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Coefficient of x^n in Product_{k>=1} 1/(1+x^k)^n.
13

%I #13 Oct 03 2023 03:28:01

%S -1,1,-4,17,-56,172,-547,1809,-6061,20316,-68135,229244,-774372,

%T 2624119,-8912759,30328593,-103382254,352975681,-1206921212,

%U 4132159452,-14163858895,48601267199,-166930975524,573872089212,-1974472043081,6798561779868,-23425506369715

%N Coefficient of x^n in Product_{k>=1} 1/(1+x^k)^n.

%H Vaclav Kotesovec, <a href="/A255526/b255526.txt">Table of n, a(n) for n = 1..500</a>

%F a(n) ~ (-1)^n * c * d^n / sqrt(n), where d = A318204 = 3.5097543279497033404372735..., c = 0.23322106096789389697797... .

%t Table[SeriesCoefficient[Product[1/(1+x^k)^n,{k,1,n}],{x,0,n}],{n,1,30}]

%t (* Calculation of constant c: *) 1/Sqrt[(4 - r^2*s^3*Derivative[0, 2][QPochhammer][-1, r*s])*Pi] /. FindRoot[{QPochhammer[-1, r*s] == 2/s, 2/s + r*s*Derivative[0, 1][QPochhammer][-1, r*s] == 0}, {r, -1/3}, {s, 2}, WorkingPrecision -> 120] (* _Vaclav Kotesovec_, Oct 03 2023 *)

%Y Cf. A008485, A270913, A278428.

%K sign

%O 1,3

%A _Vaclav Kotesovec_, Feb 24 2015