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Exercise 602Combinatorics › Counting principlesM1Added 4 Nov 2026

Dominoes on a mutilated chessboard

#colouring#tilings#dominoes

Statement

Two opposite corner squares are removed from an 8×88\times8 chessboard. Can the remaining 6262 squares be tiled by 3131 dominoes (1×21\times2 rectangles)?

Hint

Use the usual black and white colouring and count the squares of each colour.

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Solution

Idea. Every domino covers exactly one black and one white square.

Colouring. Colour the board like a chessboard. Two opposite corners, for instance (1,1)(1,1) and (8,8)(8,8), have the same colour: 1+11+1 and 8+88+8 are both even.

Counting. The full board has 3232 black and 3232 white squares. Removing two squares of the same colour leaves 3232 of one colour and 3030 of the other.

Contradiction. Any tiling by 3131 dominoes covers exactly 3131 black and 3131 white squares. Impossible.

Answer. No.

Remark. If the two removed squares have different colours, a tiling always exists (Gomory's theorem).

Lesson. Colouring: choose a colouring in which every tile covers a prescribed number of squares of each colour, then compare with the colour counts of the region.