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 A052623 #15 Jun 03 2022 18:35:20
%S A052623 0,1,2,18,192,2520,39600,725760,15200640,358162560,9376819200,
%T A052623 270037152000,8483597337600,288734500454400,10582834303641600,
%U A052623 415593298568448000,17408598098411520000,774797125808369664000
%N A052623 E.g.f. x(1-x)^2/(1-3x+x^2).
%H A052623 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=569">Encyclopedia of Combinatorial Structures 569</a>
%F A052623 E.g.f.: x*(-1+x)^2/(1-3*x+x^2)
%F A052623 Recurrence: {a(1)=1, a(0)=0, a(2)=2, (n^2+3*n+2)*a(n)+(-6-3*n)*a(n+1)+a(n+2)=0, a(3)=18}
%F A052623 Sum(-1/5*(-3+7*_alpha)*_alpha^(-1-n), _alpha=RootOf(_Z^2-3*_Z+1))*n!
%F A052623 a(n)=n!*A088305(n-1). - _R. J. Mathar_, Jun 03 2022
%p A052623 spec := [S,{S=Prod(Z,Sequence(Prod(Z,Sequence(Z),Sequence(Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052623 With[{nn=20},CoefficientList[Series[x (1-x)^2/(1-3x+x^2),{x,0,nn}],x] Range[ 0,nn]!] (* _Harvey P. Dale_, May 30 2021 *)
%K A052623 easy,nonn
%O A052623 0,3
%A A052623 encyclopedia(AT)pommard.inria.fr, Jan 25 2000