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 A052660 #13 Apr 18 2017 07:03:59
%S A052660 2,2,6,24,144,1080,10080,110880,1411200,20321280,326592000,5787936000,
%T A052660 112086374400,2353813862400,53265935923200,1291982275584000,
%U A052660 33434618241024000,919452001628160000
%N A052660 E.g.f. (2-2x-x^2)/((1-x)(1-x-x^2)).
%H A052660 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=607">Encyclopedia of Combinatorial Structures 607</a>
%F A052660 E.g.f.: -(-2+x^2+2*x)/(-1+x)/(-1+x+x^2)
%F A052660 Recurrence: {a(1)=2, a(2)=6, a(0)=2, (n^3+6*n^2+11*n+6)*a(n)+(-2*n-6)*a(n+2)+a(n+3)=0}
%F A052660 (1+Sum(1/5*(1+2*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+_Z^2)))*n!
%F A052660 a(n) = n!*A001611(n+1). - _R. J. Mathar_, Nov 27 2011
%p A052660 spec := [S,{S=Union(Sequence(Z),Sequence(Union(Z,Prod(Z,Z))))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%K A052660 easy,nonn
%O A052660 0,1
%A A052660 encyclopedia(AT)pommard.inria.fr, Jan 25 2000