VitrychenkoI. E. “On the Functional Polystability of Certain Essentially Nonlinear Nonautonomous Differential Systems”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 2 (February 25, 2000): 197-207. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4408.