1. Q. Chuanxi, G. Ladas, “Oscillation in differential equations with positive and negative coefficients,” Can. Math. Bull. 33, 442–450 (1990).
2. R. E. Edwards, Functional analysis, Holt, Rinehart and Winston Inc., New York (1965).
3. I. Gyori, G. Ladas, Oscillation theory of delay differential equation with application, Clarendon Press, Oxford (1991).
4. W. T. Li, H. S. Quan, “Oscillation of higher order neutral differential equations with positive and negative coefficients,” Ann. Different. Equat. 2, 70–76 (1995).
5. W. T. Li, J. Yan, “Oscillation of first-order neutral differential equations with positive and negative coefficients,” Collect. Math. 50, 199–209 (1999).