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 A052661 #19 Nov 25 2023 13:22:49
%S A052661 2,1,4,12,72,480,4320,45360,564480,7983360,127008000,2235340800,
%T A052661 43110144000,902918016000,20399720140800,494300911104000,
%U A052661 12783824621568000,351419178958848000,10230993181753344000
%N A052661 E.g.f. (2-3x)/((1-x)(1-x-x^2)).
%H A052661 Harvey P. Dale, <a href="/A052661/b052661.txt">Table of n, a(n) for n = 0..417</a>
%H A052661 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=608">Encyclopedia of Combinatorial Structures 608</a>
%F A052661 E.g.f.: -(-2+3*x)/(-1+x+x^2)/(-1+x)
%F A052661 Recurrence: {a(1)=1, a(2)=4, a(0)=2, (n^3+6*n^2+11*n+6)*a(n)+(-2*n-6)*a(n+2)+a(n+3)=0}
%F A052661 (1+Sum(1/5*(-1+3*_alpha)*_alpha^(-1-n), _alpha =RootOf(-1+_Z+_Z^2)))*n!
%F A052661 a(n) = n!*A001611(n-1), n>0. - _R. J. Mathar_, Nov 27 2011
%p A052661 spec := [S,{S=Union(Sequence(Z), Sequence(Prod(Z,Z, Sequence(Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%t A052661 With[{nn=20},CoefficientList[Series[(2-3x)/((1-x)(1-x-x^2)),{x,0,nn}],x] Range[0,nn]!] (* _Harvey P. Dale_, Nov 25 2023 *)
%K A052661 easy,nonn
%O A052661 0,1
%A A052661 encyclopedia(AT)pommard.inria.fr, Jan 25 2000