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A052621
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E.g.f. (2+x+x^2+x^3)/(1-x^4).
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0
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2, 1, 2, 6, 48, 120, 720, 5040, 80640, 362880, 3628800, 39916800, 958003200, 6227020800, 87178291200, 1307674368000, 41845579776000, 355687428096000, 6402373705728000, 121645100408832000, 4865804016353280000
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OFFSET
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0,1
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LINKS
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FORMULA
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E.g.f.: -(x^3+x^2+x+2)/(-1+x^4)
Recurrence: {a(1)=1, a(3)=6, a(2)=2, a(0)=2, (-n^4-35*n^2-50*n-24-10*n^3)*a(n)+a(n+4)=0}
Sum(1/4*(_alpha^3+_alpha^2+2*_alpha+1)*_alpha^(-1-n), _alpha=RootOf(-1+_Z^4))*n!
2n! if n is 0 mod 4, n! otherwise.
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MAPLE
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spec := [S, {S=Union(Sequence(Z), Sequence(Prod(Z, Z, 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[(2+x+x^2+x^3)/(1-x^4), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Oct 10 2023 *)
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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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