OFFSET
0,5
FORMULA
a(n) = (n!)^2 * Sum_{k=0..floor(n/4)} Stirling2(n-3*k,k)/( (n-3*k)! * (n-k+1)! ).
E.g.f.: (1/x) * Series_Reversion( x/(1 + x^3*(exp(x) - 1)) ).
PROG
(PARI) a(n) = n!^2*sum(k=0, n\4, stirling(n-3*k, k, 2)/((n-3*k)!*(n-k+1)!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Sep 19 2024
STATUS
approved