SamusenkoP. F., and ShkiIN. I. “On Asymptotic Formulas for Solutions of Systems of Linear Differential Equations With a Degerate Matrix With Derivatives”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 9, Sept. 1996, pp. 1278–1285, https://umj.imath.kiev.ua/index.php/umj/article/view/5241.