Problem

Source: KJMO 2019 p1

Tags: algebraic geometry, combinatorical geometry, combinatorics, KJMO, Coloring



Each integer coordinates are colored with one color and at least 5 colors are used to color every integer coordinates. Two integer coordinates $(x, y)$ and $(z, w)$ are colored in the same color if $x-z$ and $y-w$ are both multiples of 3. Prove that there exists a line that passes through exactly three points when five points with different colors are chosen randomly.