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%I #18 Aug 15 2019 16:49:26
%S 0,0,0,0,1,2,15,85,650,5800,65243,931258,17182237,414512599,
%T 13161989059
%N Number of connected triangle-free graphs on n nodes with chromatic number 3.
%H Keith M. Briggs, <a href="http://keithbriggs.info/cgt.html">Combinatorial Graph Theory</a>
%e Table of number of connected triangle-free graphs on n nodes with chromatic number k begins:
%e (The first row is the same as the number of connected bipartite graphs on n nodes, A005142)
%e n=...1...2...3...4...5....6....7.....8.....9.....10......11.......12.........13
%e k.-----------------------------------------------------------------------------
%e 2|...0...1...1...3...5...17...44...182...730...4032...25598...212780....2241730 (A005142)
%e 3|...0...0...0...0...1....2...15....85...650...5800...65243...931258...17182237 (the present sequence)
%e 4|...0...0...0...0...0....0....0.....0.....0......0.......1.......23.......1085 (A126741)
%K nonn
%O 1,6
%A _N. J. A. Sloane_, based on email from _Keith Briggs_, Mar 21 2006
%E a(14) from _Michael Sollami_, Jan 29 2012
%E a(15) from _Michael Sollami_, Feb 04 2012
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