1. A. Wintner, ?A criterion of oscillatory stability,? Q. Appl. Math.,7, No. 1, 115?117 (1949).
2. P. Hartman, Ordinary Differential Equations, S. M. Hartman (1973).
3. C. A. Swanson, Comparison and Oscillation Theory of Linear Differential Equations, Academic Press, New York (1968).
4. P. Hartman, ?On nonoscillatory linear differential equations of second order,? Am. J. Math.,74, No. 2, 389?400 (1952).
5. V. A. Kondrat'ev, ?Sufficient conditions for oscillating and nonoscillating solutions of the equations y? + p(x)y = 0,? Dokl. Akad. Nauk SSSR,113, No. 4, 742?745 (1957).