HANATANI Yoichi   HORIYAMA Takashi   IWAMA Kazuo   TAMAKI Suguru   
IEICE transactions on fundamentals of electronics, communications and computer sciences E91-A(9) 2301-2307 2008年9月 [査読有り]
The Hajós calculus is a nondeterministic procedure which generates the class of non-3-colorable graphs. If all non-3-colorable graphs can be constructed in polynomial steps by the calculus, then NP=co-NP holds. Up to date, however, it remains open...