A very short proof of the Borisov-Nuer conjecture

Abstract

We prove a conjecture of Borisov and Nuer, which states that every element in the even unimodular lattice of signature (1,9) can be expressed as the difference of two elements of squared length -2. As a consequence, every unnodal Enriques surface admits an Ulrich line bundle.

Withdrawn due to a gap in the proof for the case d>0. I am grateful to Shaul Zemel for pointing this out