configuration of several checkers on grid paper is boring
Source: Israel Joseph Gillis Mathematical Olympiad 1998 p3
February 16, 2020
combinatoricscombinatorial geometry
Problem Statement
A configuration of several checkers at the centers of squares on a rectangular sheet of grid paper is called boring if some four checkers occupy the vertices of a rectangle with sides parallel to those of the sheet.
(a) Prove that any configuration of more than checkers on an grid is boring.
(b) Prove that any configuration of checkers on a grid is boring.