OFFSET
1,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..200
N. J. A. Sloane and Thomas Wieder, The Number of Hierarchical Orderings, Order 21 (2004), 83-89.
FORMULA
See A083384 for formula.
a(n) = A261781(n+1,n)/2. - Alois P. Heinz, Aug 10 2016
a(n) ~ n! * n^2 / (8 * (log(2))^(n+2)). - Vaclav Kotesovec, Jul 01 2018
MATHEMATICA
a[n_] := n Sum[1/2 (k-1) k! StirlingS2[n, k], {k, 1, n}] + n Sum[(-1)^(k-j) Binomial[k, j] j^n, {j, 0, n}, {k, 0, n}];
Array[a, 20] (* Jean-François Alcover, Sep 01 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 07 2003
STATUS
approved