Covering gonality of symmetric products of curves and Cayley-Bacharach condition on Grassmannians

Given an irreducible projective variety $X$, the covering gonality of $X$ is the least gonality of an irreducible curve $E\subset X$ passing through a general point of $X$. In this paper we study the covering gonality of the $k$-fold symmetric product $C^{(k)}$ of a smooth complex projective curve $C$ of genus $g\ge k+1$. It follows from a previous work of the first author that the covering gonality of the second symmetric product of $C$ equals the gonality of $C$. Using a similar approach, we prove the same for the $3$-fold and the $4$-fold symmetric product of $C$. A crucial point in the proof is the study of Cayley-Bacharach condition on Grassmannians. In particular, we describe the geometry of linear subspaces of ${\mathbb{P}}^n$ satisfying this condition and we prove a result bounding the dimension of their linear span.