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 A121227 #20 Jan 19 2025 11:29:28
%S A121227 1,2,14,150,2210,41642,956878,25955630,811819826,28764498386,
%T A121227 1138755852990,49817190098694,2386544217733906,124257113538066522,
%U A121227 6986465328614267742,421887743219324342110,27231714819135144778722,1871047822756547798671074
%N A121227 Number of labeled multigraphs with loops and with n edges and no vertex of degree 0.
%H A121227 Seiichi Manyama, <a href="/A121227/b121227.txt">Table of n, a(n) for n = 0..364</a> (terms 0..75 from Nathaniel Johnston)
%F A121227 a(n) = Sum_{k=0..2*n} binomial(k*(k+1)/2+n-1, n)*(Sum_{r=k..2*n} binomial(r, k)*(-1)^(r-k)). - _Andrew Howroyd_, Sep 15 2018
%p A121227 seq(sum(binomial(m*(m+1)/2+n-1,n)/2^(m+1),m=0..infinity),n=0..10);
%o A121227 (PARI) a(n)={sum(k=0, 2*n, binomial(k*(k+1)/2+n-1, n)*sum(r=k, 2*n, binomial(r, k)*(-1)^(r-k)) )} \\ _Andrew Howroyd_, Sep 15 2018
%Y A121227 Row n=2 of A331315.
%Y A121227 Unlabeled analog is A007717.
%K A121227 easy,nonn
%O A121227 0,2
%A A121227 _Vladeta Jovovic_, Sep 06 2006