OFFSET
0,2
FORMULA
a(n) ~ 4 * exp(-gamma/2) * sqrt(n) * n! / Pi, where gamma is the Euler-Mascheroni constant A001620.
MATHEMATICA
nmax = 30; CoefficientList[Series[Product[(1+x^(4*k-3)/(4*k-3))/(1-x^(4*k-3)/(4*k-3)), {k, 1, Floor[nmax/4]+1}], {x, 0, nmax}], x] * Range[0, nmax]!
CROSSREFS
KEYWORD
nonn
AUTHOR
Vaclav Kotesovec, Jul 27 2019
STATUS
approved