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 A326877 #11 Oct 28 2023 12:08:10
%S A326877 1,0,1,8,381,252080,18687541309
%N A326877 Number of connectedness systems covering n vertices without singletons.
%C A326877 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. It is covering if every vertex belongs to some edge.
%H A326877 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 A326877 The a(3) = 8 covering connectedness systems without singletons:
%e A326877 {{1,2,3}}
%e A326877 {{1,2},{1,2,3}}
%e A326877 {{1,3},{1,2,3}}
%e A326877 {{2,3},{1,2,3}}
%e A326877 {{1,2},{1,3},{1,2,3}}
%e A326877 {{1,2},{2,3},{1,2,3}}
%e A326877 {{1,3},{2,3},{1,2,3}}
%e A326877 {{1,2},{1,3},{2,3},{1,2,3}}
%t A326877 Table[Length[Select[Subsets[Subsets[Range[n],{2,n}]],Union@@#==Range[n]&&SubsetQ[#,Union@@@Select[Tuples[#,2],Intersection@@#!={}&]]&]],{n,0,4}]
%Y A326877 Inverse binomial transform of A072446 (the non-covering case).
%Y A326877 Exponential transform of A072447 if we assume A072447(1) = 0 (the connected case).
%Y A326877 The case with singletons is A326870.
%Y A326877 The BII-numbers of these set-systems are A326873.
%Y A326877 Cf. A072444, A072445, A102896, A323818, A326866, A326867, A326868, A326871, A326872.
%K A326877 nonn,more
%O A326877 0,4
%A A326877 _Gus Wiseman_, Jul 30 2019
%E A326877 a(6) corrected by _Christian Sievers_, Oct 28 2023