A132220 Number of line graphs on n unlabeled nodes.

1, 2, 4, 10, 24, 63, 166, 471, 1408, 4436, 14719, 51468, 188754, 725010, 2907576, 12141462, 52654529, 236584104, 1098926868, 5266708497, 25997964452, 131975740172, 688019138485, 3678898458587, 20153878941460, 113000861804642, 647866385202660, 3794917399892511, 22693153274994534, 138436809489370914, 860962282687080614

Offset

1,2

Links

Max Alekseyev, Table of n, a(n) for n = 1..60

Eric Weisstein's World of Mathematics, Line Graph.

Formula

For n>=3, a(n) = A000664(n) - A000664(n-3). - Max Alekseyev, Sep 03 2016

Mathematica

<< Combinatorica`; With[{e = Table[NumberOfGraphs[2 n, n], {n, 12}]}, e - Join[{0, 0, 1}, Drop[e, -3]]] (* Eric W. Weisstein, Oct 30 2017 *)
<< Combinatorica`; With[{e = Table[Coefficient[GraphPolynomial[2 n, x], x, n], {n, 12}], e - Join[{0, 0, 1}, Drop[e, -3]]] (* Eric W. Weisstein, Oct 30 2017 *)

Crossrefs

Euler transform of A003089.

Cf. A132219 (labeled case).

Cf. A000664.

Sequence in context: A121691 A124499 A303840 * A007874 A294410 A121704

Adjacent sequences: A132217 A132218 A132219 * A132221 A132222 A132223

Keyword

nonn

Author

Vladeta Jovovic, Aug 14 2007

Extensions

a(25)-a(60) from Max Alekseyev, Sep 07 2016

Status

approved