Canonical subsheaves of torsionfree semistable sheaves

Abstract

Let $F$ be a torsionfree semistable coherent sheaf on a polarized normal projective variety. We prove that $F$ has a unique maximal locally free subsheaf $V$ such that $F/V$ is torsionfree and $V$ admits a filtration of subbundles for which each successive quotient is stable whose slope is $\mu(F)$. We also prove that $F$ has a unique maximal reflexive subsheaf $W$ such that $F/W$ is torsionfree and $W$ admits a filtration of subsheaves for which each successive quotient is a stable reflexive sheaf whose slope is $\mu(F)$. We show that these canonical subsheaves behave well with respect to the pullback operation by \'etale Galois covering maps. Given a separable finite surjective map $\phi :Y \, \rightarrow X$ between normal projective varieties, we give a criterion for the induced homomorphism of \'etale fundamental groups $\phi_* : \pi^{\rm et}_{1}(Y) \rightarrow \pi^{\rm et}_{1}(X)$ to be surjective; this criterion is in terms of the above mentioned unique maximal locally free subsheaf associated to $\phi_*{\mathcal O}_Y$.