Four color theorem
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The four color theorem is a theorem of mathematics. It says that in any plane surface with regions in it, the regions can be colored with no more than four colors. Two regions that have a common border must not get the same color. They are called adjacent (next to each other) if they share a segment of the border, not just a point.
This was the first theorem to be proved by a computer. Not all mathematicians accept the proof. The proof can not be done by hand.
It is quite obvious, that there are maps that cannot be colored in this way with only three colors. On the other hand, five colors are enough for every map.
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