On the direct images of parabolic vector bundles and parabolic connections

Abstract

Let $\varphi : Y \rightarrow X$ be a finite surjective morphism between smooth complex projective curves, where $X$ is irreducible but $Y$ need not be so. Let $V_*$ be a parabolic vector bundle on $Y$. We construct a parabolic structure on the direct image $\varphi_* V$ on $X$, where $V$ is the vector bundle underlying $V_*$. The parabolic vector bundle $\varphi_* V_*$ on $X$ obtained this way has a ramified torus sub-bundle; it is a torus bundle of $\text{Ad}(\varphi_* V)$ outside the parabolic divisor for $\varphi_* V_*$ that satisfies certain conditions at the parabolic points. Conversely, given a parabolic vector bundle $E_*$ on $X$, and a ramified torus sub-bundle $\mathcal T$ for it, we construct a ramified covering $Z$ of $X$ and a parabolic vector bundle $W_*$ on $Z$, such that the parabolic bundle $E_*$ is the direct image of $W_*$. A connection on $V_*$ produces a connection on $\varphi_* V_*$. The ramified torus sub-bundle for $\varphi_* V_*$ is preserved by the logarithmic connection on $\text{End}(\varphi_* V)$ induced by this connection on $\varphi_* V_*$. If the parabolic vector bundle $E_*$ on $X$ is equipped with a connection $D$ such that the connection on the endomorphism bundle induced by it preserves the ramified torus sub-bundle $\mathcal T$, then we prove that the corresponding parabolic vector bundle $W_*$ on $Z$ has a connection that produces the connection $D$ on the direct image $E_*$.