Interlacing property of a family of generating polynomials over Dyck paths

Abstract

In the study of a tantalizing symmetry on Catalan objects, Bóna et al. introduced a family of polynomials $\{W_{n,k}(x){\}}_{n\ge k\ge 0}$ defined by \begin{align*} W_{n,k}(x)=\sum_{m=0}^{k}w_{n,k,m}x^{m}, \end{align*} where $w_{n,k,m}$ counts the number of Dyck paths of semilength $n$ with $k$ occurrences of $UD$ and $m$ occurrences of $UUD$. They proposed two conjectures on the interlacing property of these polynomials, one of which states that $\{W_{n,k}(x){\}}_{n\ge k}$ is a Sturm sequence for any fixed $k\ge 1$, and the other states that $\{W_{n,k}(x){\}}_{1\le k\le n}$ is a Sturm-unimodal sequence for any fixed $n\ge 1$. In this paper, we obtain certain recurrence relations for $W_{n,k}(x)$, and further confirm their conjectures.