TOT 1998 Autumn OS3 Knight moves on 17 marked cells of 8x8 chessboard
Source:
May 11, 2020
combinatoricsChessboard
Problem Statement
On an chessboard, cells are marked. Prove that one can always choose two cells among the marked ones so that a Knight will need at least three moves to go from one of the chosen cells to the other. (R Zhenodarov)