SamoilenkoA. M. “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, Vol. 54, no. 11, Nov. 2002, pp. 1505-17, https://umj.imath.kiev.ua/index.php/umj/article/view/4188.