Four color theorem
statement in mathematics
In mathematics, the four color theorem, or the four-color map theorem, states that no more than four colors are required to color the regions of any map so that no two adjacent regions have the same color. Adjacent means that two regions share a common boundary of non-zero length (i.e., not merely a corner where three or more regions meet). It was the first major theorem to be proved by use of a computer. Initially, this proof was not accepted by all mathematicians because the computer-assisted proof was infeasible for a human to check by hand. The proof has gained wide acceptance since then, although some reservations remain.
The theorem is a stronger version of the five color theorem, which can be shown using a significantly simpler argument. Although the weaker five color theorem was proven already in the 1800s, the four color theorem resisted until 1976, when it was proven by Kenneth Appel and Wolfgang Haken in a computer-aided proof. This came after many false proofs and mistaken counterexamples in the preceding decades.
The Appel–Haken proof proceeds by analyzing a very large number of "reducible configurations", examples of maps with a particular property. This was improved upon in 1997 by Robertson, Sanders, Seymour, and Thomas, who have managed to decrease the number of such configurations to 633 – still an extremely long case analysis.
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