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 A326215 #6 Jun 17 2019 21:47:20
%S A326215 0,2,0,4,20
%N A326215 Number of Hamiltonian unlabeled n-vertex graphs with loops.
%C A326215 A graph is Hamiltonian if it contains a cycle passing through every vertex exactly once.
%H A326215 Wikipedia, <a href="https://en.wikipedia.org/wiki/Hamiltonian_path">Hamiltonian path</a>
%H A326215 Gus Wiseman, <a href="/A326215/a326215.png">Non-isomorphic representatives of the a(4) = 20 Hamiltonian graphs with loops</a>.
%e A326215 Non-isomorphic representatives of the a(3) = 4 edge-sets:
%e A326215 {12,13,23}
%e A326215 {12,13,23,33}
%e A326215 {12,13,22,23,33}
%e A326215 {11,12,13,22,23,33}
%Y A326215 The labeled case is A326240.
%Y A326215 The directed case is A326226 (with loops) or A326225 (without loops).
%Y A326215 The case without loops A003216.
%Y A326215 Cf. A000088, A006125, A057864, A283420, A326239.
%K A326215 nonn,more
%O A326215 0,2
%A A326215 _Gus Wiseman_, Jun 17 2019