S-equivalence is an equivalence relation on the families of semistable vector bundles on an algebraic curve.

Definition

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Let X be a projective curve over an algebraically closed field k. A vector bundle on X can be considered as a locally free sheaf. Every semistable locally free E on X admits a Jordan-Hölder filtration with stable subquotients, i.e.

 

where   are locally free sheaves on X and   are stable. Although the Jordan-Hölder filtration is not unique, the subquotients are, which means that   is unique up to isomorphism.

Two semistable locally free sheaves E and F on X are S-equivalent if gr Egr F.