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A326906
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Number of sets of subsets of {1..n} that are closed under union and cover all n vertices.
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10
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OFFSET
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0,1
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COMMENTS
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Differs from A102895 in having a(0) = 2 instead of 1.
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LINKS
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FORMULA
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EXAMPLE
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The a(0) = 2 through a(2) = 8 sets of subsets:
{} {{1}} {{1,2}}
{{}} {{},{1}} {{},{1,2}}
{{1},{1,2}}
{{2},{1,2}}
{{},{1},{1,2}}
{{},{2},{1,2}}
{{1},{2},{1,2}}
{{},{1},{2},{1,2}}
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MATHEMATICA
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Table[Length[Select[Subsets[Subsets[Range[n]]], Union@@#==Range[n]&&SubsetQ[#, Union@@@Tuples[#, 2]]&]], {n, 0, 3}]
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CROSSREFS
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The case without empty sets is A102894.
The case with a single covering edge is A102895.
The case also closed under intersection is A326878 for n > 0.
The same for intersection instead of union is (also) A326906.
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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