

A126750


Number of 2connected (or biconnected) graphs on n nodes with chromatic number 2.


1



0, 0, 0, 1, 1, 5, 8, 42, 146, 956, 6643, 65921, 818448, 13442572, 287665498, 8099980771, 300760170216, 14791653463768, 967055338887805, 84368806391412395, 9855854129239183783, 1546801291978378704267
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OFFSET

1,6


COMMENTS

Equally, number of 2connected (or biconnected) bipartite graphs on n nodes.
Also number of biconnected trianglefree graphs on n nodes with chromatic number 2, since chromatic number 2 implies trianglefree.  Gordon Royle, Apr 11 2007
See the GainerDewar/Gessel reference for formulas and Sage code.  Ralf Stephan, May 18 2013


LINKS

Table of n, a(n) for n=1..22.
Keith M. Briggs, Combinatorial Graph Theory
C. J. Colbourn and C. Huybrechts, Fully gated graphs,: recognition and convex operations, Discrete Math., 308 (2008), 51845195.
A. GainerDewar and I. M. Gessel, Enumeration of bipartite graphs and bipartite blocks, arXiv:1304.0139 [math.CO], 20132014.


CROSSREFS

A diagonal of triangle in A126749.
Sequence in context: A192272 A073930 A130223 * A256401 A109292 A275571
Adjacent sequences: A126747 A126748 A126749 * A126751 A126752 A126753


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Feb 18 2007, Oct 01 2008


STATUS

approved



