Preservation of log-concavity on gamma polynomials

Luis Ferroni, Greta Panova, Lorenzo Venturello

Published 2025-02-13Version 1

Every symmetric polynomial $h(x) = h_0 + h_1\,x + \cdots + h_n\,x^n$, where $h_i = h_{n-i}$ for each $i$, can be expressed as a linear combination in the basis $\{x^i(1+x)^{n-2i}\}_{i=0}^{\lfloor n/2\rfloor}$. The polynomial $\gamma_h(x) = \gamma_0 + \gamma_1 \,x+ \cdots + \gamma_{\lfloor n/2\rfloor}\, x^{\lfloor n/2\rfloor}$, commonly referred to as the $\gamma$-polynomial of $h(x)$, records the coefficients of the aforementioned linear combination. Two decades ago, Br\"and\'en and Gal independently showed that if $\gamma_h(x)$ has nonpositive real roots only, then so does $h(x)$. More recently, Br\"and\'en, Ferroni, and Jochemko proved using Lorentzian polynomials that if $\gamma_h(x)$ is ultra log-concave, then so is $h(x)$, and they raised the question of whether a similar statement can be proved for the usual notion of log-concavity. The purpose of this article is to show that the answer to the question of Br\"anden, Ferroni, and Jochemko is affirmative. One of the crucial ingredients of the proof is an inequality involving binomial numbers that we establish via a path-counting argument.