OFFSET
0,4
FORMULA
E.g.f.: exp( x/8 * (exp(4 * x) - 1) ).
a(0) = 1; a(n) = (1/2) * Sum_{k=2..n} k * 4^(k-2) * binomial(n-1,k-1) * a(n-k).
a(n) = n! * Sum_{k=0..floor(n/2)} 4^(n-2*k) * Stirling2(n-k,k)/(2^k * (n-k)!).
MATHEMATICA
Table[n!*Sum[4^(n - 2*k)*StirlingS2[n - k, k]/(2^k*(n - k)!), {k, 0, Floor[n/2]}], {n, 0, 25}] (* Wesley Ivan Hurt, May 25 2024 *)
PROG
(PARI) my(N=30, x='x+O('x^N)); Vec(serlaplace(exp(x/8*(exp(4*x)-1))))
(PARI) a_vector(n) = my(v=vector(n+1)); v[1]=1; for(i=1, n, v[i+1]=sum(j=2, i, j*4^(j-2)*binomial(i-1, j-1)*v[i-j+1])/2); v;
(PARI) a(n) = n!*sum(k=0, n\2, 4^(n-2*k)*stirling(n-k, k, 2)/(2^k*(n-k)!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, May 24 2022
STATUS
approved