Positive solutions Boundary value problems for second-order elliptic equations Ecuaciones diferenciales ordinarias Ecuaciones diferenciales parciales Indefinite nonliearities No linealidades de signo indefinido Nonlinear boundary value problems One dimensional problems Ordinary differential equations Partial differential equations Periodic solutions Positive solutions of nonlinear boundary value problems Problemas elípticos Problemas singulares Semilinear elliptic equations Singular nonlinear boundary value problems Soluciones estrictamente positivas Sub and supersolutions Sub y super soluciones p-Laplaciano Indefnite nonlinearities One-dimensional singular problems p-Laplacian