On correspondences of a K3 surface with itself. III

Abstract

Let $X$ be a K3 surface, and $H$ its primitive polarization of the degree $H^2=2rs$, $r,s\ge 1$. The moduli space of sheaves over $X$ with the isotropic Mukai vector $(r,H,s)$ is again a K3 surface, $Y$. In math.AG/0206158, math.AG/0304415 and math.AG/0307355 (in general) we gave necessary and sufficient conditions in terms of Picard lattice $N(X)$ of $X$ when $Y$ is isomorphic to $X$, under the additional condition $H\cdot N(X)=\bz$. Here we show that these conditions imply existence of an isomorphism between $Y$ and $X$ which is a composition of some universal isomorphisms between moduli of sheaves over $X$, and Tyurin's isomorphsim between moduli of sheaves over $X$ and $X$ itself. It follows that for a general K3 surface $X$ with $H\cdot N(X)=\bz$ and $Y\cong X$, there exists an isomorphism $Y\cong X$ which is a composition of the universal and the Tyurin's isomorphisms. This generalizes our recent results math.AG/0605362 for $r=s=2$ on similar subject.