Identifier
- St000255: Permutations ⟶ ℤ
Values
[1] => 1
[1,2] => 1
[2,1] => 1
[1,2,3] => 1
[1,3,2] => 2
[2,1,3] => 1
[2,3,1] => 1
[3,1,2] => 1
[3,2,1] => 1
[1,2,3,4] => 1
[1,2,4,3] => 3
[1,3,2,4] => 2
[1,3,4,2] => 3
[1,4,2,3] => 3
[1,4,3,2] => 5
[2,1,3,4] => 1
[2,1,4,3] => 3
[2,3,1,4] => 1
[2,3,4,1] => 1
[2,4,1,3] => 2
[2,4,3,1] => 2
[3,1,2,4] => 1
[3,1,4,2] => 2
[3,2,1,4] => 1
[3,2,4,1] => 1
[3,4,1,2] => 1
[3,4,2,1] => 1
[4,1,2,3] => 1
[4,1,3,2] => 2
[4,2,1,3] => 1
[4,2,3,1] => 1
[4,3,1,2] => 1
[4,3,2,1] => 1
[1,2,3,4,5] => 1
[1,2,3,5,4] => 4
[1,2,4,3,5] => 3
[1,2,4,5,3] => 6
[1,2,5,3,4] => 6
[1,2,5,4,3] => 14
[1,3,2,4,5] => 2
[1,3,2,5,4] => 8
[1,3,4,2,5] => 3
[1,3,4,5,2] => 4
[1,3,5,2,4] => 8
[1,3,5,4,2] => 11
[1,4,2,3,5] => 3
[1,4,2,5,3] => 8
[1,4,3,2,5] => 5
[1,4,3,5,2] => 7
[1,4,5,2,3] => 6
[1,4,5,3,2] => 9
[1,5,2,3,4] => 4
[1,5,2,4,3] => 11
[1,5,3,2,4] => 7
[1,5,3,4,2] => 10
[1,5,4,2,3] => 9
[1,5,4,3,2] => 14
[2,1,3,4,5] => 1
[2,1,3,5,4] => 4
[2,1,4,3,5] => 3
[2,1,4,5,3] => 6
[2,1,5,3,4] => 6
[2,1,5,4,3] => 14
[2,3,1,4,5] => 1
[2,3,1,5,4] => 4
[2,3,4,1,5] => 1
[2,3,4,5,1] => 1
[2,3,5,1,4] => 3
[2,3,5,4,1] => 3
[2,4,1,3,5] => 2
[2,4,1,5,3] => 5
[2,4,3,1,5] => 2
[2,4,3,5,1] => 2
[2,4,5,1,3] => 3
[2,4,5,3,1] => 3
[2,5,1,3,4] => 3
[2,5,1,4,3] => 8
[2,5,3,1,4] => 3
[2,5,3,4,1] => 3
[2,5,4,1,3] => 5
[2,5,4,3,1] => 5
[3,1,2,4,5] => 1
[3,1,2,5,4] => 4
[3,1,4,2,5] => 2
[3,1,4,5,2] => 3
[3,1,5,2,4] => 5
[3,1,5,4,2] => 8
[3,2,1,4,5] => 1
[3,2,1,5,4] => 4
[3,2,4,1,5] => 1
[3,2,4,5,1] => 1
[3,2,5,1,4] => 3
[3,2,5,4,1] => 3
[3,4,1,2,5] => 1
[3,4,1,5,2] => 2
[3,4,2,1,5] => 1
[3,4,2,5,1] => 1
[3,4,5,1,2] => 1
[3,4,5,2,1] => 1
[3,5,1,2,4] => 2
[3,5,1,4,2] => 4
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Description
The number of reduced Kogan faces with the permutation as type.
This is equivalent to finding the number of ways to represent the permutation $\pi \in S_{n+1}$ as a reduced subword of $s_n (s_{n-1} s_n) (s_{n-2} s_{n-1} s_n) \dotsm (s_1 \dotsm s_n)$, or the number of reduced pipe dreams for $\pi$.
This is equivalent to finding the number of ways to represent the permutation $\pi \in S_{n+1}$ as a reduced subword of $s_n (s_{n-1} s_n) (s_{n-2} s_{n-1} s_n) \dotsm (s_1 \dotsm s_n)$, or the number of reduced pipe dreams for $\pi$.
References
[1] Kirichenko, V. A., Smirnov, E. Y., Timorin, V. A. Schubert calculus and Gelfand-Zetlin polytopes DOI:10.1070/rm2012v067n04abeh004804 arXiv:1101.0278
Code
def statistic(pi):
n = len(pi)
if pi == Permutations(n).one():
return 1
else:
S = Word([j for i in range(n, 0, -1) for j in range(i, n+1)])
return sum(S.number_of_subword_occurrences(Word(w)) for w in pi.reduced_words())
Created
Jun 15, 2015 at 18:00 by Per Alexandersson
Updated
May 19, 2023 at 14:57 by Martin Rubey
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