OFFSET
0,3
COMMENTS
In general, if d >= 1, b > 0 and g.f. = Sum_{k>=0} d^k * x^(b*k^2 + c*k) * Product_{j=1..k} (1 + x^j), then a(n) ~ r^c * (1+r) * exp(sqrt((2*log(d)^2 + 8*b*log(d)*log(r) + 4*b*(2*b+1)*log(r)^2 + 4*polylog(2, 1/(1+r)) - Pi^2/3)*n)) / (2*sqrt((r + 2*b*(1+r))*n)), where r is the smallest positive real root of the equation d*r^(2*b)*(1+r) = 1.
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 0..10000
FORMULA
a(n) ~ r * (1+r) * exp(sqrt((2*log(2)^2 + 8*log(2)*log(r) + 12*log(r)^2 + 4*polylog(2, 1/(1+r)) - Pi^2/3)*n)) / (2*sqrt((3*r + 2)*n)), where r = ((46 - 6*sqrt(57))^(1/3) + (46 + 6*sqrt(57))^(1/3) - 2)/6 is the real root of the equation 2*r^2*(1+r) = 1 (A273065).
MATHEMATICA
nmax = 80; CoefficientList[Series[Sum[2^k * x^(k*(k+1)) * Product[1+x^j, {j, 1, k}], {k, 0, Sqrt[nmax]}], {x, 0, nmax}], x]
nmax = 80; p = 1; s = 1; Do[p = Normal[Series[2*p*(1 + x^k) * x^(2*k), {x, 0, nmax}]]; s += p; , {k, 1, Sqrt[nmax]}]; Take[CoefficientList[s, x], nmax + 1]
CROSSREFS
KEYWORD
nonn
AUTHOR
Vaclav Kotesovec, Oct 10 2024
STATUS
approved