SamoilenkoA. M. 2002. “On the Asymptotic Integration of a System of Linear Differential Equations With a Small Parameter in the Coefficients of a Part of Derivatives”. Ukrains’kyi Matematychnyi Zhurnal 54 (11), 1505-17. https://umj.imath.kiev.ua/index.php/umj/article/view/4188.