1. Arutyunov, A.V., Covering mappings in metric spaces and fixed points, Dokl. Math., 2007, vol. 76, no. 2, pp. 665–668.
2. Krasnosel’skii, M.A., Vainikko, G.M., Zabreiko, P.P., et al., Priblizhennoe reshenie operatornykh uravnenii (Approximate Solution of Operator Equations), Moscow: Nauka, 1969.
3. Trenogin, V.A., Funktsional’nyi analiz (Functional Analysis), Moscow: Nauka, 1980.
4. Perov, A.I., Cauchy problem for systems of ordinary differential equations, in Priblizhennye metody resheniya differentsial’nykh uravnenii (Approximate Solution Methods for Differential Equations), Kiev: Naukova Dumka, 1964, vol. 2, pp. 115–134.
5. Perov, A.I., Generalized contraction mapping principle, Vestn. Voronezh. Gos. Univ. Ser. Fiz. Mat., 2005, no. 1, pp. 196–207.