Identifier
- St001076: Permutations ⟶ ℤ
Values
[1,2] => 0
[2,1] => 1
[1,2,3] => 0
[1,3,2] => 1
[2,1,3] => 1
[2,3,1] => 2
[3,1,2] => 2
[3,2,1] => 1
[1,2,3,4] => 0
[1,2,4,3] => 1
[1,3,2,4] => 1
[1,3,4,2] => 2
[1,4,2,3] => 2
[1,4,3,2] => 3
[2,1,3,4] => 1
[2,1,4,3] => 2
[2,3,1,4] => 2
[2,3,4,1] => 3
[2,4,1,3] => 3
[2,4,3,1] => 2
[3,1,2,4] => 2
[3,1,4,2] => 3
[3,2,1,4] => 3
[3,2,4,1] => 2
[3,4,1,2] => 4
[3,4,2,1] => 3
[4,1,2,3] => 3
[4,1,3,2] => 2
[4,2,1,3] => 2
[4,2,3,1] => 1
[4,3,1,2] => 3
[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] => 2
[1,2,5,3,4] => 2
[1,2,5,4,3] => 3
[1,3,2,4,5] => 1
[1,3,2,5,4] => 2
[1,3,4,2,5] => 2
[1,3,4,5,2] => 3
[1,3,5,2,4] => 3
[1,3,5,4,2] => 4
[1,4,2,3,5] => 2
[1,4,2,5,3] => 3
[1,4,3,2,5] => 3
[1,4,3,5,2] => 4
[1,4,5,2,3] => 4
[1,4,5,3,2] => 5
[1,5,2,3,4] => 3
[1,5,2,4,3] => 4
[1,5,3,2,4] => 4
[1,5,3,4,2] => 3
[1,5,4,2,3] => 5
[1,5,4,3,2] => 4
[2,1,3,4,5] => 1
[2,1,3,5,4] => 2
[2,1,4,3,5] => 2
[2,1,4,5,3] => 3
[2,1,5,3,4] => 3
[2,1,5,4,3] => 4
[2,3,1,4,5] => 2
[2,3,1,5,4] => 3
[2,3,4,1,5] => 3
[2,3,4,5,1] => 4
[2,3,5,1,4] => 4
[2,3,5,4,1] => 3
[2,4,1,3,5] => 3
[2,4,1,5,3] => 4
[2,4,3,1,5] => 4
[2,4,3,5,1] => 3
[2,4,5,1,3] => 5
[2,4,5,3,1] => 4
[2,5,1,3,4] => 4
[2,5,1,4,3] => 5
[2,5,3,1,4] => 3
[2,5,3,4,1] => 2
[2,5,4,1,3] => 4
[2,5,4,3,1] => 3
[3,1,2,4,5] => 2
[3,1,2,5,4] => 3
[3,1,4,2,5] => 3
[3,1,4,5,2] => 4
[3,1,5,2,4] => 4
[3,1,5,4,2] => 5
[3,2,1,4,5] => 3
[3,2,1,5,4] => 4
[3,2,4,1,5] => 4
[3,2,4,5,1] => 3
[3,2,5,1,4] => 5
[3,2,5,4,1] => 4
[3,4,1,2,5] => 4
[3,4,1,5,2] => 5
[3,4,2,1,5] => 5
[3,4,2,5,1] => 4
[3,4,5,1,2] => 6
[3,4,5,2,1] => 5
[3,5,1,2,4] => 5
[3,5,1,4,2] => 4
[3,5,2,1,4] => 4
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Description
The minimal length of a factorization of a permutation into transpositions that are cyclic shifts of (12).
In symbols, for a permutation $\pi$ this is
$$\min\{ k \mid \pi = \tau_{i_1} \cdots \tau_{i_k}, 1 \leq i_1,\ldots,i_k \leq n\},$$
where $\tau_a = (a,a+1)$ for $1 \leq a \leq n$ and $n+1$ is identified with $1$.
Put differently, this is the number of cyclically simple transpositions needed to sort a permutation.
In symbols, for a permutation $\pi$ this is
$$\min\{ k \mid \pi = \tau_{i_1} \cdots \tau_{i_k}, 1 \leq i_1,\ldots,i_k \leq n\},$$
where $\tau_a = (a,a+1)$ for $1 \leq a \leq n$ and $n+1$ is identified with $1$.
Put differently, this is the number of cyclically simple transpositions needed to sort a permutation.
References
[1] Winter, M. A graph similar to the Bruhat graph, what is it called? MathOverflow:366504
Code
def statistic(pi):
n = len(pi)
a = Permutation([2,1])
b = Permutation([i for i in range(2,n+1)] + [1])
powers = [Permutation([])]
for k in range(n-1):
powers.append(powers[-1]*b)
gens = [p.inverse()*a*p for p in powers]
G = PermutationGroup(gens)
pi = G(pi)
w = gap.Factorization(G._gap_(), pi._gap_())
return w.Length().sage()
Created
Jan 08, 2018 at 17:14 by Martin Rubey
Updated
Jan 22, 2024 at 15:11 by Martin Rubey
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