On The Cohomology of $N_C(-2)$ in Positive Characteristic

Abstract

Let $C\subset {\mathbb{P}}^3$ be a general Brill--Noether curve. A classical problem is to determine when $H^0(N_C(-2))=0$, which controls the quadric section of $C$. So far this problem has only been solved in characteristic zero, in which case $H^0(N_C(-2))=0$ with finitely many exceptions. In this note, we extend these results to positive characteristic, uncovering a wealth of new exceptions in characteristic 2.