@article{NUMA_1983__42_132861,
     author = {W. Hackbusch and H.D. Mittelmann},
     title = {On {Multi-Grid} {Methods} for {Variational} {Inequalities.}},
     journal = {Numerische Mathematik},
     pages = {65--76},
     publisher = {mathdoc},
     volume = {42},
     year = {1983},
     zbl = {0497.65042},
     url = {http://geodesic.mathdoc.fr/item/NUMA_1983__42_132861/}
}