... conditions, we make use of the fixed point theory. So, in the case of a Dirichlet problem, the set of solutions of 1.1 coincide with the set of the fixedpoints of the operator T : C1I → C1I, ... books of Bernfeld and Lakshmikantham 6 and Ladde et al. 7 the classical theory of the method of lower and upper solutions and the monotone iterativetechnique are given. This gives the solution ... g,z,w,andr monotone functions in some of their variables. The first case covers the periodic case and the second one separated boundaryconditions.In the proofs, a more general definition of lower and...