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Problem of the Day #266: Odd Products in a Line December 10, 2011

Posted by Albert in : potd , trackback

For how many permutations $P(\{1,2,\ldots,n\}) = \{a_1,a_2,\ldots,a_n\}$ is the following expression odd, in terms of $n$?

$$(a_1 – 1)(a_2 – 2) \cdots (a_n – n)$$

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