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%I #4 Mar 30 2012 16:48:55
%S 0,1,3,6,15,36,99,270,783,2298,6936,21204,65895,206862,656253,2098602,
%T 6761028,21917364,71450229,234070806,770216253,2544458592,8435990916,
%U 28060099692,93612265143,313153860210,1050194570445,3530080085868
%N A 3-way generalization of series-parallel networks with n unlabeled edges.
%C Compare the combstruct construction here with that for A000084.
%p 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)} ]; [seq(combstruct[count](spec,size=n), n=0..40)]; # N = A058534, S=A000669
%Y Cf. A000084, A058540, A000669, A058561.
%K nonn
%O 0,3
%A _N. J. A. Sloane_, Dec 24 2000
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