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Double factorial

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Calculates the double factorial x!!.
\( n!!=\left\{{n(n-2)...3\cdot1\hspace{20px} odd\ \atop n(n-2)...4\cdot2\hspace{20px} even}\right.\\\)
Variables x
x!!  
\({\normalsize Double\ factorial}\ x!!\\ (1)\ n!!=\left\{{n(n-2)...3\cdot1\hspace{20px} n\gt0\ odd\ \atop n(n-2)...4\cdot2\hspace{20px}n\gt0\ even}\right.\\ (2)\ (2n)!!=2^nn!,\hspace{20px} (2n-1)!!=\frac{(2n)!}{2^nn!}\\ (3)\ x!!=(\frac{2}{\pi})^{\frac{1}{4}(1-\cos(\pi x))}2^{\frac{x}{2}}\Gamma{(\frac{x}{2}+1)}\\\)

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