Hongfei Fu, Hongxing Rui & Hui Guo. (1970). A Characteristic Finite Element Method for Constrained Convection-Diffusion-Reaction Optimal Control Problems.
            <i>Journal of Computational Mathematics</i>. <i>31</i> (1).
            88-106.
         doi:10.4208/jcm.1210-m3966