OFFSET
0,5
COMMENTS
We consider the empty graph to be neither connected (one component) nor disconnected (more than one component).
FORMULA
EXAMPLE
The a(4) = 3 graphs:
{{1,2},{3,4}}
{{1,3},{2,4}}
{{1,4},{2,3}}
MATHEMATICA
csm[s_]:=With[{c=Select[Tuples[Range[Length[s]], 2], And[OrderedQ[#], UnsameQ@@#, Length[Intersection@@s[[#]]]>0]&]}, If[c=={}, s, csm[Sort[Append[Delete[s, List/@c[[1]]], Union@@s[[c[[1]]]]]]]]];
Table[Length[Select[Subsets[Subsets[Range[n], {2}]], Union@@#==Range[n]&&Length[csm[#]]!=1&]], {n, 0, 5}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 24 2019
STATUS
approved