1. Zeros of solutions of a second order nonlinear differential inequality;Wong,1991
2. Maximum Principles in Differential Equations;Protter,1967
3. A comparison principle for second order differential equations;Levin;Sov. Math. Dokl.,1960
4. Generalized maximum principle for higher order differential equations;Sheng;J. Math. Anal. and Appl.,1993
5. LaSalle's inequality and uniqueness theorems;Wong;Applicable Anal.,1992