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 A052601 #16 Apr 18 2017 07:03:55
%S A052601 1,0,0,12,48,240,4320,50400,564480,9434880,166924800,2953843200,
%T A052601 60354201600,1357490534400,31907254579200,808142759424000,
%U A052601 22052620541952000,635257746579456000,19347973338710016000
%N A052601 E.g.f. (1-x)/(1-x-2x^3).
%H A052601 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=546">Encyclopedia of Combinatorial Structures 546</a>
%F A052601 E.g.f.: (-1+x)/(-1+x+2*x^3)
%F A052601 Recurrence: {a(1)=0, a(0)=1, a(2)=0, (-12*n^2-22*n-12-2*n^3)*a(n) +(-n-3)*a(n+2) +a(n+3)=0}
%F A052601 Sum(-1/29*(1+3*_alpha^2-10*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+2*_Z^3))*n!
%F A052601 a(n)= n!*A052537(n). - _R. J. Mathar_, Nov 27 2011
%p A052601 spec := [S,{S=Sequence(Prod(Z,Z,Sequence(Z),Union(Z,Z)))},labeled]: seq(combstruct[count](spec,size=n), n=0..20);
%K A052601 easy,nonn
%O A052601 0,4
%A A052601 encyclopedia(AT)pommard.inria.fr, Jan 25 2000