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A072446
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Number of connectedness systems on n vertices that contain all singletons.
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15
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OFFSET
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1,2
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COMMENTS
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If we define a connectedness system to be a set of finite nonempty sets (edges) that is closed under taking the union of any two overlapping edges, then a(n) is the number of connectedness systems on n vertices without singleton edges. The BII-numbers of these set-systems are given by A326873. The a(3) = 12 connectedness systems without singletons are:
{}
{{1,2}}
{{1,3}}
{{2,3}}
{{1,2,3}}
{{1,2},{1,2,3}}
{{1,3},{1,2,3}}
{{2,3},{1,2,3}}
{{1,2},{1,3},{1,2,3}}
{{1,2},{2,3},{1,2,3}}
{{1,3},{2,3},{1,2,3}}
{{1,2},{1,3},{2,3},{1,2,3}}
(End)
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LINKS
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FORMULA
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EXAMPLE
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a(3)=12 because of the 12 sets:
{{1}, {2}, {3}};
{{1}, {2}, {3}, {1, 2}};
{{1}, {2}, {3}, {1, 3}};
{{1}, {2}, {3}, {2, 3}};
{{1}, {2}, {3}, {1, 2, 3}};
{{1}, {2}, {3}, {1, 2}, {1, 2, 3}};
{{1}, {2}, {3}, {1, 3}, {1, 2, 3}};
{{1}, {2}, {3}, {2, 3}, {1, 2, 3}};
{{1}, {2}, {3}, {1, 2}, {1, 3}, {1, 2, 3}};
{{1}, {2}, {3}, {1, 2}, {2, 3}, {1, 2, 3}};
{{1}, {2}, {3}, {1, 3}, {2, 3}, {1, 2, 3}};
{{1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}}.
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MATHEMATICA
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Table[Length[Select[Subsets[Subsets[Range[n], {2, n}]], SubsetQ[#, Union@@@Select[Tuples[#, 2], Intersection@@#!={}&]]&]], {n, 0, 3}] (* Gus Wiseman, Jul 31 2019 *)
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CROSSREFS
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Exponential transform of A072447 (the connected case).
The case with singletons is A326866.
Binomial transform of A326877 (the covering case).
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KEYWORD
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nonn
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AUTHOR
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Wim van Dam (vandam(AT)cs.berkeley.edu), Jun 18 2002
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EXTENSIONS
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STATUS
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approved
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