OFFSET
0,4
FORMULA
a(n) = (n!)^2 * Sum_{k=0..floor(n/3)} Stirling2(n-2*k,k)/( (n-2*k)! * (n-k+1)! ).
E.g.f.: (1/x) * Series_Reversion( x/(1 + x^2*(exp(x) - 1)) ). - Seiichi Manyama, Sep 19 2024
PROG
(PARI) a(n) = n!^2*sum(k=0, n\3, stirling(n-2*k, k, 2)/((n-2*k)!*(n-k+1)!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Mar 12 2024
STATUS
approved