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 A052640 #15 Oct 10 2023 16:35:56
%S A052640 0,1,2,18,144,1680,22320,352800,6330240,128096640,2877638400,
%T A052640 71131737600,1917922406400,56024506137600,1762396334899200,
%U A052640 59401108166400000,2135568241078272000,81575844571533312000
%N A052640 E.g.f. x*(1-x)/(1-2*x-x^2+x^3).
%H A052640 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=586">Encyclopedia of Combinatorial Structures 586</a>
%F A052640 E.g.f.: -x*(-1+x)/(x^3-x^2-2*x+1)
%F A052640 Recurrence: {a(1)=1, a(0)=0, a(2)=2, (n^3+6*n^2+11*n+6)*a(n)+(-n^2-5*n-6)*a(n+1)+(-2*n-6)*a(n+2)+a(n+3)=0}
%F A052640 Sum(1/7*(-_alpha+_alpha^2+1)*_alpha^(-1-n), _alpha=RootOf(_Z^3-_Z^2-2*_Z+1))*n!
%F A052640 a(n) = n!*A077998(n-1), n>0. - _R. J. Mathar_, Nov 27 2011
%p A052640 spec := [S,{S=Prod(Z,Sequence(Prod(Z,Union(Z,Sequence(Z)))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052640 With[{nn=20},CoefficientList[Series[-x*(-1+x)/(x^3-x^2-2*x+1),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Oct 10 2023 *)
%K A052640 easy,nonn
%O A052640 0,3
%A A052640 encyclopedia(AT)pommard.inria.fr, Jan 25 2000