A076278 Number of 2-chromatic (i.e., chromatic number equals 2) simple graphs on n nodes.

0, 1, 2, 6, 12, 34, 87, 302, 1118, 5478, 32302, 251134, 2527711, 33985852, 611846939, 14864650923, 488222721991, 21712049275197, 1308300679611468, 106897965189674290, 11852113048215107821, 1784730721403509209214, 365323537513403184463272

Offset: 1

Author

Eric W. Weisstein, Oct 06 2002

Keywords

• nonn

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..50
• Keith M. Briggs, Combinatorial Graph Theory
• Eric Weisstein's World of Mathematics, n-Chromatic Graph

Crossrefs

Column k=2 of A084268.

Cf. A076279, A076280, A076281, A076282, A115597.

Programs

Mathematica

A005142 = Import["https://oeis.org/A005142/b005142.txt", "Table"][[All, 2]];
etr[p_] := Module[{b}, b[n_] := b[n] = If[n == 0, 1, Sum[Sum[d*p[d], {d, Divisors[j]}]*b[n - j], {j, 1, n}]/n]; b];
a = etr[A005142[[# + 1]]&][#] - 1&;
Array[a, 23] (* Jean-François Alcover, Sep 03 2019 *)

Formula

a(n) = A033995(n)-1.

Extensions

More terms from Vladeta Jovovic, Jul 31 2003

Terms a(21) and beyond from Andrew Howroyd, Sep 05 2018