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A326965
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Number of set-systems on n vertices where every covered vertex is the unique common element of some subset of the edges.
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20
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1, 2, 5, 46, 19181, 2010327182, 9219217424630040409, 170141181796805106025395618012972506978, 57896044618658097536026644159052312978532934306727333157337631572314050272137
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OFFSET
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0,2
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COMMENTS
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A set-system is a finite set of finite nonempty sets. The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. An antichain is a set-system where no edge is a subset of any other. This sequence counts set-systems whose dual is a (strict) antichain, also called T_1 set-systems.
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LINKS
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FORMULA
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EXAMPLE
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The a(0) = 1 through a(2) = 5 set-systems:
{} {} {}
{{1}} {{1}}
{{2}}
{{1},{2}}
{{1},{2},{1,2}}
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MATHEMATICA
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tmQ[eds_]:=Union@@Select[Intersection@@@Rest[Subsets[eds]], Length[#]==1&]==Union@@eds;
Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], tmQ]], {n, 0, 3}]
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CROSSREFS
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The version with empty edges allowed is A326967.
Set-systems whose dual is a weak antichain are A326968.
The BII_numbers of these set-systems are A326979.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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