Superconvergence of least-squares mixed finite element for symmetric elliptic problems Original Research Article

We formulate a least-squares mixed finite element method over quadrilaterals. The superconvergence result obtaineded between the finite element approximation and our appropriate-chosen interpolation of the exact solution indicates an accuracy of O(hr+2) for the least-squares mixed finite element approximation if Raviart–Thomas or Brezzi–Douglas–Fortin–Marini elements of order r are employed with optimal error estimate of O(hr+1). Numerical results are included.