Bounds on multisecant lines

Abstract

The purpose of this paper is two fold. First, we give an upper bound on the order of a multisecant line to an integral arithmetically Cohen-Macaulay subscheme in Pn of codimension two in terms of the Hilbert function. Secondly, we give an explicit description of the singular locus of the blow up of an arbitrary local ring at a complete intersection ideal. This description is used to refine standard linking theorem. These results are tied together by the construction of sharp examples for the bound, which uses the linking theorems.