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 A326949 #13 Oct 11 2023 22:22:28
%S A326949 2,4,10,68,3838,37320356
%N A326949 Number of unlabeled T_0 sets of subsets of {1..n}.
%C A326949 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}}. The T_0 condition means that the dual is strict (no repeated edges).
%F A326949 a(n) = 2 * A326946(n).
%e A326949 Non-isomorphic representatives of the a(0) = 2 through a(2) = 10 sets of sets:
%e A326949 {} {} {}
%e A326949 {{}} {{}} {{}}
%e A326949 {{1}} {{1}}
%e A326949 {{},{1}} {{},{1}}
%e A326949 {{1},{2}}
%e A326949 {{2},{1,2}}
%e A326949 {{},{1},{2}}
%e A326949 {{},{2},{1,2}}
%e A326949 {{1},{2},{1,2}}
%e A326949 {{},{1},{2},{1,2}}
%Y A326949 The non-T_0 version is A003180.
%Y A326949 The labeled version is A326941.
%Y A326949 The covering case is A326942 (first differences).
%Y A326949 The case without empty edges is A326946.
%Y A326949 Cf. A000371, A000612, A003181, A059052, A245567, A316978, A319559, A319564, A319637, A326939, A326940.
%K A326949 nonn,more
%O A326949 0,1
%A A326949 _Gus Wiseman_, Aug 08 2019
%E A326949 a(5) from _Max Alekseyev_, Oct 11 2023