On Parity Unimodality of $q$-Catalan Polynomials

TL;DR

This paper explores the parity unimodality of rational $q$-Catalan polynomials, proposing a conjecture and verifying it for small values using generating functions and constant term methods.

Contribution

It introduces a conjecture on parity unimodality of $q$-Catalan polynomials and verifies it for cases where $m \

Findings

Conjecture that $(1+q)C_{m+n}(q)$ is unimodal.

Verification of the conjecture for $m \

demonstrates the validity for small $m$ values.

Abstract

A polynomial $A(q)=\sum_{i=0}^n a_iq^i$ is said to be unimodal if $a_0\le a_1\le \cdots \le a_k\ge a_{k+1} \ge \cdots \ge a_n$. We investigate the unimodality of rational $q$-Catalan polynomials, which is defined to be $C_{m,n}(q)= \frac{1}{[n+m]} \left[ m+n \atop n\right]$ for a coprime pair of positive integers $(m,n)$. We conjecture that they are unimodal with respect to parity, or equivalently, $(1+q)C_{m+n}(q)$ is unimodal. By using generating functions and the constant term method, we verify our conjecture for $m\le 5$ in a straightforward way.