OFFSET
0,5
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..576
Wikipedia, Partition of a set
MAPLE
b:= proc(n, k) option remember; `if`(n=0, 1, add(
`if`(j=k, 0, b(n-j, k)*binomial(n-1, j-1)), j=1..n))
end:
a:= n-> b(n, 0)-b(n, 3):
seq(a(n), n=0..27);
MATHEMATICA
b[n_, k_] := b[n, k] = If[n==0, 1, Sum[If[j==k, 0, b[n-j, k] Binomial[n-1, j-1]], {j, 1, n}]];
a[n_] := b[n, 0] - b[n, 3];
a /@ Range[0, 27] (* Jean-François Alcover, May 02 2020, after Maple *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 05 2019
STATUS
approved