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Statistic identifier: St001960

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Collection: Permutations

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Description: The number of descents of a permutation minus one if its first entry is not one.

This statistic appears in [1, Theorem 2.3] in a gamma-positivity result, see also [2].

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References: [1]   Athanasiadis, C. A. Gamma-positivity in combinatorics and geometry [[MathSciNet:3878174]]
[2]   Shareshian, J., Wachs, M. L. Gamma-positivity of variations of Eulerian polynomials [[MathSciNet:4015851]]

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Code:
def statistic(pi):
    if pi(1) == 1:
        return pi.number_of_descents()
    else:
        return pi.number_of_descents()-1

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Statistic values:

[1,2]       => 0
[2,1]       => 0
[1,2,3]     => 0
[1,3,2]     => 1
[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]   => 1
[1,3,2,4]   => 1
[1,3,4,2]   => 1
[1,4,2,3]   => 1
[1,4,3,2]   => 2
[2,1,3,4]   => 0
[2,1,4,3]   => 1
[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]   => 1
[3,2,1,4]   => 1
[3,2,4,1]   => 1
[3,4,1,2]   => 0
[3,4,2,1]   => 1
[4,1,2,3]   => 0
[4,1,3,2]   => 1
[4,2,1,3]   => 1
[4,2,3,1]   => 1
[4,3,1,2]   => 1
[4,3,2,1]   => 2
[1,2,3,4,5] => 0
[1,2,3,5,4] => 1
[1,2,4,3,5] => 1
[1,2,4,5,3] => 1
[1,2,5,3,4] => 1
[1,2,5,4,3] => 2
[1,3,2,4,5] => 1
[1,3,2,5,4] => 2
[1,3,4,2,5] => 1
[1,3,4,5,2] => 1
[1,3,5,2,4] => 1
[1,3,5,4,2] => 2
[1,4,2,3,5] => 1
[1,4,2,5,3] => 2
[1,4,3,2,5] => 2
[1,4,3,5,2] => 2
[1,4,5,2,3] => 1
[1,4,5,3,2] => 2
[1,5,2,3,4] => 1
[1,5,2,4,3] => 2
[1,5,3,2,4] => 2
[1,5,3,4,2] => 2
[1,5,4,2,3] => 2
[1,5,4,3,2] => 3
[2,1,3,4,5] => 0
[2,1,3,5,4] => 1
[2,1,4,3,5] => 1
[2,1,4,5,3] => 1
[2,1,5,3,4] => 1
[2,1,5,4,3] => 2
[2,3,1,4,5] => 0
[2,3,1,5,4] => 1
[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] => 1
[2,4,3,1,5] => 1
[2,4,3,5,1] => 1
[2,4,5,1,3] => 0
[2,4,5,3,1] => 1
[2,5,1,3,4] => 0
[2,5,1,4,3] => 1
[2,5,3,1,4] => 1
[2,5,3,4,1] => 1
[2,5,4,1,3] => 1
[2,5,4,3,1] => 2
[3,1,2,4,5] => 0
[3,1,2,5,4] => 1
[3,1,4,2,5] => 1
[3,1,4,5,2] => 1
[3,1,5,2,4] => 1
[3,1,5,4,2] => 2
[3,2,1,4,5] => 1
[3,2,1,5,4] => 2
[3,2,4,1,5] => 1
[3,2,4,5,1] => 1
[3,2,5,1,4] => 1
[3,2,5,4,1] => 2
[3,4,1,2,5] => 0
[3,4,1,5,2] => 1
[3,4,2,1,5] => 1
[3,4,2,5,1] => 1
[3,4,5,1,2] => 0
[3,4,5,2,1] => 1
[3,5,1,2,4] => 0
[3,5,1,4,2] => 1
[3,5,2,1,4] => 1
[3,5,2,4,1] => 1
[3,5,4,1,2] => 1
[3,5,4,2,1] => 2
[4,1,2,3,5] => 0
[4,1,2,5,3] => 1
[4,1,3,2,5] => 1
[4,1,3,5,2] => 1
[4,1,5,2,3] => 1
[4,1,5,3,2] => 2
[4,2,1,3,5] => 1
[4,2,1,5,3] => 2
[4,2,3,1,5] => 1
[4,2,3,5,1] => 1
[4,2,5,1,3] => 1
[4,2,5,3,1] => 2
[4,3,1,2,5] => 1
[4,3,1,5,2] => 2
[4,3,2,1,5] => 2
[4,3,2,5,1] => 2
[4,3,5,1,2] => 1
[4,3,5,2,1] => 2
[4,5,1,2,3] => 0
[4,5,1,3,2] => 1
[4,5,2,1,3] => 1
[4,5,2,3,1] => 1
[4,5,3,1,2] => 1
[4,5,3,2,1] => 2
[5,1,2,3,4] => 0
[5,1,2,4,3] => 1
[5,1,3,2,4] => 1
[5,1,3,4,2] => 1
[5,1,4,2,3] => 1
[5,1,4,3,2] => 2
[5,2,1,3,4] => 1
[5,2,1,4,3] => 2
[5,2,3,1,4] => 1
[5,2,3,4,1] => 1
[5,2,4,1,3] => 1
[5,2,4,3,1] => 2
[5,3,1,2,4] => 1
[5,3,1,4,2] => 2
[5,3,2,1,4] => 2
[5,3,2,4,1] => 2
[5,3,4,1,2] => 1
[5,3,4,2,1] => 2
[5,4,1,2,3] => 1
[5,4,1,3,2] => 2
[5,4,2,1,3] => 2
[5,4,2,3,1] => 2
[5,4,3,1,2] => 2
[5,4,3,2,1] => 3

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Created: Jan 08, 2025 at 11:27 by Christian Stump

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Last Updated: Jan 08, 2025 at 11:27 by Christian Stump