four-color problem[¦fȯr ′kəl·ər ‚präb·ləm]
the problem of whether four different colors are sufficient to color any map so that no two regions having a common boundary segment have the same color. Although the conjecture that four colors are enough was proved for all known special cases, the problem long remained unsolved. Not until 1976 was a report of a rigorous mathematical proof published.
First formulated as a mathematical problem in the mid-19th century, the four-color problem became widely known through the lectures of the British mathematician A. de Morgan. A rigorous formulation of the problem requires that the regions in question be bounded by Jordan curves—that is, by simple closed curves. It can be easily proved that five colors are always sufficient to color such a map.
In the corresponding problem for space, no number of colors is sufficient.