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 A326973 #6 Aug 12 2019 22:32:39
%S A326973 1,1,3,19,1243
%N A326973 Number of unlabeled set-systems covering n vertices whose dual is a weak antichain.
%C A326973 A set-system is a finite set of finite nonempty sets. The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. A weak antichain is a multiset of sets, none of which is a proper subset of any other.
%e A326973 Non-isomorphic representatives of the a(0) = 1 through a(3) = 19 set-systems:
%e A326973 {} {{1}} {{1,2}} {{1,2,3}}
%e A326973 {{1},{2}} {{1},{2,3}}
%e A326973 {{1},{2},{1,2}} {{1},{2},{3}}
%e A326973 {{1,2},{1,3},{2,3}}
%e A326973 {{1},{2,3},{1,2,3}}
%e A326973 {{1},{2},{3},{2,3}}
%e A326973 {{1},{2},{1,3},{2,3}}
%e A326973 {{1},{2},{3},{1,2,3}}
%e A326973 {{3},{1,2},{1,3},{2,3}}
%e A326973 {{1},{2},{3},{1,3},{2,3}}
%e A326973 {{1,2},{1,3},{2,3},{1,2,3}}
%e A326973 {{1},{2},{3},{2,3},{1,2,3}}
%e A326973 {{2},{3},{1,2},{1,3},{2,3}}
%e A326973 {{1},{2},{1,3},{2,3},{1,2,3}}
%e A326973 {{1},{2},{3},{1,2},{1,3},{2,3}}
%e A326973 {{3},{1,2},{1,3},{2,3},{1,2,3}}
%e A326973 {{1},{2},{3},{1,3},{2,3},{1,2,3}}
%e A326973 {{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
%e A326973 {{1},{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
%Y A326973 Unlabeled covering set-systems are A055621.
%Y A326973 The labeled version is A326970.
%Y A326973 The non-covering case is A326971 (partial sums).
%Y A326973 The case that is also T_0 is the T_1 case A326974.
%Y A326973 Cf. A000612, A059523, A319637, A326966, A326968, A326972, A326975, A326978.
%K A326973 nonn,more
%O A326973 0,3
%A A326973 _Gus Wiseman_, Aug 11 2019