Buchsbaum–Rim multiplicities as Hilbert–Samuel multiplicities

Abstract

We study the Buchsbaum–Rim multiplicity br ( M ) of a finitely generated module M over a regular local ring R of dimension 2 with maximal ideal m . The module M under consideration is of finite colength in a free R-module F. Write F / M ≅ I / J , where J ⊂ I are m -primary ideals of R. We first investigate the colength ℓ ( R / a ) of any m -primary ideal a and its Hilbert–Samuel multiplicity e ( a ) using linkage theory. As an application, we establish several multiplicity formulas that express the Buchsbaum–Rim multiplicity of the module M in terms of the Hilbert–Samuel multiplicities of ideals related to I, J and a minimal reduction of M. The motivation comes from work by E. Jones, who applied graphical computations of the Hilbert–Samuel multiplicity to the Buchsbaum–Rim multiplicity [E. Jones, Computations of Buchsbaum–Rim multiplicities, J. Pure Appl. Algebra 162 (2001) 37–52].