V. N. Monakov, “A reduction principle for certain strongly nonlinear systems of differential equations”, Differ. Uravn., 8:12 (1972), 2255–2256
2.
V. N. Monakov, “An analogue of Perron's theorem for strongly nonlinear systems of differential equations”, Differ. Uravn., 8:11 (1972), 2096–2097
3.
V. N. Monakov, “The existence of a locally invariant surface for certain strongly nonlinear systems of differential equations”, Differ. Uravn., 8:10 (1972), 1772–1778