Hal A. Kierstead Bojan Mohar Simon Spacapan Daqing Yang Xuding Zhu The Two-Coloring Number and Degenerate Colorings of Planar Graphs. 1548-1560 2009 23 SIAM J. Discrete Math. 3 http://dx.doi.org/10.1137/070703697 db/journals/siamdm/siamdm23.html#KiersteadMSYZ09