95.A Six-by-Six of Plus and Minus One
★★★combinatoricsJane Street
How many 6x6 matrices with every entry equal to +1 or -1 have the product of the entries in every row equal to 1 and the product in every column equal to 1?
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Fill in the top-left 5x5 block freely. Then ask how much freedom is left in the final row and column.
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