Wednesday, 4 August 2010

complex geometry - question about torsion sheaf

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.

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