Martynyuk, A. A., and Yu. A. Martynyuk-Chernienko. “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.