6b

Part of 1996 German National Olympiad

Problems(1)

each point of a plane is colored in one of three colors: red, black and blue

Source: Germany 1996 p6b

Each point of a plane is colored in one of three colors: red, black and blue. Prove that there exists a rectangle in this plane whose vertices all have the same color.

Coloringcombinatoricsrectanglecombinatorial geometry