Xuying Zhao, Shipeng Mao and Zhong-Ci Shi. (2010). Adaptive Quadrilateral and Hexahedral Finite Element Methods with Hanging Nodes and Convergence Analysis.
            <i>Journal of Computational Mathematics</i>. <i>28</i> (5).
            621-644.
         doi:10.4208/jcm.1001-m3006