We present in this work two schemes of approximation for numerical solutions of PDEs. The first one is the maximum entropy method (max-ent) and the second one is the b-spline method. These methods let us impose a special kind of boundary conditions: periodic boundary conditions for unbounded domains. Some experiments need a large domain (or unbounded domain), however, this domain is divdided into some periodic cells. We develop a technique that let us simulate in the whole domain only doing a simulation in one cell. We apply this method for the resolution of second and fourth order problems (with periodic boundary conditions) like: Laplace, Kirchhoff plate and flexoelectricity.