A275539 a(n) = n! + n*(n-1)!!.

1, 2, 4, 12, 36, 160, 810, 5376, 41160, 366336, 3638250, 39959040, 479126340, 6227619840, 87180183090, 1307684044800, 20922822320400, 355687603568640, 6402374325997650, 121645103938928640, 2432902021271221500, 51090942249743155200, 1124000728080092512650

Offset: 0

Author

Olivier Gérard, Aug 01 2016

Keywords

• nonn
• easy

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..449

Crossrefs

Cf. A000142, A080497.

Programs

Maple

a:= proc(n) option remember; `if`(n<5, [1, 2, 4, 12, 36]
      [n+1], ((n-3)^2*n*a(n-1) +(n-4)*n*(n-1)*a(n-2)
       -(n-3)*n*(n-1)*(n-2)*a(n-3))/((n-4)*(n-2)))
    end:
seq(a(n), n=0..25);  # Alois P. Heinz, Dec 07 2016

Mathematica

Table[n! + n (n-1)!!, {n, 0, 20}] (* Bruno Berselli, Aug 11 2016 *)

Formula

a(n) = n! + n*(n-1)!! = n*((n-1)! + (n-1)!!).