OFFSET
1,2
FORMULA
From Vaclav Kotesovec, Nov 14 2023: (Start)
a(n) = binomial(2*n,n) * binomial(2*n-1,n) * (2*n)! / 4^n.
a(n) ~ 2^(4*n) * n^(2*n - 1/2) / (sqrt(Pi) * exp(2*n)). (End)
EXAMPLE
EllipticK(x) = 1/2*Pi + 1/8*Pi*x^2 + 9/128*Pi*x^4 + 25/512*Pi*x^6 + 1225/32768*Pi*x^8 + 3969/131072*Pi*x^10 + O(x^12).
MATHEMATICA
Table[Binomial[2*n, n]*Binomial[2*n - 1, n]*(2*n)!/4^n, {n, 1, 20}] (* Vaclav Kotesovec, Nov 14 2023 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Apr 14 2001
STATUS
approved