Samoilenko , A. M. and Kulik , V. L. (1992) “Concerning existence of the Lyapunov sign-variable functions for linear systems of differential equations”, Ukrains’kyi Matematychnyi Zhurnal, 44(1), pp. 100-106. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/7800 (Accessed: 2June2024).