Further results on ultraconvergence derivative recovery for odd-order rectangular finite elements

For rectangular finite element, we give a superconvergence method by SPR technique based on the generalization of a new ultraconvergence record and the sharp Green function estimates, by which we prove that the derivative has ultra-convergence of order O(hk+3) (k ⩾ 3 being odd) and displacement has order of O(hk+4) (k ⩾ 4 being even) at the locally symmetry points.