“…The supercloseness property, which follows from the supraconvergence results, plays an important rôle in postprocessing, recovery techniques for the gradient, and a posteriori error estimates (see [1,2,8,9,[16][17][18][19]29,30,35,36,38,52] and the references cited therein). In all these papers, either a uniform grid or a smooth transformation of a uniform grid (cf., e.g., [35]) has to be assumed in order to obtain optimal second-order convergence for the gradient in the L 2 (Ω)-norm if u ∈ H 3 (Ω).…”