A320759 Number of ordered set partitions of [n] where the maximal block size equals three.

1, 8, 80, 860, 10290, 136080, 1977360, 31365600, 539847000, 10026139200, 199937337600, 4262167509600, 96744738090000, 2329950823200000, 59348032327584000, 1594257675506496000, 45047749044458160000, 1335740755933584000000, 41473196779273459200000

Offset: 3

Author

Alois P. Heinz, Oct 20 2018

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 3..424

Crossrefs

Column k=3 of A276922.

Cf. A080599, A189886.

Programs

Maple

b:= proc(n, k) option remember; `if`(n=0, 1, add(
      b(n-i, k)*binomial(n, i), i=1..min(n, k)))
    end:
a:= n-> (k-> b(n, k) -b(n, k-1))(3):
seq(a(n), n=3..25);

Mathematica

b[n_, k_] := b[n, k] = If[n == 0, 1, Sum[b[n - i, k] Binomial[n, i], {i, 1, Min[n, k]}]];
a[n_] := With[{k = 3}, b[n, k] - b[n, k-1]];
a /@ Range[3, 25] (* Jean-François Alcover, Dec 14 2020, after Alois P. Heinz *)

Formula

E.g.f.: 1/(1-Sum_{i=1..3} x^i/i!) - 1/(1-Sum_{i=1..2} x^i/i!).

a(n) = A189886(n) - A080599(n).