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Expansion of Product_{k>=1} (1 + 3^(k-1)*x^k).
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%I #9 May 09 2021 03:32:03

%S 1,1,3,12,36,135,432,1539,4860,17496,55404,192456,623295,2125764,

%T 6849684,23442453,75110328,252965916,822670668,2735858268,8838926712,

%U 29501352792,95090206689,314068876416,1018141045092,3342663979092,10798571289897,35481518064576

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

%F a(n) = Sum_{k=0..A003056(n)} q(n,k) * 3^(n-k), where q(n,k) is the number of partitions of n into k distinct parts.

%F a(n) ~ (-polylog(2, -1/3))^(1/4) * 3^n * exp(2*sqrt(-polylog(2, -1/3)*n)) / (4*sqrt(Pi/3)*n^(3/4)). - _Vaclav Kotesovec_, May 09 2021

%t nmax = 27; CoefficientList[Series[Product[(1 + 3^(k - 1) x^k), {k, 1, nmax}], {x, 0, nmax}], x]

%t Table[Sum[Length[Select[IntegerPartitions[n, {k}], UnsameQ @@ # &]] 3^(n - k), {k, 0, Floor[(Sqrt[8 n + 1] - 1)/2]}], {n, 0, 27}]

%o (PARI) seq(n)={Vec(prod(k=1, n, 1 + 3^(k-1)*x^k + O(x*x^n)))} \\ _Andrew Howroyd_, May 08 2021

%Y Cf. A003056, A008289, A032308, A300579, A304961, A340103, A344063, A344064, A344065, A344066, A344067, A344068.

%K nonn

%O 0,3

%A _Ilya Gutkovskiy_, May 08 2021