[1] Kiguradze I. T.: Zametka ob ograničennosti rešenij differenciaľnych uravnenij. Trudy Tbilisskogo gosudarstvennogo universiteta, t. 110, 1965 (103-107). MR 0208089

[2] Wong J. S.: Boundedness theorems for certain general second order non-linear differential equations. Monatshefte für Mathematik, 71 (1967) (80-86). MR 0204768 | Zbl 0163.10603

[3] Wong J. S., Burton T. A.: Some properties of solutions of $u" + a (t) f(u) g (u') = 0$. Monatshefte für Mathematik, (69) 1965, (368-374). MR 0186885

[4] Waltman P.: Some properties of solutions of $u" + a(t) f(u)= 0$. Monatshefte für Mathematik, 67 (1963), (50-54). MR 0147700

[5] Bellman R.: Stability theory of ordinary differential equations. McGraw-Hill, New York, 1953. MR 0061235