Moduli Spaces of Reflexive Sheaves of Rank 2

Let F be a coherent rank 2 sheaf on a scheme Y in P^n of dimension at least two. In this paper we study the relationship between the functor which deforms a pair (F,s), s in H^0(F), and the functor which deforms the corresponding pair (X,e) given as in the Serre correspondence. We prove that the scheme structure of e.g. the moduli scheme M_Y(P) of stable sheaves on a threefold Y at (F), and the scheme structure at (X) of the Hilbert scheme of curves on Y are closely connected via a local scheme theoretic approach to the Serre correspondence and related forgetful maps. Using this relationship we get criteria for the dimension and smoothness of M_Y(P) at (F), without assuming Ext^2(F,F) = 0. For reflexive sheaves on Y = P^3 whose deficiency module M = H_{*}^1(F) satisfies _0Ext^2(M,M) = 0 (e.g. of diameter at most 2), we get necessary and sufficient conditions of unobstructedness which coincide in the diameter one case. The conditions are further equivalent to the vanishing of certain graded Betti numbers of the free graded minimal resolution of H_{*}^0(F). Moreover we show that every irreducible component of M_{P^3}(P) containing a reflexive sheaf of diameter one is reduced (generically smooth) and we compute its dimension. We also determine a good lower bound for the dimension of any component of M_{P^3}(P) which contains a reflexive stable sheaf with “small” deficiency module M