Number of permutations with same peak set for signed permutations

2013/08/29 by Castro-Velez, Francis, Diaz-Lopez, Alexander, Orellana, Rosa +2
#05A05 #05A10 #05A15 #Combinatorics (math.CO) #FOS: Mathematics

paper · doi:10.48550/arxiv.1308.6621

Abstract

A signed permutation π= π₁π₂ … π_n in the hyperoctahedral group B_n is a word such that each π_i ∈ -n, …, -1, 1, …, n and |π₁|, |π₂|, …, |π_n| = 1,2,…,n. An index i is a peak of πif π_i-1π_i+1 and P_B(π) denotes the set of all peaks of π. Given any set S, we define P_B(S,n) to be the set of signed permutations π∈ B_n with P_B(π) = S. In this paper we are interested in the cardinality of the set P_B(S,n). In 2012, Billey, Burdzy and Sagan investigated the analogous problem for permutations in the symmetric group, S_n. In this paper we extend their results to the hyperoctahedral group; in particular we show that #P_B(S,n) = p(n)2^2n-|S|-1 where p(n) is the same polynomial found in by Billey, Burdzy and Sagan which leads to the explicit computation of interesting special cases of the polynomial p(n). In addition we have extended these results to the case where we add π₀=0 at the beginning of the permutations, which gives rise to the possibility of a peak at position 1, for both the symmetric and the hyperoctahedral groups.

Related

• Permutations with given peak set
• The most frequent peak set of a random permutation
• Pinnacle Set Properties
• The pinnacle set of a permutation
• Pinnacle sets of signed permutations