Zheng-Jian Bai, Michael K. Ng and Liqun Qi. (2009). A Coordinate Gradient Descent Method for Nonsmooth Nonseparable Minimization.
            <i>Numerical Mathematics: Theory, Methods and Applications</i>. <i>2</i> (4).
            377-402.
         doi:10.4208/nmtma.2009.m9002s