Carla Binucci Walter Didimo Francesco Giordano Maximum upward planar subgraphs of embedded planar digraphs. 230-246 2008 41 Comput. Geom. 3 http://dx.doi.org/10.1016/j.comgeo.2008.02.001 db/journals/comgeo/comgeo41.html#BinucciDG08