References
- Willet , D. 1976 . Some nonoscillation criteria for higher order nonlinear differential equations . Pacific J. Math. , 66 : 125 – 129 .
- Kartsatos , A. G. 1980 . Advanced Ordinary Differential Equations , Tampa : Mariner .
- Willet , D. 1980 . Disconjugacy tests for singular differential equations . SIAM J. Math. Anal. , 2