Lefschetz Pencils on surfaces and an application to Seshadri constants

Abstract

Let $X$ be a nonsingular complex projective surface and let $L$ be a very ample line bundle on $X$. We study the question: when does a general pencil in the linear system $|L|$ consist of only irreducible and reduced curves? We give a precise condition which guarantees an affirmative answer: in fact, this holds if and only if the genus of a general member of $|L|$ exceeds the irregularity of $X$. The proof is topological, using vanishing cycles and the theory of Lefschetz pencils. As an application, combined with a construction due to Miranda, we prove that for any positive rational number $δ$ there exist a blow up $Y \to X$ at finitely many points, an ample line bundle $M$ on $Y$ and a point $x \in Y$ such that $\varepsilon(Y,M,x) = δ$; Miranda proved this when $X = \mathbb{P}^2$.