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A338463
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Expansion of g.f.: (-1 + Product_{k>=1} 1 / (1 + (-x)^k))^2.
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11
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1, 0, 2, 2, 3, 4, 5, 8, 9, 12, 15, 20, 23, 28, 36, 44, 52, 62, 76, 90, 106, 124, 149, 176, 203, 236, 279, 324, 372, 430, 499, 576, 657, 752, 867, 992, 1124, 1280, 1463, 1662, 1876, 2124, 2410, 2722, 3061, 3446, 3889, 4374, 4896, 5490, 6166, 6900, 7700, 8600
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OFFSET
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2,3
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LINKS
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FORMULA
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G.f.: (-1 + Product_{k>=1} (1 + x^(2*k - 1)))^2.
a(n) = [x^n]( (2/QPochhammer(-1,-x) - 1)^2 ). - G. C. Greubel, Sep 07 2023
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MATHEMATICA
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nmax = 55; CoefficientList[Series[(-1 + Product[1/(1 + (-x)^k), {k, 1, nmax}])^2, {x, 0, nmax}], x] // Drop[#, 2] &
With[{k=2}, Drop[CoefficientList[Series[(2/QPochhammer[-1, -x] -1)^k, {x, 0, 80}], x], k]] (* G. C. Greubel, Sep 07 2023 *)
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PROG
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(Magma)
m:=80;
R<x>:=PowerSeriesRing(Integers(), m);
Coefficients(R!( (-1 + (&*[1+x^(2*j+1): j in [0..m+2]]) )^2 )); // G. C. Greubel, Sep 07 2023
(SageMath)
m=80
def f(x): return (-1 + product(1+x^(2*j-1) for j in range(1, m+3)) )^2
P.<x> = PowerSeriesRing(QQ, prec)
return P( f(x) ).list()
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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