Let's assume for simplicity that M is a smooth, complex, projective variety.
The set of points where the coherent subsheaf mathcalF is not locally free is a proper closed subset of M (Hartshorne, Algebraic Geometry, Chapter II, ex. 5.8), so the stalk of ker(det(j)) at the generic point is zero, i.e. it is a torsion sheaf.
Moreover, you can say more. Indeed, since mathcalE is locally free and mathcalE/mathcalF is torsion-free, it follows that mathcalF is a reflexive sheaf (Hartshorne, Stable Reflexive Sheaves, Theorem 1.1), so it is locally free except along a closed subset of codimension geq3 (same reference, Corollary 1.4).
In particular, if M is a curve or a surface then ker(det(j)) is zero.
No comments:
Post a Comment