OFFSET
0,2
FORMULA
G.f.: sqrt( Sum_{k>=0} (2*k)!/k! * (2*x)^k ).
a(n) ~ 2^(3*n - 1/2) * n^n / exp(n). - Vaclav Kotesovec, Feb 07 2023
MATHEMATICA
nmax = 20; CoefficientList[Series[Sqrt[Sum[(2*k)!/k!*(2*x)^k, {k, 0, nmax}]], {x, 0, nmax}], x] (* Vaclav Kotesovec, Feb 07 2023 *)
PROG
(PARI) a(n) = my(A=1+x*O(x^n)); for(i=1, n, A=1-4*(n-i+1)*x/A); polcoeff(1/sqrt(A), n);
(PARI) my(N=20, x='x+O('x^N)); Vec(sqrt(sum(k=0, N, (2*k)!/k!*(2*x)^k)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Feb 02 2023
STATUS
approved