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A355089
Number of parity-alternating permutations of [n] avoiding the pattern 123.
0
1, 1, 1, 1, 3, 3, 10, 11, 37, 44, 146, 185, 603, 808, 2576, 3635, 11294, 16736, 50545, 78466, 230012, 373203, 1061236, 1795611, 4953447, 8721086, 23350320, 42691298, 111013825, 210379132, 531720722
OFFSET
0,5
COMMENTS
A permutation is parity-alternating if it sends odd integers to odd integers, and even integers to even integers. It avoids 123 if there is no subsequence a..b..c with a < b < c. The values are computed by Michael Albert, see MathOverflow link.
The odd-indexed entries agree with the odd-indexed entries in A354208. A bijection is given by reversing the permutation.
LINKS
Per Alexandersson, Samuel Asefa Fufa, Frether Getachew and Dun Qiu, Pattern-avoidance and Fuss-Catalan numbers, arXiv:2201.08168 [math.CO], 2022. See also J. Int. Seq. (2023) Vol. 26, Art. 23.4.2.
EXAMPLE
For n=4, the three permutations are 3412, 3214, 1432.
For n=5, we have 54321, 52143, 32541.
For n=6, we have 563412, 563214, 543612, 543216, 561432, 541632, 365412, 365214, 321654, 165432.
CROSSREFS
Cf. A000108 (123-avoiding permutations), A010551 (parity-alternating permutations), A354208 (parity-alternating 321-avoiding permutations).
Sequence in context: A128015 A233256 A332955 * A218953 A081809 A359894
KEYWORD
nonn
AUTHOR
STATUS
approved