A245855 Number of preferential arrangements of n labeled elements such that the minimal number of elements per rank equals 2.

1, 0, 6, 20, 120, 672, 5516, 40140, 368640, 3521870, 37445298, 422339502, 5215454426, 68144100780, 954428684280, 14160968076584, 222769496190060, 3692874342747114, 64493471050666430, 1181830474135532130, 22692074431844298558, 455404848204906308984

Offset: 2

Author

Alois P. Heinz, Aug 04 2014

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 2..400

Crossrefs

Column k=2 of A245733.

Cf. A032032, A102233, A245732.

Programs

Maple

b:= proc(n, k) option remember; `if`(n=0, 1,
      add(b(n-j, k)*binomial(n, j), j=k..n))
    end:
a:= n-> b(n, 2) -b(n, 3):
seq(a(n), n=2..25);

Formula

E.g.f.: 1/(2-exp(x)+x) -1/(2-exp(x)+x+x^2/2).

a(n) = A032032(n) - A102233(n) = A245732(n,2) - A245732(n,3).