This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A076278 #22 Feb 16 2025 08:32:47
%S A076278 0,1,2,6,12,34,87,302,1118,5478,32302,251134,2527711,33985852,
%T A076278 611846939,14864650923,488222721991,21712049275197,1308300679611468,
%U A076278 106897965189674290,11852113048215107821,1784730721403509209214,365323537513403184463272
%N A076278 Number of 2-chromatic (i.e., chromatic number equals 2) simple graphs on n nodes.
%H A076278 Andrew Howroyd, <a href="/A076278/b076278.txt">Table of n, a(n) for n = 1..50</a>
%H A076278 Keith M. Briggs, <a href="http://keithbriggs.info/cgt.html">Combinatorial Graph Theory</a>
%H A076278 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/n-ChromaticGraph.html">n-Chromatic Graph</a>
%F A076278 a(n) = A033995(n)-1.
%t A076278 A005142 = Import["https://oeis.org/A005142/b005142.txt", "Table"][[All, 2]];
%t A076278 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];
%t A076278 a = etr[A005142[[# + 1]]&][#] - 1&;
%t A076278 Array[a, 23] (* _Jean-François Alcover_, Sep 03 2019 *)
%Y A076278 Column k=2 of A084268.
%Y A076278 Cf. A076279, A076280, A076281, A076282, A115597.
%K A076278 nonn
%O A076278 1,3
%A A076278 _Eric W. Weisstein_, Oct 06 2002
%E A076278 More terms from _Vladeta Jovovic_, Jul 31 2003
%E A076278 Terms a(21) and beyond from _Andrew Howroyd_, Sep 05 2018