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

1,3

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

Formula

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

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 *)

Crossrefs

Column k=2 of A084268.

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

Sequence in context: A096610 A164099 A088808 * A221989 A349002 A204512

Adjacent sequences: A076275 A076276 A076277 * A076279 A076280 A076281

Keyword

nonn

Author

Eric W. Weisstein, Oct 06 2002

Extensions

More terms from Vladeta Jovovic, Jul 31 2003

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

Status

approved