A270919 Coefficient of x^n in Product_{k>=1} ((1 + x^k) / (1 - x^k))^n.
1, 2, 12, 80, 552, 3912, 28224, 206208, 1520784, 11297546, 84413912, 633713808, 4776117216, 36115518376, 273868321536, 2081866609920, 15859616674336, 121046064563376, 925411686479820, 7085465166635440, 54323193841192752, 416993869451825424, 3204447137019290944
Offset: 0
Keywords
Links
- Seiichi Manyama, Table of n, a(n) for n = 0..1118 (terms 0..500 from Vaclav Kotesovec)
Crossrefs
Programs
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Mathematica
Table[SeriesCoefficient[Product[((1+x^k)/(1-x^k))^n, {k, 1, n}], {x, 0, n}], {n, 0, 25}] Table[SeriesCoefficient[(QPochhammer[-1, x]/QPochhammer[x, x])^n, {x, 0, n}]/2^n, {n, 0, 25}] (* Calculation of constants {d,c}: *) eq = FindRoot[{2*s*QPochhammer[r*s] == QPochhammer[-1, r*s], (Log[1 - r*s] + QPolyGamma[0, 1, r*s])/Log[r*s] + r*((Derivative[0, 1][QPochhammer][-1, r*s] - 2*s*Derivative[0, 1][QPochhammer][r*s, r*s]) / (2*QPochhammer[r*s])) == 1}, {r, 1/8}, {s, 2}, WorkingPrecision -> 1000]; {N[1/r /. eq, 120], val = Sqrt[(1 - r*s)*Log[r*s]^2*(QPochhammer[r*s] / (Pi*(-r*s*(-1 + r*s) * Log[r*s]*(4*(2*ArcTanh[1 - 2*r*s] + QPolyGamma[0, 1, r*s])* Derivative[0, 1][QPochhammer][r*s, r*s] + r*Log[r*s]*(Derivative[0, 2][QPochhammer][-1, r*s] - 2*s*Derivative[0, 2][QPochhammer][r*s, r*s])) + 2*QPochhammer[r*s] * (4*r*s*ArcTanh[1 - 2*r*s] + 2*(-1 + (-1 + r*s)*ArcTanh[1 - 2*r*s])*Log[1 - r*s] - (-1 + r*s)*(-2 + Log[r*s] - 2*Log[1 - r*s])*QPolyGamma[0, 1, r*s] + (-1 + r*s) * QPolyGamma[0, 1, r*s]^2 + (-1 + r*s)*(QPolyGamma[1, 1, r*s] - 2*r*s*Log[r*s]* Derivative[0, 0, 1][QPolyGamma][0, 1, r*s])))))] /. eq; N[Chop[val], -Floor[Log[10, Abs[Im[val]]]] - 3]} (* Vaclav Kotesovec, Oct 03 2023 *)
Formula
a(n) ~ c * d^n / sqrt(n), where d = 7.862983395705905261519347909953827161057584... and c = 0.299856802806668079413694689903953367699319...
a(n) = [x^n] 1/theta_4(x)^n, where theta_4() is the Jacobi theta function. - Ilya Gutkovskiy, Nov 03 2017
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