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A319783
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Number of set systems spanning n vertices with empty intersection whose dual is also a set system with empty intersection.
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4
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OFFSET
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0,5
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COMMENTS
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The dual of a multiset partition has, for each vertex, one part consisting of the indices (or positions) of the parts containing that vertex, counted with multiplicity. For example, the dual of {{1,2},{2,2}} is {{1},{1,2,2}}.
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LINKS
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EXAMPLE
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The a(3) = 1 set system is {{1,2},{1,3},{2,3}}.
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CROSSREFS
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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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