1. M. Fedoryuk, Asymptotic Analysis. Linear Ordinary Differential Equations, Springer-Verlag, Berlin, Heidelberg, 2009.
2. Y. Sibuya, Global Theory of a Second Order Linear Ordinary Differential Equation with a Polynomial Coefficient, North Holland/American Elsevier, 1975.
3. W. Wasow, Asymptotic Expansions for Ordinary Differential Equations, Dover Publications, New York, 1987.
4. V. S. Buslaev and A. A. Fedotov, “The Complex WKB Method for the Harper Equation”, St. Petersburg Math. J., 6 (1995), 495–517.
5. A. Fedotov and E. Shchetka, “The Complex WKB Method for Difference Equations in Bounded Domains”, J. Math. Sci, 224 (2017), 157–169.