“…The effectiveness of this technique relies heavily on the existence of an asymptotic expansion for the error. This technique has been well demonstrated in its applications to the finite element and the mixed finite element methods for elliptic partial differential equations [4,6,22,34,35], parabolic partial differential equations [18], integral and integro-differential equations [22,24,25,26,38], and to the boundary element methods and collocation methods in [37] and [21], respectively. The defect correction (Galerkin and Petrov-Galerkin) finite element by means of an interpolation postprocessing technique is another numerical method to obtain approximations of higher accuracy, which has been proved for a wide variety of models, see, for example, [22,23,5], and the references cited therein.…”