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A052645
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E.g.f. 2*x^2*(1+x-x^2)/(1-x).
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0
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0, 0, 4, 24, 48, 240, 1440, 10080, 80640, 725760, 7257600, 79833600, 958003200, 12454041600, 174356582400, 2615348736000, 41845579776000, 711374856192000, 12804747411456000, 243290200817664000
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OFFSET
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0,3
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LINKS
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FORMULA
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E.g.f.: 2*x^2*(-x+x^2-1)/(-1+x)
Recurrence: {a(1)=0, a(0)=0, (-1-n)*a(n)+a(n+1)=0, a(2)=4, a(3)=24, a(4)=48}
2*n!, n>3.
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MAPLE
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spec := [S, {S=Prod(Z, Union(Z, Z), Union(Z, Sequence(Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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MATHEMATICA
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With[{nn=20}, CoefficientList[Series[2x^2 (1+x-x^2)/(1-x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, May 14 2017 *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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STATUS
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approved
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