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 A215851 #13 Dec 04 2014 06:50:23 %S A215851 1,1,4,19,137,1356,17167,264664,4803129,100181440,2359762091, %T A215851 61937322624,1792399894837,56697025885696,1946238657504975, %U A215851 72058247875111936,2862433512904759793,121439708940308299776,5480390058971655049939,262144060822550204416000 %N A215851 Number of simple labeled graphs on n nodes with exactly 1 connected component that is a tree or a cycle. %H A215851 Alois P. Heinz, <a href="/A215851/b215851.txt">Table of n, a(n) for n = 1..150</a> %F A215851 a(1) = a(2) = 1, a(n) = A000272(n) + A001710(n-1) = n^(n-2) + (n-1)!/2 for n>2. %e A215851 a(3) = 4: %e A215851 .1-2. .1-2. .1-2. .1 2. %e A215851 .|/ . .| . . / . .|/ . %e A215851 .3... .3... .3... .3... %p A215851 a:= n-> `if`(n<3, 1, (n-1)!/2+n^(n-2)): %p A215851 seq(a(n), n=1..25); %Y A215851 Column k=1 of A215861. %Y A215851 The unlabeled version is A215981. %Y A215851 Cf. A000272, A001710. %K A215851 nonn %O A215851 1,3 %A A215851 _Alois P. Heinz_, Aug 25 2012