On correspondences of a K3 surface with itself. IV

Abstract

Let $X$ be a K3 surface with a polarization $H$ of the degree $H^2=2rs$, $r,s\ge 1$, and the isotropic Mukai vector $v=(r,H,s)$ is primitive. The moduli space of sheaves over $X$ with the isotropic Mukai vector $(r,H,s)$ is again a K3 surface, $Y$. In \cite{Nik2} the second author gave necessary and sufficient conditions in terms of Picard lattice $N(X)$ of $X$ when $Y$ is isomorphic to $X$ (some important particular cases were also considered in math.AG/0206158, math.AG/0304415 and math.AG/0307355). Here we show that these conditions imply existence of an isomorphism between $Y$ and $X$ which is a composition of some universal geometric isomorphisms between moduli of sheaves over $X$, and geometric Tyurin's isomorphsim between moduli of sheaves over $X$ and $X$ itself. It follows that for a general K3 surface $X$ with $\rho(X)=\text{rk\}N(X)\le 2$ and $Y\cong X$, there exists an isomorphism $Y\cong X$ which is a composition of the geometric universal and the Tyurin's isomorphisms. This generalizes our recent results math.AG/0605362 and math.AG/0606239 to a general case.