L2-error of extrapolation cascadic multigrid (excmg)

Based on an asymptotic expansion of finite element, an extrapolation cascadic multigrid method (EXCMG) is proposed, in which the new extrapolation and quadratic interpolation are used to provide a better initial value on refined grid. In the case of multiple grids, both superconvergence error in H1-norm and the optimal error in /2-norm are analyzed. The numerical experiment shows the advantage of EXCMG in comparison with CMG.