1. A. S. Andreev, Prikl. Mat. Mekh. 48, 225 (1984).
2. Z. Artstein, J. Different. Equations 23(2), 216 (1977).
3. Method of Lyapunov Vector Functions in Stability Theory, Ed. by A. A. Voronov and V. M. Matrosov (Nauka, Moscow, 1987).
4. V. V. Rumyantsev and A. S. Oziraner, Stability and Stabilization of Motion with Respect to a Part of Variables (Nauka, Moscow, 1987).
5. J. P. La Salle and S. Lefshets, Stability by the Direct Liapunov Method with Applications (Academic, New York, 1961; Mir, Moscow, 1964).