On the diagonals of a Rees algebra

Abstract

The aim of this work is to study the ring-theoretic properties of the diagonals of a Rees algebra, which from a geometric point of view are the homogeneous coordinate rings of embeddings of blow-ups of projective varieties along a subvariety. In Chapter 1 we extend the definitions and results about the biprojective scheme and the Hilbert polynomial of finitely generated bigraded modules defined over standard bigraded k-algebras to finitely generated bigraded S-modules, for S=k[X_1,..., X_n,Y_1,...,Y_r] the polynomial ring bigraded by deg X_i=(1,0), deg Y_j=(d_j,1), d_j \geq 0. We also relate the shifts in the bigraded minimal free resolution of any finitely generated bigraded S-module to its a-invariants. In Chapter 2 we are concerned with the diagonal functor in the category of bigraded S-modules. We compare the local cohomology modules of a finitely generated bigraded S-module with the local cohomology modules of its diagonals. In Chapter 3 we study in detail the Cohen-Macaulay property of the rings k[(I^e)_c]. Chapter 4 is devoted to study the Gorenstein property of those rings. As a somehow unexpected by-product, the methods used to study the diagonals of a Rees algebra also allow to study the a-invariants of the powers of an ideal and their asymptotic properties (Chapters 5 and 6)