Radzievskii, G. V. 1995. “Asymptotics of the Fundamental System of Solutions of a Linear Functional-Differential Equation With Respect to a Parameter”.
Ukrains’kyi Matematychnyi Zhurnal 47 (6): 811–836.
https://umj.imath.kiev.ua/index.php/umj/article/view/5475.