OFFSET
0,4
FORMULA
a(n) = Sum_{k=0..floor(n/2)} 3^(n-2*k) * |Stirling1(n-k,k)|.
MATHEMATICA
a[n_] := Sum[3^(n-2*k) * Abs[StirlingS1[n - k, k]], {k, 0, Floor[n/2]}]; Array[a, 20, 0] (* Amiram Eldar, Apr 09 2022 *)
PROG
(PARI) my(N=20, x='x+O('x^N)); Vec(sum(k=0, N, x^k*prod(j=0, k-1, 3*j+x)))
(PARI) a(n) = sum(k=0, n\2, 3^(n-2*k)*abs(stirling(n-k, k, 1)));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 08 2022
STATUS
approved