1. I. I. Gikhman, “On one N. N. Bogolyubov’s theorem,” Ukr. Mat. Zh., No. 4, 215–219 (1952).
2. M. A. Krasnosel’skii and S. G. Krein, “On the averaging principle in nonlinear mechanics,” Usp. Mat. Nauk, 3, No. 10, 147–153 (1955).
3. J. Kurzweil and Z. Vorel, “On the continuous dependence of solutions of differential equations on a parameter,” Czech. Math. J., 7, No. 4, 568–583 (1957).
4. A. M. Samoilenko, “On one case of the continuous dependence of solutions of differential equations on a parameter,” Ukr. Mat. Zh., 14, No. 3, 289–298 (1962).
5. A. Yu. Levin, “Limit transition for nonsingular systems $$ \dot{X} $$ = An(t)X,” Dokl. Akad. Nauk SSSR, 176, No. 4, 774–777 (1967).