Connectedness Bertini Theorem via numerical equivalence

Abstract

Let $X$ be an irreducible projective variety and $f$ a morphism $X\to {\mathbb{P}}^n$. We give a new proof of the fact that the preimage of any linear variety of dimension $k\ge n+1-\dim f(X)$ is connected. We prove that the statement is a consequence of the Generalized Hodge Index Theorem using easy numerical arguments that hold in any characteristic. We also prove the connectedness Theorem of Fulton and Hansen as application of our main theorem.

11 pages, New version, incorporating the referee's suggestions