1. Cabada, A.: Green’s Functions in the Theory of Ordinary Differential Equations. Springer Briefs in Mathematics. Springer, New York (2014)
2. Cabada, A., López-Somoza, L.: Relationship between Green’s functions for even order linear boundary value problems. In: Nonlinear Analysis and Boundary Value Problems, pp. 243–263 Springer Proceedings in Mathematics & Statistics, vol. 292. Springer, Cham (2019)
3. Cabada, A., López-Somoza, L.: Spectral characterization of the constant sign Green’s functions for periodic and Neumann boundary value problems of even order. Differ. Equ. Appl. 14(2), 335–347 (2022)
4. De Coster, C., Habets, P.: Two-Point Boundary Value Problems: Lower and Upper Solutions. Mathematics in Science and Engineering, vol. 205. Elsevier, Amsterdam (2006)
5. Heikkilä, S., Lakshmikantham, V.: Monotone Iterative Techniques for Discontinuous Nonlinear Differential Equations. Monographs and Textbooks in Pure and Applied Mathematics. Marcel Dekker, New York (1994)