OFFSET
0,3
FORMULA
G.f.: Sum_{k>=0} 2^(k*(k-1)) * (x/(1 - x))^k.
a(n) = Sum_{k=0..n} binomial(n-1,k-1) * 2^(k*(k-1)).
MATHEMATICA
nmax = 13; A[_] = 0; Do[A[x_] = 1 + x A[4 x/(1 + 3 x)]/(1 - x) + O[x]^(nmax + 1) // Normal, nmax + 1]; CoefficientList[A[x], x]
Table[Sum[Binomial[n - 1, k - 1] 2^(k (k - 1)), {k, 0, n}], {n, 0, 13}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Apr 19 2022
STATUS
approved