OFFSET
0,3
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..647
FORMULA
a(n)=sum{k=0..floor(n/2), sum{i=0..k, (-1)^(k+i)i^(n-k)/(i!(k-i)!)}3^k }
G.f.: Sum_{k>=0} 3^k * x^(2*k)/Product_{j=1..k} (1 - j * x). - Seiichi Manyama, Apr 09 2022
MATHEMATICA
Table[Sum[StirlingS2[n-k, k]3^k, {k, 0, Floor[n/2]}], {n, 0, 30}] (* Harvey P. Dale, May 24 2012 *)
PROG
(PARI) my(N=40, x='x+O('x^N)); Vec(sum(k=0, N, 3^k*x^(2*k)/prod(j=1, k, 1-j*x))) \\ Seiichi Manyama, Apr 09 2022
(PARI) a(n) = sum(k=0, n\2, 3^k*stirling(n-k, k, 2)); \\ Seiichi Manyama, Apr 09 2022
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Aug 05 2004
STATUS
approved