OFFSET
0,2
COMMENTS
More formulas and information can be derived from entry A000041.
This sequence is obtained from the generalized Euler transform in A266964 by taking f(n) = 1/2, g(n) = 4^n. - Seiichi Manyama, Apr 20 2018
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 0..1000
FORMULA
G.f.: Product_{n>=1} 1/sqrt(1 - (4*x)^n).
Sum_{k=0..n} a(k) * a(n-k) = 4^n * A000041(n), for n>=0, where A000041(n) equals the number of partitions of n.
a(n) ~ 4^(n-1) * exp(sqrt(n/3)*Pi) / (3^(3/8) * n^(7/8)). - Vaclav Kotesovec, Apr 02 2016
EXAMPLE
G.f.: A(x) = 1 + 2*x + 14*x^2 + 68*x^3 + 406*x^4 + 1820*x^5 + 10892*x^6 + 48008*x^7 + 266214*x^8 + 1248044*x^9 + 6454116*x^10 +...
where A(x)^2 = P(4*x).
RELATED SERIES.
P(x) = 1 + x + 2*x^2 + 3*x^3 + 5*x^4 + 7*x^5 + 11*x^6 + 15*x^7 + 22*x^8 + 30*x^9 + 42*x^10 + 56*x^11 + 77*x^12 + 101*x^13 + 135*x^14 +...+ A000041(n)*x^n +...
1/A(x)^6 = 1 - 12*x + 320*x^3 - 28672*x^6 + 9437184*x^10 - 11811160064*x^15 + 57174604644352*x^21 +...+ (-1)^n*(2*n+1)*(4*x)^(n*(n+1)/2) +...
MATHEMATICA
nmax = 30; CoefficientList[Series[Product[1/Sqrt[1 - (4*x)^k], {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Apr 02 2016 *)
PROG
(PARI) {a(n) = polcoeff( prod(k=1, n, 1/sqrt(1 - (4*x)^k +x*O(x^n))), n)}
for(n=0, 30, print1(a(n), ", "))
(PARI) {a(n) = if( n<0, 0, polcoeff( sum( k=1, sqrtint(n+1), (4*x)^(k^2) / prod(j=1, k, 1 - (4*x)^j, 1 + x*O(x^n))^2, 1)^(1/2), n))};
for(n=0, 30, print1(a(n), ", "))
(PARI) N=99; x='x+O('x^N); Vec(prod(k=1, N, 1/(1-(4*x)^k)^(1/2))) \\ Altug Alkan, Apr 20 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 02 2016
STATUS
approved