English: A 9-colouring of the queen's graph, where no two squares with the same colour can be reached from each other by a queen move in chess. No colouring with fewer colours is possible. The image is based on "On Coloring the n × n Chessboard" (M. R. Iyer and V. V. Menon), who provide a mathematical description of this colouring.
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Captions
A colouring of the queen's graph: no two squares with the same colour are on the same row, column or diagonal