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 A052666 #13 Apr 18 2017 07:03:59
%S A052666 1,1,8,42,456,4800,69840,1093680,20482560,420577920,9736070400,
%T A052666 245887488000,6806133734400,203555082931200,6565920180019200,
%U A052666 226728504946944000,8355118608764928000,327047476385710080000
%N A052666 E.g.f. 1/(1-x-3x^2).
%H A052666 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=613">Encyclopedia of Combinatorial Structures 613</a>
%F A052666 E.g.f.: -1/(-1+x+3*x^2)
%F A052666 Recurrence: {a(1)=1, a(0)=1, (-3*n^2-9*n-6)*a(n)+(-2-n)*a(n+1)+a(n+2)=0}
%F A052666 Sum(1/13*(1+6*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+3*_Z^2))*n!
%F A052666 a(n) = n!*A006130(n). - _R. J. Mathar_, Nov 27 2011
%p A052666 spec := [S,{S=Sequence(Union(Z,Prod(Z,Union(Z,Z,Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%K A052666 easy,nonn
%O A052666 0,3
%A A052666 encyclopedia(AT)pommard.inria.fr, Jan 25 2000