A349456 - OEIS

A349456

Number of singular positroid varieties corresponding to derangements in S_n.

0, 0, 0, 0, 4, 30, 225, 1736, 14476, 132396

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OFFSET

0,5

COMMENTS

a(n) is also the number of derangements whose chordal diagrams have crossed alignments.

a(n) counts the complement of A349413 in the set of all derangements of S_n (A000166).

LINKS

S. Corteel, Crossings and alignments of permutations, arXiv:math/0601469 [math.CO], 2006.

A. Knutson, T. Lam and D. Speyer, Positroid varieties: juggling and geometry, Compos. Math. 149 (2013), no. 10, 1710-1752.

A. Postnikov, Total positivity, Grassmannians, and networks, arXiv:math/0609764 [math.CO], 2006.

FORMULA

a(n) = A000166(n) - A349413(n).

EXAMPLE

For n=4 the a(4)=4 derangements in one-line notation corresponding to singular positroid varieties are 2413, 3421, 3142, and 4312.

CROSSREFS

KEYWORD

nonn,more

AUTHOR

STATUS

approved