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 A304998 #6 May 23 2018 23:13:44
%S A304998 1,0,1,3,15,160,15963,489980652
%N A304998 Number of unlabeled antichains of finite sets spanning n vertices without singletons.
%F A304998 a(n > 0) = A261005(n) - A261005(n - 1).
%e A304998 Non-isomorphic representatives of the a(4) = 15 antichains:
%e A304998 {{1,2,3,4}}
%e A304998 {{1,2},{3,4}}
%e A304998 {{1,4},{2,3,4}}
%e A304998 {{1,3,4},{2,3,4}}
%e A304998 {{1,2},{1,3,4},{2,3,4}}
%e A304998 {{1,3},{1,4},{2,3,4}}
%e A304998 {{1,3},{2,4},{3,4}}
%e A304998 {{1,4},{2,4},{3,4}}
%e A304998 {{1,2,4},{1,3,4},{2,3,4}}
%e A304998 {{1,2},{1,3},{1,4},{2,3,4}}
%e A304998 {{1,2},{1,3},{2,4},{3,4}}
%e A304998 {{1,4},{2,3},{2,4},{3,4}}
%e A304998 {{1,2,3},{1,2,4},{1,3,4},{2,3,4}}
%e A304998 {{1,3},{1,4},{2,3},{2,4},{3,4}}
%e A304998 {{1,2},{1,3},{1,4},{2,3},{2,4},{3,4}}
%Y A304998 Cf. A006126, A261005, A304996, A304997, A304998, A304999, A305000, A305001.
%K A304998 nonn
%O A304998 0,4
%A A304998 _Gus Wiseman_, May 23 2018