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URL: https://oeis.org/A132220

⇱ A132220 - OEIS

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A132220
Number of line graphs on n unlabeled nodes.
2
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
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
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Aug 14 2007
EXTENSIONS
a(25)-a(60) from Max Alekseyev, Sep 07 2016
STATUS
approved