Linear stability of coherent systems and applications to Butler's conjecture

Abstract

The notion of linear stability of a variety in projective space was introduced by Mumford in the context of GIT. It has subsequently been applied by Mistretta and others to Butler's conjecture on stability of the dual span bundle (DSB) $M_{V, E}$ of a general generated coherent system $( E, V )$. We survey recent progress in this direction on rank one coherent systems, prove a new result for hyperelliptic curves, and state some open questions. We then extend the definition of linear stability to generated coherent systems of higher rank. We show that various coherent systems with unstable DSB studied by Brambila-Paz, Mata-Gutierrez, Newstead and Ortega are also linearly unstable. We show that linearly stable coherent systems of type $(2, d, 4)$ for low enough $d$ have stable DSB, and use this to prove a particular case of a generalized Butler conjecture. We then exhibit a linearly stable generated coherent system with unstable DSB, confirming that linear stability of $( E, V )$ in general remains weaker than semistability of $M_{V, E}$ in higher rank. We end with a list of open questions on the higher rank case.