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 A319253 Number of doubly sorted permutations of [2n]. 0
 1, 3, 31, 1186 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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. LINKS C. Defant, M. Engen, and J. A. Miller, Stack-sorting, set partitions, and Lassalle's sequence, arXiv:1809.01340 [math.CO], 2018. EXAMPLE 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}. CROSSREFS Cf. A180874, A317551. Sequence in context: A022514 A094579 A029729 * A328811 A136584 A194937 Adjacent sequences: A319250 A319251 A319252 * A319254 A319255 A319256 KEYWORD nonn,more AUTHOR Colin Defant, Sep 15 2018 STATUS approved

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Last modified March 24 12:09 EDT 2023. Contains 361479 sequences. (Running on oeis4.)