1. Agarwal R.P., Grace S.R., O’Regan D., Existence of positive solutions of semipositone Fredholm integral equations, Funkcial. Ekvac. 45 (2002), 223–235.
2. Agarwal R.P., O’Regan D., Wang P.J.Y., Positive Solutions of Differential, Difference and Integral Equations, Kluwer Academic Publishers, Dordrecht, 1999.
3. Agarwal R.P., O’Regan D., Infinite Interval Problems for Differential, Difference and Integral Equations, Kluwer Academic Publishers, Dordrecht, 2001.
4. Cabada A., The method of lower and upper solutions for second, third, fourth and higher order boundary value problems, J. Math. Anal. Appl. 185 (1994), 302–320.
5. Deimling K., Nonlinear Functional Analysis, Springer, New York, 1985.
6. Guo D., Laksmikannthan V., Nonlinear Problems in Abstract Cones, Academic Press, San Diego, 1988.
7. Lasota A., Opial Z., Sur les solutions périodiques des équations différentielles ordinaires, Ann. Polon. Math. 16 (1964), 69–94.
8. Rektorys K., Variational Methods in Mathematics, Science and Engineering, D. Reidel Publishing Co., Dordrecht, 1980.
9. Śeda V., Nieto J.J., Gera M., Periodic boundary value problems for nonlinear higher order ordinary differential equations, Appl. Math. Comput. 48 (1992), 71–82.
10. Torres P.J., Existence of one–signed periodic solution of some second–order differential equations via a Krasnosielskii fixed point theorem, J. Differential Equations 190 (2003), 643–662.
11. Zima M., Positive Operators in Banach Spaces and Their Applications, Wydawnictwo Uniwersytetu Rzeszowskiego, Rzeszów, 2005.
Google Scholar