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A129831
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Alternating sum of double factorials: n!! - (n-1)!! + (n-2)!! - ... 1!!.
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2
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1, 1, 2, 6, 9, 39, 66, 318, 627, 3213, 7182, 38898, 96237, 548883, 1478142, 8843778, 25615647, 160178913, 494550162, 3221341038, 10527969537, 71221636863, 245012506362, 1716978047238, 6188875533387, 44822878860213
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OFFSET
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1,3
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LINKS
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FORMULA
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a(n) = n!! - (n-1)!! + (n-2)!! - ... 1!! = A006882(n) - a(n-1).
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EXAMPLE
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a(5) = 5!! - 4!! + 3!! - 2!! + 1!! = 15 - 8 + 3 - 2 + 1 = 9.
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MAPLE
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add( (-1)^i*doublefactorial(n-i), i=0..n-1) ;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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