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A326946
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Number of unlabeled T_0 set-systems on n vertices.
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17
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OFFSET
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0,2
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COMMENTS
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The dual of a multiset partition has, for each vertex, one block consisting of the indices (or positions) of the blocks containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. The T_0 condition means that the dual is strict (no repeated edges).
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LINKS
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FORMULA
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EXAMPLE
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Non-isomorphic representatives of the a(0) = 1 through a(2) = 5 set-systems:
{} {} {}
{{1}} {{1}}
{{1},{2}}
{{2},{1,2}}
{{1},{2},{1,2}}
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MATHEMATICA
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dual[eds_]:=Table[First/@Position[eds, x], {x, Union@@eds}];
Table[Length[Union[normclut/@Select[Subsets[Subsets[Range[n], {1, n}]], UnsameQ@@dual[#]&]]], {n, 0, 3}]
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CROSSREFS
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The version with empty edges allowed is A326949.
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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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