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Another 3-way generalization of series-parallel networks with n unlabeled edges.
4

%I #4 Mar 30 2012 16:48:56

%S 0,1,3,9,36,144,651,3015,14634,72654,369063,1904985,9971889,52788393,

%T 282161025,1520597895,8253281871,45075359277,247534382298,

%U 1365994896264,7571065357620,42127865408028,235246997219400,1317894484506336

%N Another 3-way generalization of series-parallel networks with n unlabeled edges.

%C Compare the combstruct construction here with those for A000084 and A058534.

%p 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 Cf. A000084, A058534, A058371.

%K nonn

%O 0,3

%A _N. J. A. Sloane_, Dec 24 2000