Yau-Zaslow formula on K3 surfaces for non-primitive classes
  We compute the genus zero family Gromov-Witten invariants for K3 surfaces using the topological recursion formula and the symplectic sum formula for a degeneration of elliptic K3 surfaces. In particular we verify the Yau-Zaslow formula for non-primitive classes of index two. 
Junho Lee, Naichung Conan Leung