Double factorial

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In mathematics, the double factorial, or semifactorial, n‼ of a positive integer n is the product of all the positive integers up to n that have the same parity (odd or even) as n. That is, n ! ! = ∏ k = 0 ⌈ n 2 ⌉ − 1 ( n − 2 k ) = n ( n − 2 ) ( n − 4 ) ⋯ . {\displaystyle n!!=\prod _{k=0}^{\left\lceil {\frac {n}{2}}\right\rceil -1}(n-2k)=n(n-2)(n-4)\cdots .} Restated, this says that for even n, the double factorial is n ! ! =...

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