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A261386 Expansion of Product_{k>=1} ((1+x^k)/(1-x^k))^(2*k). 13
1, 4, 16, 56, 176, 520, 1456, 3896, 10048, 25100, 60960, 144440, 334752, 760456, 1696464, 3722224, 8043040, 17135624, 36031104, 74840568, 153680928, 312198160, 627828272, 1250540024, 2468443296, 4830809868, 9377190336, 18061370288, 34531009760, 65552873736 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Convolution of A161870 and A026011.

LINKS

Seiichi Manyama, Table of n, a(n) for n = 0..10000

Vaclav Kotesovec, A method of finding the asymptotics of q-series based on the convolution of generating functions, arXiv:1509.08708 [math.CO], Sep 30 2015, p. 19.

FORMULA

a(n) ~ exp(1/6 + 3/2*(7*Zeta(3))^(1/3) * n^(2/3)) * (7*Zeta(3))^(2/9) / (A^2 * 2^(2/3) * n^(13/18) * sqrt(3*Pi)), where Zeta(3) = A002117 and A = A074962 is the Glaisher-Kinkelin constant.

G.f.: exp(Sum_{k>=1} (sigma_2(2*k) - sigma_2(k))*x^k/k). - Ilya Gutkovskiy, Apr 14 2019

MATHEMATICA

nmax = 40; CoefficientList[Series[Product[(1+x^k)^(2*k) / (1-x^k)^(2*k), {k, 1, nmax}], {x, 0, nmax}], x]

CROSSREFS

Cf. A015128, A026011, A156616, A161870, A216406, A261384, A261389.

Sequence in context: A239988 A308288 A340257 * A073388 A109634 A026126

Adjacent sequences:  A261383 A261384 A261385 * A261387 A261388 A261389

KEYWORD

nonn

AUTHOR

Vaclav Kotesovec, Aug 17 2015

STATUS

approved

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Last modified August 5 17:08 EDT 2021. Contains 346488 sequences. (Running on oeis4.)