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 A058540 #4 Mar 30 2012 16:48:56
%S A058540 0,1,3,9,36,144,651,3015,14634,72654,369063,1904985,9971889,52788393,
%T A058540 282161025,1520597895,8253281871,45075359277,247534382298,
%U A058540 1365994896264,7571065357620,42127865408028,235246997219400,1317894484506336
%N A058540 Another 3-way generalization of series-parallel networks with n unlabeled edges.
%C A058540 Compare the combstruct construction here with those for A000084 and A058534.
%p A058540 spec := [ N, {N=Union(Z,S,P,Q), S=Set(Union(Z,P,Q),card>=2), P=Set(Union(Z,S,Q),card>=2), Q=Set(Union(Z,S,P),card>=2)} ]; [seq(combstruct[count](spec,size=n), n=0..40)]; # N=A058540, S=A058371
%Y A058540 Cf. A000084, A058534, A058371.
%K A058540 nonn
%O A058540 0,3
%A A058540 _N. J. A. Sloane_, Dec 24 2000