1. Bellman R.,Stability Theory of Differential Equations, McGraw-Hill, 1953.
2. Bellman R. and K. Cooke,Asymptotic behavior of solutions of differential-difference equations, Memoris of the Amer. Math. Soc. No. 35, 1959.
3. Bellman R. and K. Cooke,Differential-Difference Equations, Academic Press, New York, 1963.
4. Cesari L.,Asymptotic Behavior and Stability Problems in Ordinary Differential Equations, Academic Press, 1963.
5. Halanay A.,Teoria Calitativa a Ecuatiilor Diferentiale, Editura Academiei Republici Populare Romina, Bucarest, 1963. Translation published by Academic Press, 1966.