The gendarme algorithm for adaptive mesh refinement

We present an adaptive two-level finite element method. The main idea of the algorithm is to guide the local mesh refinement of the coarser mesh by a finer mesh called gendarme mesh. Two different estimators are involved in the algorithm: a standard a posteriori error estimator of residual type is used to control the error in the gendarme mesh; a simpler estimator based on the difference of the solutions corresponding to the two meshes is employed to guide the local mesh refinemen