OFFSET
0,3
COMMENTS
A graph is topologically connected if the graph whose vertices are the edges and whose edges are crossing pairs of edges is connected, where two edges cross each other if they are of the form {{x,y},{z,t}} with x < z < y < t or z < x < t < y.
FORMULA
Binomial transform of A324327.
EXAMPLE
The a(0) = 1 through a(5) = 27 graphs:
{} {} {} {} {} {}
{{12}} {{12}} {{12}} {{12}}
{{13}} {{13}} {{13}}
{{23}} {{14}} {{14}}
{{23}} {{15}}
{{24}} {{23}}
{{34}} {{24}}
{{13}{24}} {{25}}
{{34}}
{{35}}
{{45}}
{{13}{24}}
{{13}{25}}
{{14}{25}}
{{14}{35}}
{{24}{35}}
{{13}{14}{25}}
{{13}{24}{25}}
{{13}{24}{35}}
{{14}{24}{35}}
{{14}{25}{35}}
{{13}{14}{24}{25}}
{{13}{14}{24}{35}}
{{13}{14}{25}{35}}
{{13}{24}{25}{35}}
{{14}{24}{25}{35}}
{{13}{14}{24}{25}{35}}
MATHEMATICA
croXQ[stn_]:=MatchQ[stn, {___, {___, x_, ___, y_, ___}, ___, {___, z_, ___, t_, ___}, ___}/; x<z<y<t||z<x<t<y];
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]]]]]]]]];
crosscmpts[stn_]:=csm[Union[Subsets[stn, {1}], Select[Subsets[stn, {2}], croXQ]]];
Table[Length[Select[Subsets[Subsets[Range[n], {2}]], Length[crosscmpts[#]]<=1&]], {n, 0, 5}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Feb 22 2019
STATUS
approved