In the summer of 1976 Kenneth Appel and Wolfgang Haken of the University of Illinois announced that they had solved the Four-Color
Problem. Suddenly what had been known to several generations of
mathematicians as the Four-Color Conjecture had become the Four-
Color Theorem.
Since it had been a conjecture for over one hundred years that all
maps are four-colorable, and since a great deal of mathematics was
done in attempts to solve the Four-Color Conjecture, it will be called a
conjecture rather than a theorem throughout most of this book.