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A319253
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Number of doubly sorted permutations of [2n].
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0
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OFFSET
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1,2
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COMMENTS
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A permutation pi is called doubly sorted if |s^{-1}(pi)| = 2, where s is West's stack-sorting map. There are no doubly sorted permutations of odd length.
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LINKS
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Table of n, a(n) for n=1..4.
C. Defant, M. Engen, and J. A. Miller, Stack-sorting, set partitions, and Lassalle's sequence, arXiv:1809.01340 [math.CO], 2018.
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EXAMPLE
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The a(2) = 3 doubly sorted permutations of [4] are 2314, 1324, and 3124. Specifically, letting s denote West's stack-sorting map, we have s^{-1}(2314) = {2341, 3241}, s^{-1}(1324) = {1342, 3142}, and s^{-1}(3124) = {3412, 3421}.
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CROSSREFS
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Cf. A180874, A317551.
Sequence in context: A022514 A094579 A029729 * A328811 A136584 A194937
Adjacent sequences: A319250 A319251 A319252 * A319254 A319255 A319256
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KEYWORD
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nonn,more
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AUTHOR
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Colin Defant, Sep 15 2018
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STATUS
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approved
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