Double factorial (chart)

Calculates a table of the double factorial x!! and draws the chart.

 $n!!=\left\{{n(n-2)...3\cdot1\hspace{20} odd\ \atop n(n-2)...4\cdot2\hspace{20} even}\right.\\$
initial value x
 real number
[ increment
 repetition ]
 ${\normal Double\ factorial}\ x!!\\[10](1)\ n!!=\left\{{n(n-2)...3\cdot1\hspace{20} n\gt0\ odd\ \atop n(n-2)...4\cdot2\hspace{20}n\gt0\ even}\right.\\(2)\ (2n)!!=2^nn!,\hspace{20} (2n-1)!!=\frac{(2n)!}{2^nn!}\\(3)\ x!!=(\frac{2}{\pi})^{\frac{1}{4}(1-cos(\pi x))}2^{\frac{x}{2}}\Gamma{(\frac{x}{2}+1)}\\$

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