Yanping Chen, Yao Fu, Huanwen Liu, Yongquan Dai and Huayi Wei. (2009). Recovery a Posteriori Error Estimates for General Convex Elliptic Optimal Control Problems Subject to Pointwise Control Constraints.
            <i>Journal of Computational Mathematics</i>. <i>27</i> (4).
            543-560.
         doi:10.4208/jcm.2009.27.4.018