[1] W. J. Kim: 
Nonoscillatory Solutions of Class of n-th Order Linear Differential Equations. J. Differential Equations 27, 19-27 (1978). 
MR 0460792[2] D. L. Lovelady: 
On the Oscillatory Behavior of Bounded Solutions of Higher Order Differential Equations. J. Differential Equations, 19, 167-175 (1975). 
MR 0382781 | 
Zbl 0333.34030[3] M. Švec: Мonotone Solution of Some Differential Equations. Colloquium Mathematicum, XVIII, 1967, 7-21.