English

Generating functions for descents over permutations which avoid sets of consecutive patterns

Combinatorics 2015-10-16 v1

Abstract

We extend the reciprocity method of Jones and Remmel to study generating functions of the form n0tnn!σNMn(Γ)xLRmin(σ)y1+des(σ)\sum_{n \geq 0} \frac{t^n}{n!} \sum_{\sigma \in \mathcal{NM}_n(\Gamma)}x^{\mathrm{LRmin}(\sigma)}y^{1+\mathrm{des}(\sigma)} where Γ\Gamma is a set of permutations which start with 1 and have at most one descent, NMn(Γ)\mathcal{NM}_n(\Gamma) is the set of permutations σ\sigma in the symmetric group Sn\mathfrak{S}_n which have no Γ\Gamma-matches, des(σ)\mathrm{des}(\sigma) is the number of descents of σ\sigma and LRmin(σ)\mathrm{LRmin}(\sigma) is the number of left-to-right minima of σ\sigma. We show that this generating function is of the form (1UΓ(t,y))x\left( \frac{1}{U_{\Gamma}(t,y)}\right)^x where UΓ(t,y)=n0UΓ,n(y)tnn!U_{\Gamma}(t,y) = \sum_{n\geq 0}U_{\Gamma,n}(y) \frac{t^n}{n!} and the coefficients UΓ,n(y)U_{\Gamma,n}(y) satisfy some simple recursions in the case where Γ\Gamma equals {1324,123}\{1324,123\}, {1324p,12(p1)}\{1324 \cdots p,12 \cdots (p-1)\} for p5p \geq 5, or Γ\Gamma is the set of permutations σ=σ1σn\sigma = \sigma_1 \cdots \sigma_n of length n=k1+k2n=k_1+k_2 where k1,k22k_1,k_2 \geq 2, σ1=1\sigma_1 =1, σk1+1=2\sigma_{k_1+1}=2, and des(σ)=1\mathrm{des}(\sigma) =1.

Keywords

Cite

@article{arxiv.1510.04319,
  title  = {Generating functions for descents over permutations which avoid sets of consecutive patterns},
  author = {Quang T. Bach and Jeffrey B. Remmel},
  journal= {arXiv preprint arXiv:1510.04319},
  year   = {2015}
}
R2 v1 2026-06-22T11:20:41.514Z