OFFSET
0,5
FORMULA
a(n) = Sum_{k=0..floor(n/3)} (3*k)! * (n-k+1)^(k-1) * Stirling2(n,3*k)/(6^k * k!).
PROG
(PARI) a(n) = sum(k=0, n\3, (3*k)!*(n-k+1)^(k-1)*stirling(n, 3*k, 2)/(6^k*k!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Sep 11 2022
STATUS
approved