A Class of Kazhdan-Lusztig R-Polynomials and q-Fibonacci Numbers

Abstract

Let $S_n$ denote the symmetric group on $\{1,2,\ldots,n\}$. For two permutations $u, v\in S_n$ such that $u\leq v$ in the Bruhat order, let $R_{u,v}(q)$ and $\R_{u,v}(q)$ denote the Kazhdan-Lusztig $R$-polynomial and $\R$-polynomial, respectively. Let $v_n=34\cdots n\, 12$, and let $\sigma$ be a permutation such that $\sigma\leq v_n$. We obtain a formula for the $\R$-polynomials $\R_{\sigma,v_n}(q)$ in terms of the $q$-Fibonacci numbers depending on a parameter determined by the reduced expression of $\sigma$. When $\sigma$ is the identity $e$, this reduces to a formula obtained by Pagliacci. In another direction, we obtain a formula for the $\R$-polynomial $\R_{e,\,v_{n,i}}(q)$, where $v_{n,i} = 3 4\cdots i\,n\, (i+1)\cdots (n-1)\, 12$. In a more general context, we conjecture that for any two permutations $\sigma,\tau\in S_n$ such that $\sigma\leq \tau\leq v_n$, the $\R$-polynomial $\R_{\sigma,\tau}(q)$ can be expressed as a product of $q$-Fibonacci numbers multiplied by a power of $q$.