The problem of deciding if four colors are sufficient to color any map on a plane or sphere.
Guthrie's Problem
See also
Coloring, Four-Color TheoremExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Guthrie's Problem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/GuthriesProblem.html