OFFSET
0,4
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..1000
FORMULA
a(n) ~ -(-1)^n * c * 2^(2*n - 1) / (sqrt(Pi) * n^(3/2)), where c = Product_{k>=1} (1 + sqrt(1 + 4*(-1/4)^k))/2 = 0.52271977595412566689522667777276363119313248923... - Vaclav Kotesovec, May 06 2021
MAPLE
N:= 40:
P:= mul((1+sqrt(1+4*x^k))/2, k=1..N):
S:= series(P, x, N+1):
seq(coeff(S, x, j), j=0..N); # Robert Israel, Sep 22 2019
MATHEMATICA
nmax = 30; CoefficientList[Series[Product[(1+Sqrt[1+4*x^k])/2, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Sep 22 2019 *)
PROG
(PARI) N=66; x='x+O('x^N); Vec(prod(k=1, N, (1+sqrt(1+4*x^k))/2))
(PARI) N=66; x='x+O('x^N); Vec(prod(i=1, N, 1-sum(j=1, N\i, (-1)^j*binomial(2*j-2, j-1)*x^(i*j)/j)))
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Sep 22 2019
STATUS
approved