1. V. V. Rumyantsev, Vestn. Mosk. Univ., Ser. Mat. Mekh. Fiz. Astron. Khim., No. 4, 9 (1957).
2. C. Corduneanu, Rev. Roum. Math. Pures Appl. 9(3), 229 (1964).
3. A. Halanay, Differential Equations: Stability, Oscillations, Time-Lags (Academic, New York, 1966).
4. K. Peiffer and N. Rouche, J. Mec. 8(2), 323 (1969).
5. V. V. Rumyantsev and A. S. Oziraner, Stability and Stabilization of Motion Related with Respect to Some of Variables (Nauka, Moscow, 1987).