[1] Myshkis A.D. General theory of differential equations with a delayed argument // Uspekhi matematicheskikh nauk. - 1949. - 4, N5 (33). - P. 99-141.

[2] Vasilieva A.B., Rodionov A.M. Application of the perturbation method to an equation with a lagging argument in the case of a small lag // Proceedings of the seminar on the theory of differential equations with a deviating argument. - 1968. - 1. - P. 20-27.

[3] Ryabov Yu.A. The method of a small parameter in the theory of periodic solutions of differential equations with a delayed argument // Proceedings of the seminar on the theory of differential equations with a sloping argument. - 1962. -1. - P. 103-113.

[4] Kurdevanidze G.D. About the periodic mode of motion of a controlled object with a small delay. Automatic Control // Proceedings of the Institute of Systems and Control, Academy of Sciences of GSSR, Tbilisi. - 1974. - P. 168-181.

[5] Самойленко А.М., Шкiль М.I., Яковець В.П. Лiнiйнi системи диференцiальних рiвнянь з виродженням. – К.: Вища шк., 2000. – 294 с.

[6] Shkil N.I., Starun I.I., Yakovets V.P. Asymptotic integration of linear systems of ordinary differential equations. - K.:Vyshcha shk., 1989. - 287 p.

[7] Sibuya Y. Simplification of a system of linear ordinary differential equations about a singular point // Funkcialaj Ekvacioj. – 1962. – 4 . – P. 29-56.

[8] Самусенко П.Ф. Асимптотичне iнтегрування сингулярно збурених систем диференцiально-функцiональних рiвнянь з виродженнями. – К.: НПУ iменi М.П. Драгоманова, 2011. – 343 с.

[9] Kantorovich L.V., Akilov G.P. Functional Analysis. - M.: Nauka, 1977. - 744 p.

[10] Vasilieva A.B., Butuzov V.F. Singularly perturbed equations in critical cases. -M.: Izd-vo Mosk. univ., 1978. - 107 p.