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A058534
A 3-way generalization of series-parallel networks with n unlabeled edges.
3
0, 1, 3, 6, 15, 36, 99, 270, 783, 2298, 6936, 21204, 65895, 206862, 656253, 2098602, 6761028, 21917364, 71450229, 234070806, 770216253, 2544458592, 8435990916, 28060099692, 93612265143, 313153860210, 1050194570445, 3530080085868
OFFSET
0,3
COMMENTS
Compare the combstruct construction here with that for A000084.
MAPLE
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
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Dec 24 2000
STATUS
approved