(Four- (vertex) color theorem) The famous Jour-color
Chapter 23, Problem 23.8(choose chapter or problem)
(Four- (vertex) color theorem) The famous Jour-color theorem states that one can color the vertices of any planar graph (so that adjacent vertices get different colors) with at most four colors. It had been conjectured for a long time and was eventually proved in 1976 by Appel and Haken [Illinois J. Math 21 (1977), 429-5671. Can you color the complete graph K5 with four colors? Does the result contradict the four-color theorem? (For more details, see Ref. [F8] in App. I.)
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