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 A326867 #17 Oct 28 2023 23:54:55
%S A326867 1,2,6,30,466,80926,1689195482
%N A326867 Number of unlabeled connectedness systems on n vertices.
%C A326867 We define a connectedness system (investigated by Vim van Dam in 2002) to be a set of finite nonempty sets (edges) that is closed under taking the union of any two overlapping edges.
%H A326867 Gus Wiseman, <a href="http://www.mathematica-journal.com/2017/12/every-clutter-is-a-tree-of-blobs/">Every Clutter Is a Tree of Blobs</a>, The Mathematica Journal, Vol. 19, 2017.
%e A326867 Non-isomorphic representatives of the a(0) = 1 through a(3) = 30 connectedness systems:
%e A326867 {} {} {} {}
%e A326867 {{1}} {{1}} {{1}}
%e A326867 {{1,2}} {{1,2}}
%e A326867 {{1},{2}} {{1},{2}}
%e A326867 {{2},{1,2}} {{1,2,3}}
%e A326867 {{1},{2},{1,2}} {{1},{2,3}}
%e A326867 {{2},{1,2}}
%e A326867 {{1},{2},{3}}
%e A326867 {{3},{1,2,3}}
%e A326867 {{1},{2},{1,2}}
%e A326867 {{1},{3},{2,3}}
%e A326867 {{2,3},{1,2,3}}
%e A326867 {{2},{3},{1,2,3}}
%e A326867 {{1},{2,3},{1,2,3}}
%e A326867 {{1},{2},{3},{2,3}}
%e A326867 {{3},{2,3},{1,2,3}}
%e A326867 {{1},{2},{3},{1,2,3}}
%e A326867 {{1,3},{2,3},{1,2,3}}
%e A326867 {{1},{3},{2,3},{1,2,3}}
%e A326867 {{2},{3},{2,3},{1,2,3}}
%e A326867 {{2},{1,3},{2,3},{1,2,3}}
%e A326867 {{3},{1,3},{2,3},{1,2,3}}
%e A326867 {{1,2},{1,3},{2,3},{1,2,3}}
%e A326867 {{1},{2},{3},{2,3},{1,2,3}}
%e A326867 {{1},{2},{1,3},{2,3},{1,2,3}}
%e A326867 {{2},{3},{1,3},{2,3},{1,2,3}}
%e A326867 {{3},{1,2},{1,3},{2,3},{1,2,3}}
%e A326867 {{1},{2},{3},{1,3},{2,3},{1,2,3}}
%e A326867 {{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
%e A326867 {{1},{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
%Y A326867 The case without singletons is A072444.
%Y A326867 The labeled case is A326866.
%Y A326867 The connected case is A326869.
%Y A326867 Partial sums of A326871 (the covering case).
%Y A326867 Cf. A072445, A072446, A072447, A102896, A306445, A326870, A326872.
%K A326867 nonn,more
%O A326867 0,2
%A A326867 _Gus Wiseman_, Jul 29 2019
%E A326867 a(5) from _Andrew Howroyd_, Aug 10 2019
%E A326867 a(6) from _Andrew Howroyd_, Oct 28 2023