The Tangent Cone of a local ring of codimension 2

Abstract

Let $(S, \mathfrak n) $ be a regular local ring and let $I \subseteq \mathfrak n^2 $ be a perfect ideal of $S. $ Sharp upper bounds on the minimal number of generators of $I$ are known in terms of the Hilbert function of $R=S/I. $ Starting from information on the ideal $I, $ for instance the minimal number of generators, a difficult task is to find good bounds on the minimal number of generators of the leading ideal $I^* $, which defines the tangent cone of $R$ or to give information on its graded structure. Motivated by papers of S.C. Kothari, S. Goto ${\it{et al.}}$ concerning the leading ideal of a complete intersection $I=(f,g) $ in a regular local ring, we present results provided ht$(I)=2.$ If $I$ is a complete intersection, we prove that the Hilbert function of $R$ determines the graded Betti numbers of the leading ideal and, as a consequence, we recover most of the results of the previously quoted papers. The description is more complicated if $\nu(I) >2$ and a careful investigation can be provided when $\nu(I)=3. $ Several examples illustrating our results are given.