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Completions of rings and modules
*August 25, 2009*

*Posted by Akhil Mathew in algebra, commutative algebra.*

Tags: Artin-Rees lemma, completions, exact functors, Hilbert basis theorem, Noetherian rings

2 comments

Tags: Artin-Rees lemma, completions, exact functors, Hilbert basis theorem, Noetherian rings

2 comments

So, we saw in the previous post that completion can be defined generally for abelian groups. Now, to specialize to rings and modules.

**Rings **

The case in which we are primarily interested comes from a ring with a descending filtration (satisfying ), which implies the are ideals; as we saw, the completion will also be a ring. Most often, there will be an ideal such that , i.e. the filtration is -adic. We have a completion functor from filtered rings to rings, sending . Given a filtered -module , there is a completion , which is also a -module; this gives a functor from filtered -modules to -modules. (more…)

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How to tell if a ring is Noetherian
*August 9, 2009*

*Posted by Akhil Mathew in algebra, commutative algebra.*

Tags: algebra, commutative algebra, Hilbert basis theorem, localization, Noetherian rings

7 comments

Tags: algebra, commutative algebra, Hilbert basis theorem, localization, Noetherian rings

7 comments

I briefly outlined the definition and first properties of Noetherian rings and modules a while back. There are several useful and well-known criteria to tell whether a ring is Noetherian, as I will discuss in this post. Actually, I’ll only get to the first few basic ones here, though these alone give us a lot of tools for, say, algebraic geometry, when we want to show our schemes are relatively well-behaved. But there are plenty more to go.

**Hilbert’s basis theorem **

It is the following:

Theorem 1 (Hilbert)Let be a Noetherian ring. Then the polynomial ring is also Noetherian.

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Why simple modules are often finite-dimensional, I
*July 19, 2009*

*Posted by Akhil Mathew in algebra.*

Tags: algebra, Hilbert basis theorem, Nullstellensatz, polynomials, simple modules

3 comments

Tags: algebra, Hilbert basis theorem, Nullstellensatz, polynomials, simple modules

3 comments

Today I want to talk (partially) about a general fact, that first came up as a side remark in the context of my project, and which Dustin Clausen, David Speyer, and I worked out a few days ago. It was a useful bit of algebra for me to think about.

Theorem 1Let be an associative algebra with identity over an algebraically closed field ; suppose the center is a finitely generated ring over , and is a finitely generated -module.Then: all simple -modules are finite-dimensional -vector spaces.

We’ll get to this after discussing a few other facts about rings, interesting in their own right.