TOT 198 1988 Autumn S1 min no of squares in chessboard
Source:
March 7, 2021
combinatorial geometrycombinatoricsChessboard
Problem Statement
What is the smallest number of squares of a chess board that can be marked in such a manner that
(a) no two marked squares may have a common side or a common vertex, and
(b) any unmarked square has a common side or a common vertex with at least one marked square?
Indicate a specific configuration of marked squares satisfying (a) and (b) and show that a lesser number of marked squares will not suffice. (A. Andjans, Riga)