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 A304981 #8 May 23 2018 23:12:46
%S A304981 1,1,2,5,19,176,16118,489996568
%N A304981 Number of unlabeled clutters (connected antichains) spanning up to n vertices without singleton edges.
%F A304981 Partial sums of A261006(n > 0).
%e A304981 Non-isomorphic representatives of the a(3) = 5 clutters:
%e A304981 {}
%e A304981 {{1,2}}
%e A304981 {{1,2,3}}
%e A304981 {{1,3},{2,3}}
%e A304981 {{1,2},{1,3},{2,3}}
%e A304981 Non-isomorphic representatives of the a(4) = 19 clutters:
%e A304981 {}
%e A304981 {{1,2}}
%e A304981 {{1,2,3}}
%e A304981 {{1,2,3,4}}
%e A304981 {{1,3},{2,3}}
%e A304981 {{1,4},{2,3,4}}
%e A304981 {{1,3,4},{2,3,4}}
%e A304981 {{1,2},{1,3},{2,3}}
%e A304981 {{1,2},{1,3,4},{2,3,4}}
%e A304981 {{1,3},{1,4},{2,3,4}}
%e A304981 {{1,3},{2,4},{3,4}}
%e A304981 {{1,4},{2,4},{3,4}}
%e A304981 {{1,2,4},{1,3,4},{2,3,4}}
%e A304981 {{1,2},{1,3},{1,4},{2,3,4}}
%e A304981 {{1,2},{1,3},{2,4},{3,4}}
%e A304981 {{1,4},{2,3},{2,4},{3,4}}
%e A304981 {{1,2,3},{1,2,4},{1,3,4},{2,3,4}}
%e A304981 {{1,3},{1,4},{2,3},{2,4},{3,4}}
%e A304981 {{1,2},{1,3},{1,4},{2,3},{2,4},{3,4}}
%Y A304981 Cf. A048143, A198085, A261006, A304912, A304981, A304982, A304983, A304984, A304985, A304986.
%K A304981 nonn
%O A304981 0,3
%A A304981 _Gus Wiseman_, May 23 2018