1. Chuanxi Q. and Ladas, G., Oscillation of higher order neutral differential equation with variable coefficients, Math. Nachr., 1991, 150: 25–34.
2. Gopalsamy, K., Lalli, B. S. and Zhang, B. G., Oscillation of odd order neutral differential equations, Czechoslovak Math. J., 1992, 42: 313–323.
3. Grammatikopoulos, M. K., Ladas, G. and Meimarido, A., Oscillation and asymptotic behavior of higher order neutral equations with variable coefficients, Chinese Ann. Math. Ser. B, 1988, 9: 322–338.
4. Ladas, G. and Stavroulakis, I. P., On delay differential inequalities of higher order, Canad. Math. Bull., 1982, 25: 348–354.
5. Ladas, G., Qian Cheng, Yan Jurang, Oscillation of higher order neutral differential equations, Portugal Math., 1991, 48: 291–307.