Expanded Mixed Finite Element Methods for
Linear Second-Order Elliptic Problems, I

Zhangxin Chen
Department of Mathematics, Box 156
Southern Methodist University
Dallas, Texas 75275--0156, USA.


We develop a new mixed formulation for the numerical solution of second-order elliptic problems. This new formulation expands the standard mixed formulation in the sense that three variables are explicitly treated: the scalar unknown, its gradient, and its flux (the coefficient times the gradient). Based on this formulation, mixed finite element approximations of the second-order elliptic problems are considered. Optimal order error estimates in the Lp- and H-s-norms are obtained for the mixed approximations. Various implementation techniques for solving the systems of algebraic equations are discussed. A postprocessing method for improving the scalar variable is analyzed, and superconvergent estimates in the Lp-norm are derived. The mixed formulation is suitable for the case where the coefficient of differential equations is a small tensor and does not need to be inverted.

This paper will appear in RAIRO Modèl. Math. Anal. Numér.

Contributed June 23, 1997.