A271733 Number of set partitions of [n] with maximal block length multiplicity equal to four.

1, 0, 15, 35, 385, 2331, 13335, 88110, 629200, 4811235, 35992957, 276332420, 2325570065, 20036259075, 171879027000, 1583318184855, 14476456463826, 139849724906591, 1347082690705367, 13909222770509990, 144001190692525628, 1519193757875044900

Offset: 4

Author

Alois P. Heinz, Apr 13 2016

Keywords

• nonn

Comments

At least one block length occurs exactly 4 times, and all block lengths occur at most 4 times.

Links

• Alois P. Heinz, Table of n, a(n) for n = 4..612
• Wikipedia, Partition of a set

Crossrefs

Column k=4 of A271423.

Programs

Maple

with(combinat):
b:= proc(n, i, k) option remember; `if`(n=0, 1,
      `if`(i<1, 0, add(multinomial(n, n-i*j, i$j)
        *b(n-i*j, i-1, k)/j!, j=0..min(k, n/i))))
    end:
a:= n-> b(n$2, 4)-b(n$2, 3):
seq(a(n), n=4..30);

Mathematica

multinomial[n_, k_List] := n!/Times @@ (k!);
b[n_, i_, k_] := b[n, i, k] = If[n == 0, 1, If[i < 1, 0, Sum[multinomial[n, Join[{n - i*j}, Table[i, j]]]*b[n - i*j, i - 1, k]/j!, {j, 0, Min[k, n/i] }]]];
a[n_] := b[n, n, 4] - b[n, n, 3];
Table[a[n], {n, 4, 30}] (* Jean-François Alcover, May 08 2018, after Alois P. Heinz *)