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A261919
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Number of n-node unlabeled graphs without isolated nodes or endpoints (i.e., no nodes of degree 0 or 1).
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13
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1, 0, 0, 1, 3, 11, 62, 510, 7459, 197867, 9808968, 902893994, 153723380584, 48443158427276, 28363698856991892, 30996526139142442460, 63502034434187094606966, 244852545450108200518282934, 1783161611521019613186341526720, 24603891216946828886755056314074748
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OFFSET
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0,5
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REFERENCES
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F. Harary, Graph Theory, Wiley, 1969. See illustrations in Appendix 1.
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LINKS
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FORMULA
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EXAMPLE
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Non-isomorphic representatives of the a(0) = 1 through a(5) = 11 graphs (empty columns not shown):
{} {12,13,23} {12,13,24,34} {12,13,24,35,45}
{13,14,23,24,34} {12,14,25,34,35,45}
{12,13,14,23,24,34} {12,15,25,34,35,45}
{13,14,23,24,35,45}
{12,13,24,25,34,35,45}
{13,15,24,25,34,35,45}
{14,15,24,25,34,35,45}
{12,13,15,24,25,34,35,45}
{14,15,23,24,25,34,35,45}
{13,14,15,23,24,25,34,35,45}
{12,13,14,15,23,24,25,34,35,45}
(End)
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MATHEMATICA
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permcount[v_] := Module[{m = 1, s = 0, k = 0, t}, For[i = 1, i <= Length[v], i++, t = v[[i]]; k = If[i > 1 && t == v[[i - 1]], k + 1, 1]; m *= t*k; s += t]; s!/m];
edges[v_] := Sum[GCD[v[[i]], v[[j]]], {i, 2, Length[v]}, {j, 1, i - 1}] + Total[Quotient[v, 2]];
b[n_] := Sum[permcount[p]*2^edges[p]*Coefficient[Product[1-x^p[[i]], {i, 1, Length[p]}], x, n-k]/k!, {k, 1, n}, {p, IntegerPartitions[k]}]; b[0] = 1;
a[n_] := b[n] - b[n-1];
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CROSSREFS
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Cf. A004108 (connected version), A004110 (version allowing isolated nodes).
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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