1. S. F. Feshchenko, N. I. Shkil', and L. D. Nikolenko, Asymptotic Methods in the Theory of Linear Differential Equations [in Russian], Naukova Dumka, Kiev (1966).
2. M. I. Shkil', Asymptotic Methods for Differential Equations [in Ukrainian], Vishcha Shkola, Kiev (1971).
3. N. A. Sotnichenko and S. F. Feshchenko, ?Splitting of some systems of differential equation,? in: Approximate Methods of Mathematical Analysis [in Russian], Kiev Pedagogical Inst., Kiev (1979), pp. 124?133.
4. N. A. Sotnichenko and S. F. Feshchenko, ?Asymptotic splitting of some classes of singularly perturbed differential equations,? in: All-Union Conference on Asymptotic Methods in the Theory of Singularly Perturbed Equations, Alma-Ata, June 1979, abstracts of reports [in Russian], Nauka, Part 1 (1979), pp. 185?186.
5. N. A. Sotnichenko and S. F. Feshchenko, ?Asymptotic integration of differential equations,? Preprint Inst. Mat. Akad. Nauk Ukr. SSR 80.3, Kiev (1980).