Semi-continuity of Stability for Sheaves and Variation of Gieseker Moduli Spaces

TL;DR

This paper studies how the stability of sheaves varies with changes in polarization on smooth threefolds, showing that moduli spaces are connected through a finite sequence of flips called Thaddeus-flips, which are themselves moduli spaces.

Contribution

It introduces a semi-continuity property for sheaf stability and demonstrates that moduli spaces of Gieseker-stable sheaves are related by Thaddeus-flips on smooth threefolds.

Findings

Moduli spaces are connected by a finite sequence of Thaddeus-flips.

Thaddeus-flips are themselves moduli spaces of sheaves.

Semi-continuity property for stability conditions is established.

Abstract

We investigate a semi-continuity property for stability conditions for sheaves that is important for the problem of variation of the moduli spaces as the stability condition changes. We place this in the context of a notion of stability previously considered by the authors, called multi-Gieseker-stability, that generalises the classical notion of Gieseker-stability to allow for several polarisations. As such we are able to prove that on smooth threefolds certain moduli spaces of Gieseker-stable sheaves are related by a finite number of Thaddeus-flips (that is flips arising for Variation of Geometric Invariant Theory) whose intermediate spaces are themselves moduli spaces of sheaves.