Slyusarchuk, V. E. “Theorems on Instability of Systems With Respect to Linear Approximation”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 8 (August 25, 1996): 1104-1113. Accessed May 17, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5254.