References
- Bellman , R and Cooke , K . 1963 . Differential-Difference Equations , New York : Academic Press .
- Gopalsamy , K and Ladas , G . 1990 . On the oscillation and asymptotic behavior of . Quarterly of Applied Mathematics , 48 : 433 – 440 .
- Gyori , I and Ladas , G . 1991 . Oscillation Theory of Delay Differential Equations with Applications , Oxford : Clarendon Press .
- Hale , J. K . 1977 . Theory of Functional-Differential Equations , New York : Springer .