A posteriori error estimations of a coupled mixed and standard Galerkin method for second order operators

In this paper, we consider a discretization method proposed by Wieners and Wohlmuth [The coupling of mixed and conforming finite element discretizations, in: Domain Decomposition Methods, vol. 10, Contemporary Mathematics, vol. 218, American Mathematical Society, Providence RI, 1998, pp. 547–554] (see also [R.D. Lazarov, J. Pasciak, P.S. Vassilevski, Iterative solution of a coupled mixed and standard Galerkin discretization method for elliptic problems, Numer. Linear Algebra Appl. 8 (2001) 13–31]) for second order operators, which is a coupling between a mixed method in a subdomain and a standard Galerkin method in the remaining part of the domain. We perform an a posteriori error analysis of residual type of this method, by combining some arguments from a posteriori error analysis of Galerkin methods and mixed methods. The reliability and efficiency of the estimator are proved. Some numerical tests are presented and confirm the theoretical error bounds.