English

Continued fractions and generalized patterns

Combinatorics 2007-05-23 v2

Abstract

In [BS] Babson and Steingrimsson introduced generalized permutation patterns that allow the requirement that two adjacent letters in a pattern must be adjacent in the permutation. Let fτ;r(n)f_{\tau;r}(n) be the number of 1\mn3\mn21\mn3\mn2-avoiding permutations on nn letters that contain exactly rr occurrences of τ\tau, where τ\tau a generalized pattern on kk letters. Let Fτ;r(x)F_{\tau;r}(x) and Fτ(x,y)F_\tau(x,y) be the generating functions defined by Fτ;r(x)=n0fτ;r(n)xnF_{\tau;r}(x)=\sum_{n\geq0} f_{\tau;r}(n)x^n and Fτ(x,y)=r0Fτ;r(x)yrF_\tau(x,y)=\sum_{r\geq0}F_{\tau;r}(x)y^r. We find an explicit expression for Fτ(x,y)F_\tau(x,y) in the form of a continued fraction for where τ\tau given as a generalized pattern; τ=12\mn3\mn...\mnk\tau=12\mn3\mn...\mn k, τ=21\mn3\mn...\mnk\tau=21\mn3\mn...\mn k, τ=123...k\tau=123... k, or τ=k...321\tau=k... 321. In particularly, we find Fτ(x,y)F_\tau(x,y) for any τ\tau generalized pattern of length 3. This allows us to express Fτ;r(x)F_{\tau;r}(x) via Chebyshev polynomials of the second kind, and continued fractions.

Keywords

Cite

@article{arxiv.math/0110037,
  title  = {Continued fractions and generalized patterns},
  author = {T. Mansour},
  journal= {arXiv preprint arXiv:math/0110037},
  year   = {2007}
}

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16 pages