Lagrangian subspaces of the moduli space of simple sheaves on K3 surfaces

Abstract: Let $X$ be a K3 surface and let $\text{Spl}(r;c_1,c_2)$ be the moduli space of simple sheaves on $X$ of fixed rank $r$ and Chern classes $c_1$ and $c_2$. Under suitable assumptions, to a pair $(F,W)$ (respectively, $(F,V)$) where $F\in \text{Spl}(r;c_1,c_2)$ and $W\subset H^0(F)$ (resp.~$V^*\subset H^1(F^*)$) is a vector subspace, we associate a simple syzygy bundle (resp.~extension bundle) on $X$. We show that both syzygy bundles and extension bundles can be constructed in families and that the induced morphism to a different component of the moduli of simple sheaves is a locally closed embedding. We show that this construction associates to every Lagrangian (resp.~isotropic) algebraic subspace of $\text{Spl}(r;c_1,c_2)$ an induced Lagrangian (resp.~isotropic) algebraic subspace of a different component of the moduli of simple sheaves.