In mathematics, the Heawood number of a surface is a certain upper bound for the maximal number of colors needed to color any graph embedded in the surface.
In 1890 Heawood proved for all surfaces except the sphere that no more than
colors are needed to color any graph embedded in a surface of Euler characteristic . The case of the sphere is the four-color conjecture which was settled by Kenneth Appel and Wolfgang Haken in 1976. The number became known as Heawood number in 1976.
Franklin proved that the chromatic number of a graph embedded in the Klein bottle can be as large as, but never exceeds . Later it was proved in the works of Gerhard Ringel and J. W. T. Youngs that the complete graph of vertices can be embedded in the surface unless is the Klein bottle. This established that Heawood's bound could not be improved.
For example, the complete graph on vertices can be embedded in the torus as follows:
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“Even in ordinary speech we call a person unreasonable whose outlook is narrow, who is conscious of one thing only at a time, and who is consequently the prey of his own caprice, whilst we describe a person as reasonable whose outlook is comprehensive, who is capable of looking at more than one side of a question and of grasping a number of details as parts of a whole.”
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