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 A058561 #4 Mar 30 2012 16:48:56
%S A058561 0,1,3,12,78,708,8256,117624,1980096,38456736,846413472,20819693568,
%T A058561 565998548352,16852047061632,545372109629184,19061178127458816,
%U A058561 715541912895773184,28713061759257037824,1226513716981332031488
%N A058561 A 3-way generalization of series-parallel networks with n labeled edges.
%p A058561 spec := [ N, {N=Union(Z,S,P,Q), S=Set(Union(Z,P),card>=2), P=Set(Union(Z,Q),card>=2), Q=Set(Union(Z,S),card>=2)}, labeled ]; [seq(combstruct[count](spec,size=n), n=0..40)]; # N = A058561, S=A000311
%Y A058561 Cf. A058534, A000311, A006351.
%K A058561 nonn
%O A058561 0,3
%A A058561 _N. J. A. Sloane_, Dec 26 2000