Rees algebras of finitely generated torsion-free modules over a two-dimensional regular local ring

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Let (R, m) be a two-dimensional regular local ring and let A be a finitely generated torsion-free R-module. If A is a complete module, then Katz and Kodiyalam show A satisfies five conditions, one of these being that the Rees algebra A of A is Cohen-Macaulay and another being that the "associated graded ring" A/IA of A is Cohen-Macaulay. They ask whether these five conditions are equivalent without assuming A to be complete. We exhibit an example to show that A/IA may be Cohen-Macaulay while A fails to be Cohen-Macaulay, and investigate other implications among these five properties in the case where A is not complete. We prove in general that the depth of A is greater than or equal to the depth of A/IA, and that if a module has reduction number at most one, then a direct summand also has reduction number at most one. We present an example where A is a direct sum of two submodules each of which has reduction number at most one while A has reduction number at least two. In the last section of this paper, we present two sufficient conditions for modules obtained by adjoining one element to the submodule mn1 ⊕ ⋯ ⊕ mnr of Rr to be complete.

Original languageEnglish
Pages (from-to)4015-4039
Number of pages25
JournalCommunications in Algebra
Issue number12
Publication statusPublished - 1998 Jan 1


ASJC Scopus subject areas

  • Algebra and Number Theory

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