splitting and ramification in number fields and Galois extensions
Let be an extension of number fields and let and be their respective rings of integers. The ring of integers of a number field is a Dedekind domain, and these enjoy the property that every ideal factors uniquely as a finite product of prime ideals (see the entry fractional ideal (http://planetmath.org/FractionalIdeal)). Let be a prime ideal of . Then is an ideal of . Let us assume that the prime ideal factorization of into primes of is as follows:
(1) |
We say that the primes lie above and (divides). The exponent (commonly denoted as ) is the ramification index of over . Notice that for each prime ideal , the quotient ring is a finite field extension of the finite field (also called the residue field). The degree of this extension is called the inertial degree of over and it is usually denoted by:
Notice that as it is pointed out in the entry βinertial degree (http://planetmath.org/InertialDegree)β, the ramification index and the inertial degree are related by the formula:
(2) |
where is the number of prime ideals lying above (as in Eq. (1)). See the theorem below for an improvement of Eq. (2) in the case when is Galois.
Definition 1.
Let and be as above.
-
1.
If for some , then we say that is ramified over and ramifies in . If for all then we say that is unramified in .
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2.
If there is a unique prime ideal lying above (so ) and then we say that is totally ramified in . In this case .
-
3.
On the other hand, if for all , we say that is totally split (or splits completely) in . Notice that there are exactly prime ideals of lying above .
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4.
Let be the characteristic of the residue field . If and and are relatively prime, then we say that is tamely ramified. If then we say that is strongly ramified (or wildly ramified).
When the extension is a Galois extension then Eq. (2) is quite more simple:
Theorem 1.
Assume that is a Galois extension of number fields. Then all the ramification indices are equal to the same number , all the inertial degrees are equal to the same number and the ideal factors as:
Moreover:
Title | splitting and ramification in number fields and Galois extensions |
Canonical name | SplittingAndRamificationInNumberFieldsAndGaloisExtensions |
Date of creation | 2013-03-22 15:05:29 |
Last modified on | 2013-03-22 15:05:29 |
Owner | alozano (2414) |
Last modified by | alozano (2414) |
Numerical id | 11 |
Author | alozano (2414) |
Entry type | Definition |
Classification | msc 12F99 |
Classification | msc 13B02 |
Classification | msc 11S15 |
Synonym | completely split |
Synonym | strongly ramified |
Synonym | wild ramification |
Related topic | Ramify |
Related topic | InertialDegree |
Related topic | CalculatingTheSplittingOfPrimes |
Related topic | PrimeIdealDecompositionInQuadraticExtensionsOfMathbbQ |
Related topic | PrimeIdealDecompositionInCyclotomicExtensionsOfMathbbQ |
Defines | totally ramified |
Defines | totally split |
Defines | wildly ramified |
Defines | tamely ramified |