Analysis of two-grid methods for reaction-diffusion equations by expanded mixed finite element methods

We present two efficient methods of two-grid scheme for the approximation of two-dimensional semilinear reaction-diffusion equations using an expanded mixed finite element method. To linearize the discretized equations, we use two Newton iterations on the fine grid in our methods. Firstly, we solve an original non-linear problem on the coarse grid. Then we use twice Newton iterations on the fine grid in our first method, and while in second method we make a correction on the coarse grid between two Newton iterations on the fine grid. These two-grid ideas are from Xu’s work (SIAM J. Sci. Comput. 1994; 15:231–237; SIAM J. Numer. Anal. 1996; 33:1759–1777) on standard finite element method. We extend the ideas to the mixed finite element method. Moreover, we obtain the error estimates for two algorithms of two-grid method. It is showed that coarse space can be extremely coarse and we achieve asymptotically optimal approximation as long as the mesh sizes satisfy H =O(h1/4) in the first algorithm and H =O(h1/6) in second algorithm. Copyright q 2006 John Wiley & Sons, Ltd.