The ultraconvergence of derivative for bicubic finite element

It is for the first time that we obtained the O(h6)O(h6) convergence of the derivatives for bicubic finite element by a new recovery technique proposed in Meng and Zhu [L.X. Meng, Q.D. Zhu, The ultraconvergence derivative recovery technique for odd-degree elements – a computer-based test result, Natur. Sci. J. 25 (2) (2003) 117–121]. This numerical postprocessing result is theoretically justified by using projection interpolant and the theory of discrete Green’s function in this paper.