Gorenstein Biliaison and ACM Sheaves

Abstract

Let $X$ be a normal arithmetically Gorenstein scheme in ${\mathbb{P}}^n$. We give a criterion for all codimension two ACM subschemes of $X$ to be in the same Gorenstein biliaison class on $X$, in terms of the category of ACM sheaves on $X$. These are sheaves that correspond to the graded maximal Cohen--Macaulay modules on the homogeneous coordinate ring of $X$. Using known results on MCM modules, we are able to determine the Gorenstein biliaison classes of codimension two subschemes of certain varieties, including the nonsingular quadric surface in ${\mathbb{P}}^3$, and the cone over it in ${\mathbb{P}}^4$. As an application we obtain a new proof of some theorems of Lesperance about curves in ${\mathbb{P}}^4$, and answer some questions be raised.

Key words and phrases: linkage, liaison, biliaison, Gorenstein scheme, maximal Cohen-Macaulay modules; 30 pages