estimation of Solutions for a Set of Equations of Perturbed Motion”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 65, no. 2, Feb. 2013, pp. 273-95, https://umj.imath.kiev.ua/index.php/umj/article/view/2418.