1. M. V. Fedoryuk, Asymptotic Methods for Linear Ordinary Differential Equations (Nauka, Moscow, 1983) [in Russian].
2. Y. Shibuya, Global Theory of Second Order Linear Ordinary Differential Equations with a Polynomial Coefficient (Amsterdam, North-Holland, 1975).
3. W. Wasow, Asymptotic Expansions for Ordinary Differential Equations (Dover Publ., New York, 1987).
4. V. S. Buslaev and A. A. Fedotov, St. Petersburg Math. J. 6 (3), 495 (1995).
5. A. A. Fedotov and E. V. Shchetka, J. Math. Sci. (N. Y.) 224 (1), 157 (2017).