1. S. F. Freschenko, N. I. Shkil', and L. D. Nikolenko, Asymptotic Methods in the Theory of Differential Equations [in Russian], Naukova Dumka, Kiev (1966).
2. N. I. Shkil', ?Asymptotic methods in the theory of linear differential equations with slowly varying coefficients,? Author's abstract of Ph. D. thesis, Kiev (1968).
3. G. D. Birkhoff, ?On the asymptotic character of the solutions of certain linear differential equations containing a parameter,? Trans. Amer. Math. Soc.,9, 219?271 (1908).
4. I. S. Gradshtein, ?Linear equations with variable coefficients and small parameters in the higher derivatives,? Matem. Sb.,27 (69), No. 1 (1950).
5. Yu. L. Rabinovich and M. M. Khapaev, ?Linear equations with a small parameter in the higher derivative in a neighborhood of a regular singularity,? Dokl. Akad. Nauk SSSR,129, No, 2 (1959).