1. Letov, A.M., Stability in Nonlinear Control Systems, Princeton: Princeton Univ. Press, 1961.
2. Lurie, A.I. and Postnikov, V.N., On the Stability Theory of Control Systems, Prikl. Mat. Mekh., 1944, vol. 8, no. 3, pp. 246–248.
3. Malkin, I.G., Theory of Stability of Motion, Moscow: Nauka, 1966.
4. Rozenvasser, E.N., The Absolute Stability of Nonlinear Systems, Autom. Remote Control, 1963, vol. 24, no. 3, pp. 283–291.
5. Yakubovich, V.A., The Solution of Certain Matrix Inequalities in Automatic Control Theory, Sov. Math., 1962, vol. 3, no. 2, pp. 620–623.