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 A265580 #18 Feb 01 2020 22:40:13 %S A265580 1,0,0,1,1,2,4,7,13,27,54,112,243,538,1223,2875,6909,17052,43138, %T A265580 111686,295658,799684,2207356,6213391,17820961,52042771,154640528, %U A265580 467254731,1434837672,4475520062,14173115724,45548395180,148485883443,490831193397,1644581336531 %N A265580 Number of (unlabeled) loopless multigraphs with no isolated vertices such that the sum of the numbers of vertices and edges is n. %C A265580 Also the number of skeletal 2-cliquish graphs with n vertices and no isolated vertices. See Einstein et al. link below. %H A265580 Andrew Howroyd, <a href="/A265580/b265580.txt">Table of n, a(n) for n = 0..100</a> %H A265580 D. Einstein, M. Farber, E. Gunawan, M. Joseph, M. Macauley, J. Propp and S. Rubinstein-Salzedo, <a href="https://arxiv.org/abs/1510.06362">Noncrossing partitions, toggles, and homomesies</a>, arXiv:1510.06362 [math.CO], 2015. %F A265580 a(n) = A265581(n) - A265581(n-1), n>=1. %e A265580 For n = 5, the a(5) = 2 such multigraphs are the graph with three vertices and edges from one vertex to each of the other two, and the graph with two vertices connected by three edges. %Y A265580 Cf. A265581. %K A265580 nonn %O A265580 0,6 %A A265580 _Michael Joseph_, Dec 10 2015 %E A265580 Terms a(19) and beyond from _Andrew Howroyd_, Feb 01 2020