In this way, the map coloring problem becomes a graph coloring problem: Color the vertices so neighbors are different colors. The minimum number of colors is called the chromatic number of the graph.

De Grey pitched the problem of finding the minimal five-color graph to Terence Tao, a mathematician at the University of California, Los Angeles, as a potential Polymath problem. Polymath began about ...

Each 9-by-9 sudoku grid is composed of nine 3-by-3 subgrids. Initially, some of these 81 squares contain a number, from 1 through 9, but others do not.