Given an chess board, in how many ways can we select squares on the board while satisfying both of the following requirements:
1. All black squares are selected.
2. Exactly seven squares are selected in each column and in each row.
, 2014
Solution
Switching the definition of selection and non-selection, we see that an equivalent formulation is to select eight squares, with one in each row and in each column, and no black squares selected. We assume that the non-black squares are white.
Orient the chessboard so that the square in row , column is white. Let be the column of the selected white square in row , so is a permutation of and has the same parity as . There are choices for (), then for (), for , and for . After these selections, we again have choices for , for , etc. All of these sets of choices are independent of each other, so in all we have ways of performing our selection.
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