What does the !! mean in trigonometric identity?

In mathematics, the double factorial or semifactorial of a number $n$ (denoted by $n!!$) is the product of all the integers from $1$ up to $n$ that have the same parity (odd or even) as $n$.

Example: $9!! = 9 \cdot 7 \cdot 5 \cdot 3 \cdot 1$

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Factorial

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