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 A308634 #11 Jan 08 2022 22:01:47 %S A308634 1,1,2,5,18,81,475,3394,29140,293198 %N A308634 Number of triangle-free acyclic digraphs (or DAGs) up to isomorphism with n vertices, maximum indegree 2 and unique maximal element. %e A308634 a(1) = 1, a(2) = 1 and a(3) = 2 coincide with the first three terms of A001190, as all connected triangle-free DAGs with at most three vertices are necessarily trees. %e A308634 For n = 4, the a(4) = 5 solutions are the following three trees: %e A308634 (((()))) %e A308634 ((), (())) %e A308634 (((), ())) %e A308634 together with two 4-cycles. %Y A308634 Lower-bounded by A001190. %K A308634 nonn,more %O A308634 1,3 %A A308634 _Adam P. Goucher_, Jun 12 2019