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e, f, and the remainder theorem
*September 12, 2009*

*Posted by Akhil Mathew in algebra, algebraic number theory, commutative algebra, number theory.*

Tags: Chinese remainder theorem, Dedekind domains, ramification

1 comment so far

Tags: Chinese remainder theorem, Dedekind domains, ramification

1 comment so far

So, now to the next topic in introductory algebraic number theory: ramification. This is a measure of how primes “split.” (*No, definitely wrong word there…)*

**e and f**

Fix a Dedekind domain with quotient field ; let be a finite separable extension of , and the integral closure of in . We know that is a Dedekind domain.

(By the way, I’m now assuming that readers have been following the past few posts or so on these topics.)

Given a prime , there is a prime lying above . I hinted at the proof in the previous post, but to save time and avoid too much redundancy I’ll refer interested readers to this post.

Now, we can do a prime factorization of say . The primes contain and consequently lie above . Conversely, any prime of containing must lie above , since if is an ideal in a Dedekind domain contained in a prime ideal , then occurs in the prime factorization of (to see this, localize and work in a DVR). (more…)

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Extensions of discrete valuations
*September 5, 2009*

*Posted by Akhil Mathew in algebra, algebraic number theory, commutative algebra, number theory.*

Tags: absolute values, Dedekind domains, discrete valuation rings, separable extensions

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Tags: absolute values, Dedekind domains, discrete valuation rings, separable extensions

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With the school year starting, I can’t keep up with the one-post-a-day frequency anymore. Still, I want to keep plowing ahead towards class field theory.

Today’s main goal is to show that under certain conditions, we can always extend valuations to bigger fields. I’m not aiming for maximum generality here though.

**Dedekind Domains and Extensions **

One of the reasons Dedekind domains are so important is

Theorem 1Let be a Dedekind domain with quotient field , a finite separable extension of , and the integral closure of in . Then is Dedekind. (more…)

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Dedekind domains
*August 31, 2009*

*Posted by Akhil Mathew in algebra, algebraic number theory, commutative algebra, number theory.*

Tags: Dedekind domains, discrete valuation rings, Krull dimension, localization, Noetherian rings, unique factorization

2 comments

Tags: Dedekind domains, discrete valuation rings, Krull dimension, localization, Noetherian rings, unique factorization

2 comments

Today’s (quick) topic focuses on Dedekind domains. These come up when you take the ring of integers in any finite extension of (i.e. number fields). In these, you don’t necessarily have unique factorization. But you do have something close, which makes these crucial.

Definition 1A(more…)Dedekind domainis a Noetherian integral domain that is integrally closed, and of Krull dimension one—that is, each nonzero prime ideal is maximal.