Samoilenko AM, Kulik VL. Concerning existence of the Lyapunov sign-variable functions for linear systems of differential equations. Ukr. Mat. Zhurn. [Internet]. 1992Feb.4 [cited 2024Nov.24];44(1):100-6. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/7800