  In this paper we will deduce several properties of the Green's functions
related to the Hill's equation coupled to various boundary value conditions. In
particular, the idea is to study the Green's functions of the second order
differential operator coupled to Neumann, Dirichlet, Periodic and Mixed
boundary conditions, by expressing the Green's function of a given problem as a
linear combination of the Green's function of the other ones. This will allow
us to compare different Green's functions when they have constant sign.
Finally, such properties of the Green's function of the linear problem will be
fundamental to deduce the existence of solutions to the nonlinear problem. The
results are derived from the fixed point theory applied to related operators
defined on suitable cones in Banach spaces.
