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 A308111 #21 Apr 12 2020 15:45:17 %S A308111 1,2,6,24,160,2512,129816,22665792,13056562208,24953006054144, %T A308111 160860329639968800,3555065836569542246400, %U A308111 273147301191314006316868352,73832333258502021627712839197696,70920540648597652305602460997787710080,244186544390677638132290202415190606165938176,3036252267734950687777830287721323374283100639476736 %N A308111 Isomorphism classes of Eulerian digraphs with n vertices, allowing loops. %C A308111 Eulerian means that for every vertex the in-degree equals the out-degree. %H A308111 Gus Wiseman, <a href="/A308111/a308111.png">Non-isomorphic representatives of the a(3) = 24 Eulerian digraphs with loops</a>. %F A308111 Euler transform of A308128. %e A308111 For n=2 the a(2)=6 solutions are: two non-adjacent vertices with or without loops (3 cases), two vertices with or without loops connected by edges in each direction (3 cases). %Y A308111 For labeled digraphs rather than isomorphism classes see A229865. %Y A308111 For isomorphism classes with loops forbidden see A058338. %Y A308111 Cf. A308128 (connected version of this). %Y A308111 Cf. A000273, A000595, A002416, A002720, A308161. %K A308111 nonn,hard %O A308111 0,2 %A A308111 _Brendan McKay_, May 11 2019 %E A308111 Terms a(10) and beyond from _Andrew Howroyd_, Apr 12 2020