Fu , Hongfei and Rui , Hongxing. (2015). A Priori Error Estimates for Least-Squares Mixed Finite Element Approximation of Elliptic Optimal Control Problems.
            <i>Journal of Computational Mathematics</i>. <i>33</i> (2).
            113-127.
         doi:10.4208/jcm.1406-m4396