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 A129831 Alternating sum of double factorials: n!! - (n-1)!! + (n-2)!! - ... 1!!. 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS FORMULA a(n) = n!! - (n-1)!! + (n-2)!! - ... 1!! = A006882(n) - a(n-1). EXAMPLE a(5) = 5!! - 4!! + 3!! - 2!! + 1!! = 15 - 8 + 3 - 2 + 1 = 9. MAPLE A129831 := proc(n)         add( (-1)^i*doublefactorial(n-i), i=0..n-1) ; end proc: # R. J. Mathar, Aug 22 2012 CROSSREFS Cf. A005165, A006882. Sequence in context: A242421 A093397 A082459 * A101713 A240761 A117541 Adjacent sequences:  A129828 A129829 A129830 * A129832 A129833 A129834 KEYWORD nonn AUTHOR Paolo P. Lava & Giorgio Balzarotti, May 21 2007 STATUS approved

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