1. Linear Operators;Dunford,1958
2. Positive Solutions of Differential, Difference and Integral Equations;Agarwal,1999
3. Multiple positive solutions of some boundary value problems;Erbe;J. Math. Anal. Appl.,1994
4. On the existence of positive solutions of ordinary differential equations;Erbe,1994
5. On the existence of positive solutions of nonlinear second order differential equations;Lian,1996