1. R. P. Agarwal, Boundary Value Problems for Higher Order Differential Equations, World Scientific, Singapore, 1986.
2. R. P. Agarwal, D. O’Regan and P. J. Y. Wong, Positive Solutions of Differential, Difference and Integral Equations, Kluwer, Dordrecht, 1999.
3. R. P. Agarwal, D. O’Regan and P. J. Y. Wong, Constant-Sign Solutions of Systems of Integral Equations, Springer, New York, 2013.
4. C. J. Chyan and J. Henderson, Positive solutions for singular higher order nonlinear equations, Diff. Eqs. Dyn. Sys. 2, 153–160 (1994).
5. P. W. Eloe and J. Henderson, Positive solutions for higher order ordinary differential equations, Electronic J. of Differential Equations 3, 1–8 (1995).