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 A308161 #19 Apr 10 2020 21:02:51 %S A308161 1,1,1,2,3,7,24,200,5479,439517,91097868,48916220147,68628518786683, %T A308161 254305521019154638,2512451288680194070842,66741359152815902974086530, %U A308161 4802230893555589082929258033462,942013815025325986980154281918094498,506666364226468633163453153303288094604018 %N A308161 Number of isomorphism classes of Eulerian oriented graphs with n vertices. %C A308161 Loops and 2-cycles are not permitted. Eulerian means that each vertex has equal in-degree and out-degree. %e A308161 For n=4, the a(4)=3 solutions are an empty graph, a directed 3-cycle plus an isolated vertex, and a directed 4-cycle. %Y A308161 A058338 is the same allowing 2-cycles. %Y A308161 A308111 is the same allowing both loops and 2-cycles. %Y A308161 Cf. A007081 (labeled), A308239 (connected). %K A308161 nonn,hard %O A308161 0,4 %A A308161 _Brendan McKay_, May 15 2019 %E A308161 Terms a(12) and beyond from _Andrew Howroyd_, Apr 10 2020