Youichi Hanatani Takashi Horiyama Kazuo Iwama Suguru Tamaki New Graph Calculi for Planar Non-3-Colorable Graphs. 2301-2307 2008 91-A IEICE Transactions 9 http://dx.doi.org/10.1093/ietfec/e91-a.9.2301 db/journals/ieicet/ieicet91a.html#HanataniHIT08