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 A052617 #15 Jun 03 2022 18:50:26
%S A052617 1,4,18,114,936,9480,114480,1607760,25764480,464123520,9286099200,
%T A052617 204334099200,4904497382400,127523158963200,3570735629260800,
%U A052617 107123376552192000,3427968972460032000,116551300751069184000
%N A052617 E.g.f. (1+x-x^2)/((1-x)(1-2x)).
%H A052617 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=562">Encyclopedia of Combinatorial Structures 562</a>
%F A052617 E.g.f.: -(-x+x^2-1)/(-1+2*x)/(-1+x)
%F A052617 Recurrence: {a(0)=1, a(1)=4, (2*n^2+6*n+4)*a(n) +(-6-3*n)*a(n+1) +a(n+2)=0, a(2)=18}
%F A052617 (-1+5*2^(n-1))*n!, n>0.
%F A052617 a(n)=n!*A052549(n). - _R. J. Mathar_, Jun 03 2022
%p A052617 spec := [S,{S=Prod(Union(Z,Sequence(Z)),Sequence(Union(Z,Z)))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052617 With[{nn=20},CoefficientList[Series[(1+x-x^2)/((1-x)(1-2x)),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Nov 25 2018 *)
%K A052617 easy,nonn
%O A052617 0,2
%A A052617 encyclopedia(AT)pommard.inria.fr, Jan 25 2000