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 A306827 #25 Mar 28 2019 06:25:56 %S A306827 1,2,5,13,59,285,1987,16057,149430,1551863,17747299,221015026 %N A306827 Isomorphism classes of connected 2-in 2-out digraphs, allowing parallel edges but not loops. %C A306827 From _R. J. Mathar_, Mar 15 2019: (Start) %C A306827 The multiset transform (graphs with n>=2 nodes and k>=1 components) is %C A306827 1 %C A306827 2 %C A306827 5 1 %C A306827 13 2 %C A306827 59 8 1 %C A306827 285 23 2 %C A306827 1987 100 8 1 %C A306827 16057 468 27 2 %C A306827 149430 2943 115 8 1 %C A306827 The row sums (Euler transform) are A307180. (End) %H A306827 J. Hu, A. H. MacDonald and B. D. McKay, <a href="https://doi.org/10.1103/PhysRevB.49.15263">Correlations in two-dimensional vortex liquids</a>, Physical Review B, 49, 21 (1994) 15263-15270. %H A306827 R. J. Mathar, <a href="/A306827/a306827.pdf">OEIS A306827</a> %e A306827 For n=3, the a(3)=2 solutions consist of two superimposed directed triangles, one with the triangles oriented consistently and the other with the triangles oriented oppositely. %Y A306827 Cf. A219889 (no multiedges, not necessarily connected), A306892 (loops permitted), A307180 (Euler transf.) %K A306827 nonn,hard %O A306827 2,2 %A A306827 _Brendan McKay_, Mar 12 2019