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A326870
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Number of connectedness systems covering n vertices.
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13
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OFFSET
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0,3
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COMMENTS
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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.
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LINKS
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EXAMPLE
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The a(2) = 5 connectedness systems:
{{1,2}}
{{1},{2}}
{{1},{1,2}}
{{2},{1,2}}
{{1},{2},{1,2}}
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MATHEMATICA
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Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], Union@@#==Range[n]&&SubsetQ[#, Union@@@Select[Tuples[#, 2], Intersection@@#!={}&]]&]], {n, 0, 4}]
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CROSSREFS
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Inverse binomial transform of A326866 (the non-covering case).
Exponential transform of A326868 (the connected case).
The BII-numbers of these set-systems are A326872.
The case without singletons is A326877.
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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