Local Bezout estimates and multiplicities of parameter and primary ideals

Abstract

Let $\mathfrak{q}$ denote an $\mathfrak{m}$-primary ideal of a $d$-dimensional local ring $(A, \mathfrak{m}).$ Let $\underline{a} = a_1,\ldots,a_d \subset \mathfrak{q}$ be a system of parameters. Then there is the following inequality for the multiplicities $c \cdot e(\mathfrak{q};A) \leq e(\underline{a};A)$ where $c$ denotes the product of the initial degrees of $a_i$ in the form ring $G_A(\mathfrak{q}).$ The aim of the paper is a characterization of the equality as well as a description of the difference by various homological methods via Koszul homology. To this end we have to characterize when the sequence of initial elements $\underline{a^{\star}} = a_1^{\star}, \ldots,a_d^{\star}$ is a homogeneous system of parameters of $G_A(\mathfrak{q}).$ In the case of $\dim A = 2$ this leads to results on the local Bezout inequality. In particular, we give several equations for improving the classical Bezout inequality to an equality.