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A370818
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Number of sets of nonempty subsets of {1..n} with only one possible way to choose a set of different vertices of each edge.
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2
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OFFSET
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0,2
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LINKS
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FORMULA
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EXAMPLE
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The set-system {{2},{1,2},{2,4},{1,3,4}} has unique choice (2,1,4,3) so is counted under a(4).
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MATHEMATICA
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Table[Length[Select[Subsets[Subsets[Range[n]]], Length[Union[Sort/@Select[Tuples[#], UnsameQ@@#&]]]==1&]], {n, 0, 3}]
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CROSSREFS
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Factorizations of this type are counted by A370645.
A070939 gives length of binary expansion.
A096111 gives product of binary indices.
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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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