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 A116508 #38 Aug 06 2025 09:08:23
%S A116508 1,0,0,1,15,252,5005,116280,3108105,94143280,3190187286,119653565850,
%T A116508 4922879481520,220495674290430,10682005290753420,556608279578340080,
%U A116508 31044058215401404845,1845382436487682488000,116475817125419611477660,7779819801401934344268210
%N A116508 a(n) = C( C(n,2), n).
%C A116508 a(n) is the number of simple labeled graphs with n nodes and n edges. - _Geoffrey Critzer_, Nov 02 2014
%C A116508 These graphs are not necessarily covering, but the covering case is A367863, unlabeled A006649, and the unlabeled version is A001434. - _Gus Wiseman_, Dec 22 2023
%H A116508 Alois P. Heinz, <a href="/A116508/b116508.txt">Table of n, a(n) for n = 0..370</a>
%F A116508 a(n) ~ exp(n - 2) * n^(n - 1/2) / (sqrt(Pi) * 2^(n + 1/2)). - _Vaclav Kotesovec_, May 19 2020
%F A116508 a(n) = [x^n] (1+x)^(n*(n-1)/2). - _Vaclav Kotesovec_, Aug 06 2025
%e A116508 a(5) = C(C(5,2),5) = C(10,5) = 252.
%p A116508 a:= n-> binomial(binomial(n, 2), n):
%p A116508 seq(a(n), n=0..20);
%t A116508 nn = 18; f[x_, y_] :=
%t A116508 Sum[(1 + y)^Binomial[n, 2] x^n/n!, {n, 1, nn}]; Table[
%t A116508 n! Coefficient[Series[f[x, y], {x, 0, nn}], x^n y^n], {n, 1, nn}] (* _Geoffrey Critzer_, Nov 02 2014 *)
%t A116508 Table[Length[Subsets[Subsets[Range[n],{2}],{n}]],{n,0,5}] (* _Gus Wiseman_, Dec 22 2023 *)
%t A116508 Table[SeriesCoefficient[(1 + x)^(n*(n-1)/2), {x, 0, n}], {n, 0, 20}] (* _Vaclav Kotesovec_, Aug 06 2025 *)
%o A116508 (Sage) [(binomial(binomial(n+2,n),n+2)) for n in range(-1, 17)] # _Zerinvary Lajos_, Nov 30 2009
%o A116508 (Magma) [0] cat [(Binomial(Binomial(n+2, n), n+2)): n in [0..20]]; // _Vincenzo Librandi_, Nov 03 2014
%o A116508 (Python)
%o A116508 from math import comb
%o A116508 def A116508(n): return comb(n*(n-1)>>1,n) # _Chai Wah Wu_, Jul 02 2024
%Y A116508 Main diagonal of A084546.
%Y A116508 The unlabeled version is A001434, covering case A006649.
%Y A116508 The connected case is A057500, unlabeled A001429.
%Y A116508 For set-systems we have A136556, covering case A054780.
%Y A116508 The covering case is A367863.
%Y A116508 A006125 counts graphs, A000088 unlabeled.
%Y A116508 A006129 counts covering graphs, A002494 unlabeled.
%Y A116508 A133686 counts graphs satisfying a strict AOC, connected A129271.
%Y A116508 A367867 counts graphs contradicting a strict AOC, connected A140638.
%Y A116508 Cf. A001187, A003465, A143543, A305000, A367916, A367917.
%K A116508 easy,nonn
%O A116508 0,5
%A A116508 Christopher Hanusa (chanusa(AT)math.binghamton.edu), Mar 21 2006
%E A116508 a(0)=1 prepended by _Alois P. Heinz_, Feb 02 2024