[1] DŽURINA J.: Comparison theorems for nonlinear ODEs. Math. Slovaca 42 (1992), 299-315. MR 1182960

[2] KIGURADZE I. T.: On the oscillation of solutions of the equation $d^m u/dt^m +a(t)|u|^n \sgn u = 0$. (Russian), Mat. Sb. 65 (1964), 172-187. MR 0173060 | Zbl 0135.14302

[3] LADDE G. S., LAKSHMIKANTHAM V., ZHANG B. G.: Oscillation Theory of Differential Equations with Deviating Arguments. Dekker, New York, 1987. MR 1017244 | Zbl 0832.34071

[4] OHRISKA J.: On the oscillation of differential equation of second order. Czechoslovak Math. J. 39 (1989), 16-23. MR 0983480

[5] PHILOS, CH. G., SFICAS Y. G.: Oscillatory and asymptotic behavior of second and third order retarded differential equations. Czechoslovak Math. J. 32 (1982), 169-182. MR 0654054 | Zbl 0507.34062

[6] PHILOS, CH. G.: Oscillatory and asymptotic behavior of the bounded solutions of differential equations with deviating argument. Hiroshima Math. J. 8 (1978), 31-48. MR 0481368

[7] TRENCH W. F.: Canonicals form and principal systems for general disconjugate equations. Trans. Amer. Math. Soc. 189 (1974), 319-327. MR 0330632