Meaning of specific math symbol: "$!!$"

Question

Can someone please explain me, what does "$!!$" mean? Is it math symbol at all?

I know that it is not factorial, because that's just one "$!$", but I was unable to find any definition for it.

Thanks

Answer 1

$n!!$ is known as a "double factorial," or "semifactorial," and it typically represents the product of all positive integers less than or equal to $n$ with the same parity (odd or even) as $n$. For example,

$8!! = 2 \cdot 4 \cdot 6 \cdot 8$

$7!! = 1 \cdot 3 \cdot 5 \cdot 7$