4 colors in 50x50 square - All-Russian MO 1999 Regional (R4) 11.6
Source:
September 25, 2024
combinatoricsColoring
Problem Statement
The cells of a square are painted in four colors. Prove that there is a cell, on four sides of which (i.e. top, bottom, left and on the right) there are cells of the same color as it (not necessarily adjacent to this cell).