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A361863
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Number of set partitions of {1..n} such that the median of medians of the blocks is (n+1)/2.
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1
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1, 2, 3, 9, 26, 69, 335, 1018, 6629, 22805, 182988, 703745
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OFFSET
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1,2
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COMMENTS
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The median of a multiset is either the middle part (for odd length), or the average of the two middle parts (for even length).
Since (n+1)/2 is the median of {1..n}, this sequence counts "transitive" set partitions.
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LINKS
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EXAMPLE
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The a(1) = 1 through a(4) = 9 set partitions:
{{1}} {{12}} {{123}} {{1234}}
{{1}{2}} {{13}{2}} {{12}{34}}
{{1}{2}{3}} {{124}{3}}
{{13}{24}}
{{134}{2}}
{{14}{23}}
{{1}{23}{4}}
{{14}{2}{3}}
{{1}{2}{3}{4}}
The set partition {{1,4},{2,3}} has medians {5/2,5/2}, with median 5/2, so is counted under a(4).
The set partition {{1,3},{2,4}} has medians {2,3}, with median 5/2, so is counted under a(4).
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MATHEMATICA
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sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]& /@ sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];
Table[Length[Select[sps[Range[n]], (n+1)/2==Median[Median/@#]&]], {n, 6}]
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CROSSREFS
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For mean instead of median we have A361910.
A361864 counts set partitions with integer median of medians, means A361865.
A361866 counts set partitions with integer sum of medians, means A361911.
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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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