[1] G. J. Butler: Hille-Wintner comparison theorems for second order ordinary differential equations. Proc. Amer. Math. Soc. 76 (1979), 51 - 59. DOI 10.1090/S0002-9939-1979-0534390-5 | MR 0534390

[2] G. J. Etgen, R. T. Lewis: A Hille-Wintner comparison theorem for second order differential systems. Czechoslovak Math. J. 30 (1980), 98-107. MR 0565912 | Zbl 0429.34033

[3] G. J. Etgen, J. F. Pawlowski: A comparison theorem and oscillation criteria for second order differential systems. Pacific J. Math. 72 (1977), 59-69. DOI 10.2140/pjm.1977.72.59 | MR 0450675 | Zbl 0373.34016

[4] E. Hille: Lectures on ordinary differential equations. Addison-Wesley Publishing Company, Reading, Massachusetts, 1969. MR 0249698 | Zbl 0179.40301

[5] M. R. S. Kulenovič: Hille-Wintner type comparison theorems for differential inequalities. to appear. MR 0695113

[6] С. A. Swanson: Comparison and oscillation theory of linear differential equations. Academic Press, New York, 1968. MR 0463570 | Zbl 0191.09904

[7] C. T. Taam: Nonoscillatory differential equations. Duke Math. J. 19 (1952), 493 - 497. DOI 10.1215/S0012-7094-52-01951-0 | MR 0051994

[8] D. Willett: On the oscillatory behavior of the solutions of second order linear differential equations. Ann. Polon. Math. 21 (1969), 175-194. DOI 10.4064/ap-21-2-175-194 | MR 0249723 | Zbl 0174.13701