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A129831
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Alternating double factorials: n!! - (n-1)!! + (n-2)!! - ... 1!!.
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0
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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FORMULA
| a(n) = n!! - (n-1)!! + (n-2)!! - ... 1!! = n!! - a(n-1)
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EXAMPLE
| a(5)= 5!! - 4!! + 3!! - 2!! + 1!! = 15 - 8 + 3 - 2 + 1 = 9
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MAPLE
| P:=proc(n) local a, i, j, k, w; for i from 1 by 1 to n do a:=0; for j from i by -1 to 0 do k:=j; w:=j-2; while w>0 do k:=k*w; w:=w-2; od; a:=a+k*(-1)^j od; print(abs(a)); od; end: P(100);
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CROSSREFS
| Cf. A005165.
Sequence in context: A101823 A093397 A082459 * A101713 A117541 A095105
Adjacent sequences: A129828 A129829 A129830 * A129832 A129833 A129834
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KEYWORD
| nonn
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AUTHOR
| Paolo P. Lava & Giorgio Balzarotti (paoloplava(AT)gmail.com), May 21 2007
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