Holomorphic Rank Two Vector Bundles on Blow-ups

In this paper we study holomorphic rank two vector bundles on the blow up of $ {\bf C}^2$ at the origin. A classical theorem of Birchoff and Grothendieck says that any holomorphic vector bundle on the projective plane ${\bf P}^1$ splits into a sum of line bundles. If $E$ is a holomorphic vector bundle over the blow up of $ {\bf C}^2$ at the origin, then the restriction of $E$ to the exceptional divisor is a vector bundle over ${\bf P}^1$ and therefore splits. Moreover we assume that $E$ is a rank two bundle that has zero first Chern class. Hence its restriction to the exceptional divisor is of the form $ {\cal O}(j) \oplus {\cal O}(-j) $ for some integer $j.$ We denote by ${\cal M}_j$ the moduli space of equivalence classes (under holomorphic isomorphisms) of rank two holomorphic vector bundles on the blow up of $ {\bf C}^2$ at the origin whose restriction to the exceptional divisor is $ {\cal O}(j) \oplus {\cal O}(-j) .$