Chunxiao Wu & Justin W.L. Wan. (2020). Multigrid Methods with Newton-Gauss-Seidel Smoothing and Constraint Preserving Interpolation for Obstacle Problems.
            <i>Numerical Mathematics: Theory, Methods and Applications</i>. <i>8</i> (2).
            199-219.
         doi:10.4208/nmtma.2015.w08si