A303351 Expansion of Product_{n>=1} (1 + 9*x^n)^(1/3).

1, 3, -6, 57, -294, 1884, -13011, 95178, -712293, 5448495, -42444375, 335392941, -2681006280, 21639853488, -176113016241, 1443450932445, -11903668996713, 98695838478585, -822212761531101, 6878755556938029, -57767592614370576, 486792969548157129

Offset: 0

Author

Seiichi Manyama, Apr 22 2018

Keywords

• sign

Comments

This sequence is obtained from the generalized Euler transform in A266964 by taking f(n) = -1/3, g(n) = -9.

In general, if h > 1 and g.f. = Product_{k>=1} (1 + h^2*x^k)^(1/h), then a(n) ~ -(-1)^n * c^(1/h) * h^(2*n-1) / (Gamma(1 - 1/h) * n^(1 + 1/h)), where c = Product_{k>=2} (1 + (-1)^k / h^(2*k-2)). - Vaclav Kotesovec, Apr 22 2018

Crossrefs

Expansion of Product_{n>=1} (1 + b^2*x^n)^(1/b): A000009 (b=1), A303350 (b=2), this sequence (b=3).

Programs

Maple

seq(coeff(series(mul((1+9*x^k)^(1/3), k = 1..n), x, n+1), x, n), n = 0..25); # Muniru A Asiru, Apr 22 2018

Mathematica

nmax = 30; CoefficientList[Series[Product[(1 + 9*x^k)^(1/3), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Apr 22 2018 *)

PARI

N=66; x='x+O('x^N); Vec(prod(k=1, N, (1+9*x^k)^(1/3)))

Formula

a(n) ~ -(-1)^n * c^(1/3) * 3^(2*n-1) / (Gamma(2/3) * n^(4/3)), where c = Product_{k>=2} (1 + 9*(-1/9)^k) = 1.09874828793226302381837574278380702... - Vaclav Kotesovec, Apr 22 2018