Prosenyuk, L. G. 1993. “Existence and Asymptotic Properties of Solutions of a Real System of Differential Equations Unresolved With Respect to the Derivative”. Ukrains’kyi Matematychnyi Zhurnal 45 (10), 1461–1464. https://umj.imath.kiev.ua/index.php/umj/article/view/5952.