Lifts of line bundles on curves on K3 surfaces

Abstract

Let X be a K3 surface, let C be a smooth curve of genus g on X, and let A be a line bundle of degree d on C. Then a line bundle M on X with $M\otimes {\mathcal {O}}_C=A$ is called a lift of A. In this paper, we prove that if the dimension of the linear system |A| is $r\ge 2$, $g>2d-3+(r-1)^2$, $d\ge 2r+4$, and A computes the Clifford index of C, then there exists a base point free lift M of A such that the general member of |M| is a smooth curve of genus r. In particular, if |A| is a base point free net which defines a double covering $\pi :C\longrightarrow C_0$ of a smooth curve $C_0\subset {\mathbb {P}}^2$ of degree $k\ge 4$ branched at distinct 6k points on $C_0$, then, by using the aforementioned result, we can also show that there exists a 2:1 morphism ${\tilde{\pi }}:X\longrightarrow {\mathbb {P}}^2$ such that ${\tilde{\pi }}|_C=\pi$.