1. Über Differential Ungleichungen;Bohl;J. reine und angev. Math.,1913
2. On Asymptotic Stability of Solutions of a System of Differential Equations;Lyashchenko;Reports of Academy of Science of USSR,1954
3. The Asymptotic Behavior of Solutions of Weakly Nonlinear Systems of Ordinary Differential Equations;Alekseev;Reports of Academy of Science of USSR,1960