# An overpartition analogue of $q$-binomial coefficients, II: combinatorial proofs and $(q,t)$-log concavity

## TL;DR

This paper introduces a two-parameter overpartition analogue of Gaussian polynomials, extends classical $q$-series identities, and proves $(q,t)$-log concavity using combinatorial involutions, with implications for Delannoy numbers.

## Contribution

It develops a new two-parameter generalization of Gaussian polynomials, provides finite $q$-series identities, and establishes $(q,t)$-log concavity through combinatorial methods.

## Key findings

- Finite $q$-series identities derived
- $(q,t)$-log concavity proved for the new polynomial
- Connections made to Delannoy numbers and conjectures on unimodality

## Abstract

In a previous paper, we studied an overpartition analogue of Gaussian polynomials as the generating function for overpartitions fitting inside an $m \times n$ rectangle. Here, we add one more parameter counting the number of overlined parts, obtaining a two-parameter generalization $\overline{{m+n \brack n}}_{q,t}$ of Gaussian polynomials, which is also a $(q,t)$-analogue of Delannoy numbers. First we obtain finite versions of classical $q$-series identities such as the $q$-binomial theorem and the Lebesgue identity, as well as two-variable generalizations of classical identities involving Gaussian polynomials. Then, by constructing involutions, we obtain an identity involving a finite theta function and prove the $(q,t)$-log concavity of $\overline{{m+n \brack n}}_{q,t}$. We particularly emphasize the role of combinatorial proofs and the consequences of our results on Delannoy numbers. We conclude with some conjectures about the unimodality of $\overline{{m+n \brack n}}_{q,t}$.