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It is easy to place two queens on a chessboard so that each queen attacks one other queen. And it is easy to do the same with four queens. Place as many queens as you can on an 8x8 board so that each queen attacks exactly one other queen. Note: Queens can move any number of squares along their row, column, or diagonal.Source: Journal of Recreational Mathematics,13:1© Copyright 2000 Stan Wagon. Reproduced with permission.
p. 61 (1980-81),and Martin Gardner, The Last Recreations, Springer/Copernicus, p. 281(Gardner's book has further references).