[1] Choy-Tak Taam: Oscillation Theorems. Amer. J. Math. LXXIV (1952), 317-324. MR 0048669

[2] Barret J. H.: Second order complex differential equations with a real independent variable. Рас. J. Math. VIII (1958), 187-200. MR 0098213

[3] Hartman P.: Ordinary differential equations. John Wiley Sons, Inc., New York-London- Sydney, 1964. MR 0171038 | Zbl 0125.32102