Antti Hannukainen, Sergey Korotov and Michal Křížek. (2010). Nodal $\mathcal{O}(h^4)$-Superconvergence in 3D by Averaging Piecewise Linear, Bilinear, and Trilinear FE Approximations.
            <i>Journal of Computational Mathematics</i>. <i>28</i> (1).
            1-10.
         doi:10.4208/jcm.2009.09-m1004