The Picard Group of Vertex Affinoids in the First Drinfeld Covering

Abstract

Let $F$ be a finite extension of $\mathbb{Q}_p$. Let $\Omega$ be the Drinfeld upper half plane, and $\Sigma^1$ the first Drinfeld covering of $\Omega$. We study the affinoid open subset $\Sigma^1_v$ of $\Sigma^1$ above a vertex of the Bruhat-Tits tree for $\text{GL}_2(F)$. Our main result is that $\text{Pic}(\Sigma^1_v)[p] = 0$, which we establish by showing that $\text{Pic}(\mathbf{Y})[p] = 0$ for $\mathbf{Y}$ the Deligne-Lusztig variety of $\text{SL}_2(\mathbb{F}_q)$. One formal consequence is a description of the representation $H^1_{\text{\'{e}t}}(\Sigma^1_v, \mathbb{Z}_p(1))$ of $\text{GL}_2(\mathcal{O}_F)$ as the $p$-adic completion of $\mathcal{O}(\Sigma^1_v)^\times$.