1. R. P. Agarwal, M. Bohner, and W. T. Li, Nonoscillation and Oscillation : Theory of Functional differential Equations (Marcel Dekker, New York, 2004).
2. L. H. Erbe, Q. Kong, and B. G. Zhang, Oscillation Theory for Functional differential Equations (Marcel Dekker, New York, 1995).
3. I. Gyori and G. Ladas, Oscillation Theory of Delay differential Equations with Applications (Clarendon Press, Oxford, 1991).
4. J. K. Hale, Theory of Functional differential Equations (Springer-Verlag, New York, 1987).
5. Oscillation theorems for certain neutral functional differential equations