OFFSET
0,3
FORMULA
a(n) = (3*n)! * Sum_{k=0..n} |Stirling1(n,k)|/(3*n-k+1)!.
a(n) ~ (-3 - LambertW(-1, -3*exp(-4)))^(2*n+1) * (-LambertW(-1, -3*exp(-4)))^n * n^(n-1) / (sqrt(-3 - 3*LambertW(-1, -3*exp(-4))) * exp(n)). - Vaclav Kotesovec, Nov 07 2023
MATHEMATICA
Table[(3*n)! * Sum[Abs[StirlingS1[n, k]]/(3*n-k+1)!, {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Nov 07 2023 *)
PROG
(PARI) a(n) = (3*n)!*sum(k=0, n, abs(stirling(n, k, 1))/(3*n-k+1)!);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Nov 07 2023
STATUS
approved