books.google.com - The four-colour theorem is one of the famous problems of mathematics, that frustrated generations of mathematicians from its birth in 1852 to its solution (using substantial assistance from electronic computers) in 1976. The theorem asks whether four colours are sufficient to colour all conceivable maps,...http://books.google.com/books/about/Graphs_Colourings_and_the_Four_Colour_Th.html?id=iq0sSnIxJioC&utm_source=gb-gplus-shareGraphs, Colourings and the Four-Colour Theorem