Upper Bounds of Schubert Polynomials

Abstract

Let $w$ be a permutation of $\{1,2,\ldots,n \}$, and let $D(w)$ be the Rothe diagram of $w$. The Schubert polynomial $\mathfrak{S}_w(x)$ can be realized as the dual character of the flagged Weyl module associated to $D(w)$. This implies a coefficient-wise inequality \[\mathrm{Min}_w(x)\leq \mathfrak{S}_w(x)\leq \mathrm{Max}_w(x),\] where both $\mathrm{Min}_w(x)$ and $\mathrm{Max}_w(x)$ are polynomials determined by $D(w)$. Fink, Mészáros and St.$\,$Dizier found that $\mathfrak{S}_w(x)$ equals the lower bound $\mathrm{Min}_w(x)$ if and only if $w$ avoids twelve permutation patterns. In this paper, we show that $\mathfrak{S}_w(x)$ reaches the upper bound $\mathrm{Max}_w(x)$ if and only if $w$ avoids two permutation patterns 1432 and 1423. Similarly, for any given composition $α\in \mathbb{Z}_{\geq 0}^n$, one can define a lower bound $\mathrm{Min}_α(x)$ and an upper bound $\mathrm{Max}_α(x)$ for the key polynomial $κ_α(x)$. Hodges and Yong established that $κ_α(x)$ equals $\mathrm{Min}_α(x)$ if and only if $α$ avoids five composition patterns. We show that $κ_α(x)$ equals $\mathrm{Max}_α(x)$ if and only if $α$ avoids a single composition pattern $(0,2)$. As an application, we obtain that when $α$ avoids $(0,2)$, the key polynomial $κ_α(x)$ is Lorentzian, partially verifying a conjecture of Huh, Matherne, Mészáros and St.$\,$Dizier.