OFFSET
0,3
FORMULA
a(n) = n! * Sum_{k=0..n} (n+1)^(k-1) * binomial(3*k,n-k)/k!.
E.g.f.: (1/x) * Series_Reversion( x*exp(-x*(1 + x)^3) ). - _ Seiichi Manyama_, Sep 23 2024
PROG
(PARI) a(n) = n!*sum(k=0, n, (n+1)^(k-1)*binomial(3*k, n-k)/k!);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Aug 17 2023
STATUS
approved