[1] D. D. Bainov, D. P. Mishev: 
Oscillation Theory for Neutral Differential Equations with Delay. Adam Hilger, 1991. 
MR 1147908 
[2] T. A. Chanturia, R. G. Koplatadze: On Oscillatory Properties of Differential Equations with Deviating Arguments. Tbilisi, Univ. Press, Tbilisi, 1977. (Russian)
[3] J. Dzurina, B. Mihalikova: 
A note on unstable neutral differential equations of the second order. Math. Fascic. 29 (1999), 17–22. 
MR 1724462 
[4] L. H. Erbe, Q. Kong, B. G. Zhang: 
Oscillation Theory for Functional Differential Equations. Marcel Dekker, New York, 1995. 
MR 1309905 
[5] I. Győri, G. Ladas: 
Theory of Delay Differential Equations with Applications. Clarendon Press, Oxford, 1991. 
MR 1168471 
[6] G. S. Ladde, V. Lakshmikantham, B. G. Zhang: 
Oscillation Theory of Differential Equations with Deviating Arguments. Dekker, New York, 1987. 
MR 1017244 
[8] J. S. Yu, B. G. Zhang: 
The existence of positive solution and bounded oscillation for second-order neutral differential equations of unstable type. J. Syst. Sci. Math. 16 (1996), 92–96. 
MR 1404160 
[9] B. G. Zhang: 
Oscillation of second order neutral differential equations. Kexue Tongbao 34 (1989), 563–566. 
MR 1020422 | 
Zbl 0661.34074