Weierstrass points on the Drinfeld modular curve $X_0(\mathfrak{p})$

Abstract

Consider the Drinfeld modular curve $X_0(\mathfrak{p})$ for $\mathfrak{p}$ a prime ideal of ${\mathbb{F}}_q[T]$. It was previously known that if $j$ is the $j$-invariant of a Weierstrass point of $X_0(\mathfrak{p})$, then the reduction of $j$ modulo $\mathfrak{p}$ is a supersingular $j$-invariant. In this paper we show the converse: Every supersingular $j$-invariant is the reduction modulo $\mathfrak{p}$ of the $j$-invariant of a Weierstrass point of $X_0(\mathfrak{p})$.

Main theorem has been strengthened. Mistakes in the proof of Theorems 5.7 and 6.2 have been fixed. A new section on the behavior of W(z) at the cusps has been added. Various typos corrected and clarifications made