1. Chen, M.P., Lalli, B.S., Yu, J.S., Oscillation in neutral delay difference equations with variable coefficients, Comput. Math. Appl., 1995,29:5–11.
2. Chen, M.P., Zhang, B.G., Oscillation and comparison theorems of difference equations with positive and negative coefficients, Bull. Math. Inst. Academia Sinica, 1994,22:295–306.
3. Chuanxi, G., Ladas, G., Oscillation behavior of difference equations with positive and negative coefficients, Matematiche (Catania), 1989,44:293–309.
4. Gyori, I., Ladas, G., Oscillation Theory of Delay Differential Equations with Applications, Clarendon Press, Oxford, 1991.
5. Ladas, G., Oscillation of difference equations with positive and negative coefficients, Rocky Mountain J. Math., 1990,20:1051–1061.