OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..345
Wikipedia, Partition of a set
FORMULA
a(n) = A324162(2n,n).
EXAMPLE
a(2) = 10: 123/4, 124/3, 12/34, 134/2, 13/24, 14/23, 1/234, 1/2|3/4, 1/3|2/4, 1/4|2/3.
MAPLE
b:= proc(n, k) option remember; `if`(n=0, 1, add(b(n-j, k)
*binomial(n-1, j-1)*Stirling2(j, k), j=k..n))
end:
a:= n-> b(2*n, n):
seq(a(n), n=0..18);
MATHEMATICA
b[n_, k_] := b[n, k] = If[n == 0, 1, If[k == 0 || k > n, 0, Sum[b[n-j, k]* Binomial[n - 1, j - 1] StirlingS2[j, k], {j, k, n}]]];
a[n_] := b[2n, n];
a /@ Range[0, 18] (* Jean-François Alcover, May 05 2020, after Maple *)
PROG
(PARI) a(n) = if(n==0, 1, stirling(2*n, n, 2)+binomial(2*n, n)/2); \\ Seiichi Manyama, May 08 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Sep 02 2019
STATUS
approved