1. Cesari, L., Asymptotic Behavior and Stability Problems in Ordinary Differential Equations, 2nd. ed. New York: Academic Press 1963.
2. Coddington, E. A.,& N. Levinson, Theory of Ordinary Differential Equations. New York: McGraw-Hill 1955.
3. Hochstadt, H., Differential Equations — A Modern Approach. Holt, Rinehart & Winston 1964.
4. Eckweiler, H.J., Nonlinear Differential Equations of the Van der Pol type with a Variety of Periodic Solutions, Studies in Nonlinear Vibration Theory. Institute for Mathematics and Mechanics, New York University, 1946.
5. Stoker, J.J., Nonlinear Vibrations. New York: Interscience Publishers 1950.