OFFSET
0,4
LINKS
Eric Weisstein's MathWorld, Bell Polynomial.
FORMULA
Let A(0)=1, B(0)=0 and C(0)=0. Let B(n+1) = Sum_{k=0..n} binomial(n,k)*A(k), C(n+1) = Sum_{k=0..n} binomial(n,k)*B(k) and A(n+1) = 2 * Sum_{k=0..n} binomial(n,k)*C(k). A357782(n) = A(n), A357783(n) = B(n) and a(n) = C(n).
Let w = exp(2*Pi*i/3) and set F(x) = (exp(x) + w*exp(w*x) + w^2*exp(w^2*x))/3 = x^2/2! + x^5/5! + x^8/8! + ... . Then the e.g.f. for the sequence is F(2^(1/3) * (exp(x)-1))/(2^(2/3)).
a(n) = ( Bell_n(2^(1/3)) + w * Bell_n(2^(1/3)*w) + w^2 * Bell_n(2^(1/3)*w^2) )/(3*2^(2/3)), where Bell_n(x) is n-th Bell polynomial.
PROG
(PARI) a(n) = sum(k=0, (n-2)\3, 2^k*stirling(n, 3*k+2, 2));
(PARI) my(N=30, x='x+O('x^N)); concat([0, 0], Vec(serlaplace(sum(k=0, N\3, 2^k*(exp(x)-1)^(3*k+2)/(3*k+2)!))))
(PARI) Bell_poly(n, x) = exp(-x)*suminf(k=0, k^n*x^k/k!);
a(n) = my(v=2^(1/3), w=(-1+sqrt(3)*I)/2); round((Bell_poly(n, v)+w*Bell_poly(n, v*w)+w^2*Bell_poly(n, v*w^2))/(3*v^2));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Oct 13 2022
STATUS
approved