Minimal number of separating edges in a tri-colored grid
Source: China Second Round 2017(A), Test 2, Problem 3
February 22, 2018
combinatorics
Problem Statement
Each square of a square grid is colored in one of the three colors: red, yellow or blue, such that the numbers of squares in each color are the same. If two squares sharing a common edge are in different colors, call that common edge a separating edge. Find the minimal number of separating edges in the grid.