“…However, in most of the papers that do not use this theory, the results was mainly dependent of the constant sign of the associated Green's function to the periodic or Dirichlet problems 5,6 in bounded domains and, more recently, for homoclinic solutions of second-order equations. 7,8 In Cabada and Dimitrov 9 and Graef et al, 10 the authors studied the existence of positive solutions to the periodic boundary value problems. In the first one, the authors obtained existence, multiplicity, and nonexistence results for the following nonlinear fourth-order problem with parameter dependence { u(k + 4) + Mu(k) = g(k) (u(k)) + c(k), k ∈ {0, … , T − 1}, u(i) = u(T + i), i = 0, … , 3, in which the Green's function is nonnegative and attains the zero value at some points of its set of definition.…”