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For questions on modules over rings.

19 votes
Accepted

When is $A\otimes R$ a free $R$-module?

Remark on projective modules over $\mathbb{Z}/n\mathbb{Z}$. Let $n$ be a positive integer. Then the following are equivalent: The integer $n$ has no square factor. … Kaplansky, "Elementary divisors and modules", 1949. [2] H. Matsumura, "Commutative Ring Theory", 1986. …
Luc Guyot's user avatar
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8 votes
Accepted

Intersection of free/affine submodules, comparison with vector spaces

There is a natural generalization of the aforementioned dimension-based reasoning to modules $M$ over commutative domains or commutative Noetherian reduced rings. …
Luc Guyot's user avatar
  • 7,893
8 votes
Accepted

$R$ a DVR with fraction field $K.$ What are the $R$-submodules of $K^n?$

The focus of [4] is the class of purely indecomposable modules (pi-modules), i.e., torsion-free indecomposable modules $M$ of finite rank with $p\text{-rank}(M) = 1$, or equivalently indecomposable pure … Rotman, "A note on completion of modules", 1960. [2] D. Arnold, "A duality Torsion-free modules of finite rank over a discrete valuation ring", 1969. [3] H. …
Luc Guyot's user avatar
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8 votes
Accepted

When is countable direct-product of projective modules again projective ?

Under the assumption that a countable direct product of modules over $R$ means a direct product of countably many modules over $R$, I answer OP's question when $R$ is Noetherian. … Chase, "Direct product of modules", 1960. [2] H. Bass, "Big projective modules are free", 1962. [3] J. O'Neill, "When a ring is an $F$-ring", 1993. …
Luc Guyot's user avatar
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7 votes
Accepted

Global to local principle for f.g. $\mathbb{Z}[x]$ modules

Your lemma easily follows from the Smith Normal Form Theorem, a result you already referred to. The short heuristic argument that you gave can indeed be turned into a short proof. Nothing in the sequ …
Luc Guyot's user avatar
  • 7,893
6 votes
Accepted

Modules over infinite rings which can not be a finite union of their proper submodules

Gottlieb, "Modules covered by finite unions of submodules", 1998. Below lie the remains of my initial (awkward) answer. This is only a long comment. …
Luc Guyot's user avatar
  • 7,893
5 votes
Accepted

On integral domains over which special kind of modules are projective

An integral domain $R$ such that every proper non-zero $R$-submodule of $\text{Frac}(R)$ is projective is a local principal ideal ring. (The converse is David Handelman's comment above). Indeed, we h …
Luc Guyot's user avatar
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4 votes

Basis for free modules over an affine domain

, particularly in the resolution of Serre's problem on projective modules and its generalizations [9]. … Suslin, "A cancellation theorem for projective modules over algebras", 1977. [5] H. Lindel, "On the Bass-Quillen conjecture concerning projective modules over polynomial rings", 1981. [6] J. …
Luc Guyot's user avatar
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2 votes
Accepted

Invariant Factors of a product $AU$ with $U$ an invertible matrix

I'll assume that elementary row and column transformations have determinant one. Otherwise the question is trivial, as darij grinberg pointed out in his comment above. My answer expands on Mohan's com …
Luc Guyot's user avatar
  • 7,893