On the Chern number of a filtration

TL;DR

This paper investigates the Chern number (first Hilbert coefficient) of local rings, extending bounds and linking extremal values to Cohen-Macaulayness, with applications to Sally modules in non-Cohen-Macaulay contexts.

Contribution

It extends an upper bound on the Chern number for local rings, establishing Cohen-Macaulayness from extremal behavior, and applies these results to Sally modules for non-Cohen-Macaulay modules.

Findings

Extended an upper bound on $e_1$ for local rings.

Linked extremal $e_1$ values to Cohen-Macaulayness.

Provided bounds on the multiplicity of Sally modules.

Abstract

We study the first Hilbert coefficient (after the multiplicity) $e_1$ of a local ring $(A,\m). $ Under various circumstances, it is also called the {\bf Chern number} of the local ring $A.$ Starting from the work of D.G. Northcott in the 60's, several results have been proved which give some relationships between the Hilbert coefficients, but always assuming the Cohen-Macaulayness of the basic ring. Recent papers of S. Goto, K. Nishida, A. Corso and W. Vasconcelos pushed the interest toward a more general setting. In this paper we extend an upper bound on $e_1$ proved by S. Huckaba and T. Marley. Thus we get the Cohen-Macaulayness of the ring $A$ as a consequence of the extremal behavior of the integer $e_1.$ The result can be considered a confirm of the general philosophy of the paper of W. Vasconcelos where the Chern number is conjectured to be a measure of the distance from the Cohen-Macaulyness of $A.$ This main result of the paper is a consequence of a nice and perhaps unexpected property of superficial elements. It is essentially a kind of "Sally machine" for local rings. In the last section we describe an application of these results, concerning an upper bound on the multiplicity of the Sally module of a good filtration of a module which is not necessarily Cohen-Macaulay. It is an extension to the non Cohen-Macaulay case of a result of Vaz Pinto.