Pencils of minimal degree on curves on a K3 surface.
Let be a smooth projective surface, the canonical divisor, a very ample divisor and the moduli space of rank-two vector bundles, -stable with Chern classes and . We prove that, if there exists such that is numerically equivalent to and if is even, greater or equal to , then there is no Poincaré bundle on . Conversely, if there exists such that the number is odd or if is odd, then there exists a Poincaré bundle on .
We study the relationship between positivity of restriction of line bundles to general complete intersections and vanishing of their higher cohomology. As a result, we extend classical vanishing theorems of Kawamata–Viehweg and Fujita to possibly non-nef divisors.