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 A362387 #28 Jun 12 2023 14:51:54 %S A362387 1,1,6,31,198,1270,8838,63419,475796,3697876,29793899,248312750, %T A362387 2139071800,19023864211,174495408060,1648972635108,16036927429350, %U A362387 160338342813918,1646265278973845,17340402062779049,187190960572134321 %N A362387 Number of directed multigraphs including self-loops on n vertices and n edges. %D A362387 F. Harary and E. Palmer, Graphical Enumeration, Academic Press, 1973, page 88. %F A362387 a(n) = A138107(n,n). %e A362387 For n=2 we have two edges and two vertices, for a total of six graphs: %e A362387 * one vertex with two loops attached to it %e A362387 * one loop attached to each vertex %e A362387 * two vertices linked by two edges in opposite directions %e A362387 * two vertices linked by two edges in the same direction %e A362387 * one vertex with a loop being pointed at %e A362387 * one vertex with a loop being pointed away from. %Y A362387 Main diagonal of A138107. %K A362387 nonn %O A362387 0,3 %A A362387 _Marko Riedel_, Jun 09 2023