This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A275539 #10 Dec 07 2016 12:35:25
%S A275539 1,2,4,12,36,160,810,5376,41160,366336,3638250,39959040,479126340,
%T A275539 6227619840,87180183090,1307684044800,20922822320400,355687603568640,
%U A275539 6402374325997650,121645103938928640,2432902021271221500,51090942249743155200,1124000728080092512650
%N A275539 a(n) = n! + n*(n-1)!!.
%H A275539 Alois P. Heinz, <a href="/A275539/b275539.txt">Table of n, a(n) for n = 0..449</a>
%F A275539 a(n) = n! + n*(n-1)!! = n*((n-1)! + (n-1)!!).
%p A275539 a:= proc(n) option remember; `if`(n<5, [1, 2, 4, 12, 36]
%p A275539 [n+1], ((n-3)^2*n*a(n-1) +(n-4)*n*(n-1)*a(n-2)
%p A275539 -(n-3)*n*(n-1)*(n-2)*a(n-3))/((n-4)*(n-2)))
%p A275539 end:
%p A275539 seq(a(n), n=0..25); # _Alois P. Heinz_, Dec 07 2016
%t A275539 Table[n! + n (n-1)!!, {n, 0, 20}] (* _Bruno Berselli_, Aug 11 2016 *)
%Y A275539 Cf. A000142, A080497.
%K A275539 nonn,easy
%O A275539 0,2
%A A275539 _Olivier Gérard_, Aug 01 2016