A303390 Expansion of Product_{k>=1} (1 + 3*x^k)/(1 - 3*x^k).

1, 6, 24, 96, 330, 1104, 3552, 11184, 34584, 105990, 322224, 975264, 2942016, 8857680, 26631312, 80005632, 240219114, 721036320, 2163789816, 6492625152, 19480105392, 58444390176, 175340344416, 526034008752, 1578124753152, 4734415061142, 14203316252400

Offset: 0

Author

Vaclav Kotesovec, Apr 23 2018

Keywords

• nonn

Crossrefs

Cf. A032308, A242587.

Cf. A015128, A261584, A303391.

Programs

Mathematica

nmax = 30; CoefficientList[Series[Product[(1+3*x^k)/(1-3*x^k), {k, 1, nmax}], {x, 0, nmax}], x]

Formula

a(n) ~ c * 3^n, where c = QPochhammer[-1, 1/3] / QPochhammer[1/3] = 5.5877920355220979147599292926505407983327527...