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 A052580 #20 Mar 06 2022 08:36:07
%S A052580 1,0,2,12,120,1440,20880,352800,6814080,148055040,3574368000,
%T A052580 94922150400,2749948185600,86306508288000,2917072801843200,
%U A052580 105636550795776000,4080467097907200000,167469023145295872000
%N A052580 E.g.f. (1-2x)/(1-2x-x^2).
%H A052580 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=524">Encyclopedia of Combinatorial Structures 524</a>
%F A052580 E.g.f.: (-1+2*x)/(-1+2*x+x^2)
%F A052580 D-finite recurrence: {a(1)=0, a(0)=1, (-2-n^2-3*n)*a(n)+(-4-2*n)*a(n+1)+a(n+2)=0.}
%F A052580 Sum(1/4*(-1+3*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^2))*n!
%F A052580 a(n) = n!*A000129(n-1). - _R. J. Mathar_, Nov 27 2011
%p A052580 spec := [S,{S=Sequence(Prod(Z,Z,Sequence(Union(Z,Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052580 With[{nn=20},CoefficientList[Series[(1-2x)/(1-2x-x^2),{x,0,nn}],x] Range[ 0,nn]!] (* _Harvey P. Dale_, Dec 05 2018 *)
%K A052580 easy,nonn
%O A052580 0,3
%A A052580 encyclopedia(AT)pommard.inria.fr, Jan 25 2000