OFFSET
0,3
FORMULA
a(n) = n! * Sum_{k=0..n} k^(n-k) * binomial(n+3*k+1,k)/( (n+3*k+1)*(n-k)! ).
PROG
(PARI) a(n) = n!*sum(k=0, n, k^(n-k)*binomial(n+3*k+1, k)/((n+3*k+1)*(n-k)!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Aug 25 2023
STATUS
approved