English

Generating functions for permutations which avoid consecutive patterns with multiple descents

Combinatorics 2017-02-28 v1

Abstract

Let SnS_n denote the group all permutations of nn. For every permutation σ\sigma, we let des(σ)\mathrm{des}(\sigma) denote the number of descents in σ\sigma and LRMin(σ)\mathrm{LRMin}(\sigma) denote the number of left-to-right minima of σ\sigma. Given a sequence τ=τ1τn\tau = \tau_1 \cdots \tau_n of distinct positive integers, we define the reduction of τ\tau, red(τ)\mathrm{red}(\tau), to be the permutation of SnS_n that results by replacing the ii-th smallest element of τ\tau by ii. If Γ\Gamma is a set of permutations, we say that a permutation σ=σ1σnSn\sigma = \sigma_1 \ldots \sigma_n \in S_n has a Γ\Gamma-match starting at position ii if there is a i<ji < j such that red(σiσi+1σj)Γ\mathrm{red}(\sigma_i \sigma_{i+1} \ldots \sigma_j) \in \Gamma. We let Γ\Gamma-mch(σ)\mathrm{mch}(\sigma) denote the number of Γ\Gamma-matches in σ\sigma. We let NMn(Γ)\mathcal{NM}_n(\Gamma) be the set of σSn\sigma \in S_n such that Γ\Gamma-mch(σ)=0\mathrm{mch}(\sigma) = 0. In this paper, we modify Jones and Remmel's reciprocity method to study the generating function of the form \begin{equation} \mbox{NM}_{\Gamma}(t,x,y)=\sum_{n \geq 0} \frac{t^n}{n!} \mbox{NM}_{\Gamma,n}(x,y) \end{equation} where \mboxNMΓ,n(x,y)=σNMn(Γ)xLRmin(σ)y1+des(σ)\displaystyle \mbox{NM}_{\Gamma,n}(x,y) =\sum_{\sigma \in \mathcal{NM}_n(\Gamma)}x^{\mathrm{LRmin}(\sigma)}y^{1+\mathrm{des}(\sigma)} in the case where we no longer insist that all the permutations τΓ\tau \in \Gamma have at most one descent.

Keywords

Cite

@article{arxiv.1702.08125,
  title  = {Generating functions for permutations which avoid consecutive patterns with multiple descents},
  author = {Quang T. Bach and Jeffrey B. Remmel},
  journal= {arXiv preprint arXiv:1702.08125},
  year   = {2017}
}

Comments

arXiv admin note: substantial text overlap with arXiv:1510.07190