Jan O. Kleppe
Let F be a coherent rank 2 sheaf on a scheme Y in Pn of dimension at least two and let X in Y be the zero set of a section \sigma of H0(F). In this paper we study the relationship between the functor which deforms the pair (F,\sigma) and the two functors which deform F on Y, and X in Y, respectively. By imposing some conditions on two forgetful maps between the functors, we prove that the scheme structure of e.g. the moduli scheme MY(P) of stable sheaves on a threefold Y at (F), and the scheme structure at (X) of the Hilbert scheme of curves on Y become closely related. Using this relationship we get criteria for the dimension and smoothness of MY(P) at (F), without assuming Ext2(F, F) = 0. For reflexive sheaves on Y=P3 whose deficiency module M = H*1(F) satisfies 0Ext2(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 MP3(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 MP3(P) which contains a reflexive stable sheaf with ``small'' deficiency module M.