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A126750 Number of 2-connected (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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,6

COMMENTS

Equally, number of 2-connected (or biconnected) bipartite graphs on n nodes.

Also number of biconnected triangle-free graphs on n nodes with chromatic number 2, since chromatic number 2 implies triangle-free. - Gordon Royle, Apr 11 2007

See the Gainer-Dewar/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), 5184-5195.

A. Gainer-Dewar and I. M. Gessel, Enumeration of bipartite graphs and bipartite blocks, arXiv:1304.0139 [math.CO], 2013-2014.

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

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Last modified June 1 04:57 EDT 2020. Contains 334758 sequences. (Running on oeis4.)