[1] A. S. Galbraith E. J. McShane, Gene B. Parrish: On the Solutions of Linear Secondorder Differential Equations. Proc. Nat. Acad. Sci., Vol. 53 (1965) pp. 247-249. DOI 10.1073/pnas.53.2.247 | MR 0174817

[2] A. С. Lazer: On the Asymptotitc Behavior of Solutions of the Differential Equation $ y" + p(x) y = 0$. Proc. Amer. Math. Soc., Vol. 16 (1965) pp. 1295-1298. MR 0186867

[3] Walter Leighton, Zeev Nehari: On the Oscillations of Solutions of Self-adjoint Linear Differential Equations of the Fourth Order. Trans. Amer. Math. Soc., Vol. 89 (1958) pp. 325-377. DOI 10.1090/S0002-9947-1958-0102639-X | MR 0102639