A speciality Theorem for curves in $\mathbb{P}^5$ contained in Noether-Lefschetz general fourfolds

Abstract

Let $C\subset \mathbb P^r$ be an integral projective curve. We define the speciality index $e(C)$ of $C$ as the maximal integer $t$ such that $h^0(C,ω_C(-t))>0$, where $ω_C$ denotes the dualizing sheaf of $C$. In the present paper we consider $C\subset \mathbb P^5$ an integral degree $d$ curve and we denote by $s$ the minimal degree for which there exists a hypersurface of degree $s$ containing $C$. We assume that $C$ is contained in two smooth hypersurfaces $F$ and $G$, with $deg(F)=n>k=deg (G)$. We assume additionally that $F$ is Noether-Lefschetz general, i.e. that the $2$-th Néron-Severi group of $F$ is generated by the linear section class. Our main result is that in this case the speciality index is bounded as $e(C)\leq {\frac{d}{snk}}+s+n+k-6.$ Moreover equality holds if and only if $C$ is a complete intersection of $T:=F\cap G$ with hypersurfaces of degrees $s$ and ${\frac{d}{snk}}$.