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A052617
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E.g.f. (1+x-x^2)/((1-x)(1-2x)).
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0
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1, 4, 18, 114, 936, 9480, 114480, 1607760, 25764480, 464123520, 9286099200, 204334099200, 4904497382400, 127523158963200, 3570735629260800, 107123376552192000, 3427968972460032000, 116551300751069184000
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OFFSET
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0,2
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LINKS
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FORMULA
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E.g.f.: -(-x+x^2-1)/(-1+2*x)/(-1+x)
Recurrence: {a(0)=1, a(1)=4, (2*n^2+6*n+4)*a(n) +(-6-3*n)*a(n+1) +a(n+2)=0, a(2)=18}
(-1+5*2^(n-1))*n!, n>0.
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MAPLE
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spec := [S, {S=Prod(Union(Z, Sequence(Z)), Sequence(Union(Z, 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[(1+x-x^2)/((1-x)(1-2x)), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Nov 25 2018 *)
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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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