A grid is given. Answer the number of possible cases to write a number among , , or in each square so that
* no number appears twice (or more) in the same row, and
* no number appears twice (or more) in the same column.
Solution
Denote by the entry that lies in the -th column and the -th row. The given conditions yields that each , , and appear exactly once in each column.
We may assume without loss of generality that for . We would multiply the answer by afterwards. We may also assume that . We would multiply the answer by afterwards.
If , then . We can choose which of and to be , and the rest entries would be determined uniquely. So we have two ways.
Otherwise, if , there are only one way to fill the other squares. The case is similar. So we have two ways.
Therefore, the answer is .
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