Gregory Gutin Ton Kloks Chuan-Min Lee Anders Yeo Kernels in planar digraphs. 174-184 2005 71 J. Comput. Syst. Sci. 2 http://dx.doi.org/10.1016/j.jcss.2005.02.003 db/journals/jcss/jcss71.html#GutinKLY05