Genuinely ramified maps and pseudo-stable vector bundles

Abstract

Let $X$ and $Y$ be irreducible normal projective varieties, of same dimension, defined over an algebraically closed field, and let $f : Y \rightarrow X$ be a finite generically smooth morphism such that the corresponding homomorphism between the \'etale fundamental groups $f_*:\pi^{\rm et}_{1}(Y) \rightarrow\pi^{\rm et}_{1}(X)$ is surjective. Fix a polarization on $X$ and equip $Y$ with the pulled back polarization. For a point $y_0\in Y$, let $\varpi(Y, y_0)$ (respectively, $\varpi(X, f(y_0))$) be the affine group scheme given by the neutral Tannakian category defined by the strongly pseudo-stable vector bundles of degree zero on $Y$ (respectively, $X$). We prove that the homomorphism $\varpi(Y, y_0) \rightarrow \varpi(X, f(y_0))$ induced by $f$ is surjective. Let $E$ be a pseudo-stable vector bundle on $X$ and $F \subset f^*E$ a pseudo-stable subbundle with $\mu(F)= \mu(f^*E)$. We prove that $f^*E$ is pseudo-stable and there is a pseudo-stable subbundle $W \subset E$ such that $f^*W = F$ as subbundles of $f^*E$.