Identifier
- St000367: Permutations ⟶ ℤ
Values
[1] => 0
[1,2] => 0
[2,1] => 0
[1,2,3] => 0
[1,3,2] => 0
[2,1,3] => 0
[2,3,1] => 0
[3,1,2] => 0
[3,2,1] => 1
[1,2,3,4] => 0
[1,2,4,3] => 0
[1,3,2,4] => 0
[1,3,4,2] => 0
[1,4,2,3] => 0
[1,4,3,2] => 1
[2,1,3,4] => 0
[2,1,4,3] => 0
[2,3,1,4] => 0
[2,3,4,1] => 0
[2,4,1,3] => 0
[2,4,3,1] => 1
[3,1,2,4] => 0
[3,1,4,2] => 0
[3,2,1,4] => 2
[3,2,4,1] => 1
[3,4,1,2] => 0
[3,4,2,1] => 2
[4,1,2,3] => 0
[4,1,3,2] => 0
[4,2,1,3] => 1
[4,2,3,1] => 0
[4,3,1,2] => 1
[4,3,2,1] => 3
[1,2,3,4,5] => 0
[1,2,3,5,4] => 0
[1,2,4,3,5] => 0
[1,2,4,5,3] => 0
[1,2,5,3,4] => 0
[1,2,5,4,3] => 1
[1,3,2,4,5] => 0
[1,3,2,5,4] => 0
[1,3,4,2,5] => 0
[1,3,4,5,2] => 0
[1,3,5,2,4] => 0
[1,3,5,4,2] => 1
[1,4,2,3,5] => 0
[1,4,2,5,3] => 0
[1,4,3,2,5] => 2
[1,4,3,5,2] => 1
[1,4,5,2,3] => 0
[1,4,5,3,2] => 2
[1,5,2,3,4] => 0
[1,5,2,4,3] => 0
[1,5,3,2,4] => 1
[1,5,3,4,2] => 0
[1,5,4,2,3] => 1
[1,5,4,3,2] => 3
[2,1,3,4,5] => 0
[2,1,3,5,4] => 0
[2,1,4,3,5] => 0
[2,1,4,5,3] => 0
[2,1,5,3,4] => 0
[2,1,5,4,3] => 1
[2,3,1,4,5] => 0
[2,3,1,5,4] => 0
[2,3,4,1,5] => 0
[2,3,4,5,1] => 0
[2,3,5,1,4] => 0
[2,3,5,4,1] => 1
[2,4,1,3,5] => 0
[2,4,1,5,3] => 0
[2,4,3,1,5] => 2
[2,4,3,5,1] => 1
[2,4,5,1,3] => 0
[2,4,5,3,1] => 2
[2,5,1,3,4] => 0
[2,5,1,4,3] => 0
[2,5,3,1,4] => 1
[2,5,3,4,1] => 0
[2,5,4,1,3] => 1
[2,5,4,3,1] => 3
[3,1,2,4,5] => 0
[3,1,2,5,4] => 0
[3,1,4,2,5] => 0
[3,1,4,5,2] => 0
[3,1,5,2,4] => 0
[3,1,5,4,2] => 1
[3,2,1,4,5] => 3
[3,2,1,5,4] => 3
[3,2,4,1,5] => 1
[3,2,4,5,1] => 1
[3,2,5,1,4] => 2
[3,2,5,4,1] => 2
[3,4,1,2,5] => 0
[3,4,1,5,2] => 0
[3,4,2,1,5] => 3
[3,4,2,5,1] => 2
[3,4,5,1,2] => 0
[3,4,5,2,1] => 3
[3,5,1,2,4] => 0
[3,5,1,4,2] => 0
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Description
The number of simsun double descents of a permutation.
The restriction of a permutation $\pi$ to $[k] = \{1,\ldots,k\}$ is given in one-line notation by the subword of $\pi$ of letters in $[k]$.
A simsun double descent of a permutation $\pi$ is a double descent of any restriction of $\pi$ to $[1,\ldots,k]$ for some $k$. (Note here that the same double descent can appear in multiple restrictions!)
The restriction of a permutation $\pi$ to $[k] = \{1,\ldots,k\}$ is given in one-line notation by the subword of $\pi$ of letters in $[k]$.
A simsun double descent of a permutation $\pi$ is a double descent of any restriction of $\pi$ to $[1,\ldots,k]$ for some $k$. (Note here that the same double descent can appear in multiple restrictions!)
References
[1] Sundaram, S. The homology representations of the symmetric group on Cohen-Macaulay subposets of the partition lattice MathSciNet:1273390
[2] Ma, S.-M., Yeh, Y.-N. The peak statistics on simsun permutations arXiv:1601.06505
[2] Ma, S.-M., Yeh, Y.-N. The peak statistics on simsun permutations arXiv:1601.06505
Code
def restriction(pi,k):
return Permutation([pi[i] for i in range(len(pi)) if pi[i] <= k])
def double_descents(pi):
return sum( 1 for i in range(len(pi)-2) if pi[i]>pi[i+1]>pi[i+2] )
def statistic(pi):
return sum( double_descents(restriction(pi,k)) for k in [3..len(pi)] )
Created
Jan 26, 2016 at 21:14 by Christian Stump
Updated
May 10, 2019 at 17:28 by Henning Ulfarsson
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