1. Difference Equations and Inequalities;Agarwal,1992
2. Advanced Topics in Difference Equations;Agarwal,1997
3. Oscillation Theory of Delay Differential Equations with Applications;Györi,1991
4. Oscillatory properties of systems of first order linear delay differential inequalities;Gopalsamy;Pacific Math. J.,1987
5. Oscillations of delay differential equations;Gopalsamy;J. Austral. Math. Soc. Ser. B,1991