Moduli spaces of reflexive sheaves of rank 2
Abstract
Let be a coherent rank 2 sheaf on a scheme Y ⊂ n of dimension at least two. In this paper we study the relationship between the functor which deforms a pair (,σ), σ ∈ H0(), and the functor which deforms the corresponding pair (X,) given as in the Serre correspondence. We prove that the scheme structure of e.g. the moduli scheme MY(P) of stable sheaves on a threefold Y at (), and the scheme structure at (X) of the Hilbert scheme of curves on Y are closely related. Using this relationship we get criteria for the dimension and smoothness of MY(P) at (), without assuming Ext2(,) = 0. For reflexive sheaves on Y = 3 whose deficiency module M = H*1() satisfies Ext2(M,M) = 0 in degree zero (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(). It follows that every irreducible component of M3(P) containing a reflexive sheaf of diameter one is reduced (generically smooth). We also determine a good lower bound for the dimension of any component of M3(P) which contains a reflexive stable sheaf with "small" deficiency module M.