Return double factorial
n(n-2)...(5)(3)(1), n>0, odd
n!! = n(n-2)...(6)(4)(2), n>0, even
1 , n = -1, or n = 0
6!! = (6)(4)(2) = 48
Neither n = -1, nor n = 0 are tested.
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Divide elements by sum of elements
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