Monodromy of a family of hypersurfaces

Abstract

Let $Y$ be an $(m+1)$-dimensional irreducible smooth complex projective variety embedded in a projective space. Let $Z$ be a closed subscheme of $Y$, and $\delta$ be a positive integer such that ${\mathcal{I}}_{Z,Y}(\delta )$ is generated by global sections. Fix an integer $d\ge \delta +1$, and assume the general divisor $X \in |H^0(Y,\ic_{Z,Y}(d))|$ is smooth. Denote by $H^m(X;\mathbb{Q}{)}_{\perp Z}^{\text{van}}$ the quotient of $H^m(X;\mathbb{Q})$ by the cohomology of $Y$ and also by the cycle classes of the irreducible components of dimension $m$ of $Z$. In the present paper we prove that the monodromy representation on $H^m(X;\mathbb{Q}{)}_{\perp Z}^{\text{van}}$ for the family of smooth divisors $X \in |H^0(Y,\ic_{Z,Y}(d))|$ is irreducible.

arXiv comment13 pages, to appear on Ann. Scient. Ec. Norm. Sup