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[tex](2n)!!=2^nn![/tex]

for example

[tex](2)!!=2^11!=2[/tex]

[tex](4)!!=2^22!=8[/tex]

...

[tex](2n)!!=2n(2n-2)(2n-4)...=2^3n(n-1)(n-2)...[/tex]

I see that by intiution but I don't know how to write prove.

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# Double Factorial

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