

A319253


Number of doubly sorted permutations of [2n].


0




OFFSET

1,2


COMMENTS

A permutation pi is called doubly sorted if s^{1}(pi) = 2, where s is West's stacksorting map. There are no doubly sorted permutations of odd length.


LINKS

Table of n, a(n) for n=1..4.
C. Defant, M. Engen, and J. A. Miller, Stacksorting, 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 stacksorting 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



