S. G. Kondratenya, I. N. Klimashevskaya, T. I. Shilo, “Existence and construction of polar solutions of an autonomous system of two differential equations”, Differ. Uravn., 24:12 (1988), 2170–2172
2.
S. G. Kondratenya, E. G. Prolisko, T. I. Shilo, “On the existence of polar solutions to first-order differential equations”, Differ. Uravn., 24:10 (1988), 1824–1826
