norm-Euclidean number field
Definition. An algebraic number field K is a norm-Euclidean number field, if for every pair (α,β) of the integers (http://planetmath.org/AlgebraicInteger) of K, where β≠0, there exist ϰ and ϱ of the field such that
α=ϰβ+ϱ,|N(ϱ)|<|N(β)|. |
Here N means the norm function in K.
Theorem 1. A field K is norm-Euclidean if and only if each number γ of K is in the form
γ=ϰ+δ | (1) |
where ϰ is an of the field and |N(δ)|<1.
Proof. First assume the condition (1). Let α and β be integers of K, β≠0. Then there are the numbers ϰ,δ∈K such that ϰ is integer and
αβ=ϰ+δ,|N(δ)|< 1. |
Thus we have
α=ϰβ+βδ=ϰβ+ϱ. |
Here ϱ=βδ is integer, since α and ϰβ are integers. We also have
|N(ϱ)|=|N(β)|⋅|N(δ)|<|N(β)|⋅1=|N(β)|. |
Accordingly, K is a norm-Euclidean number field.
Secondly assume that K is norm-Euclidean. Let γ be an arbitrary element of the field. We can determine (http://planetmath.org/MultiplesOfAnAlgebraicNumber) a rational integer m(≠0) such that mγ is an algebraic integer of K. The assumption
guarantees the integers ϰ, ϱ of K such that
mγ=ϰm+ϱ,N(ϱ)<N(m). |
Thus
γ=mγm=ϰ+ϱm,|N(ϱm)|=|N(ϱ)||N(m)|<1, |
Q.E.D.
Theorem 2. In a norm-Euclidean number field, any two non-zero have a greatest common divisor.
Proof. We recall that the greatest common divisor of two elements of a commutative ring means such a common divisor of the elements that it is divisible by each common divisor of the elements. Let now ϱ0 and ϱ1 be two algebraic integers of a norm-Euclidean number field K. According the definition there are the integers ϰi and ϱi of K such that
{ϱ0=ϰ2ϱ1+ϱ2,|N(ϱ2)|<|N(ϱ1)|ϱ1=ϰ3ϱ2+ϱ3,|N(ϱ3)|<|N(ϱ2)|ϱ2=ϰ4ϱ3+ϱ4,|N(ϱ4)|<|N(ϱ3)|⋯⋯ϱn-2=ϰnϱn-1+ϱn,|N(ϱn)|<|N(ϱn-1)|ϱn-1=ϰn+1ϱn+0, |
The ends to the remainder 0, because the numbers |N(ϱi)| form a descending sequence of non-negative rational integers — see the entry norm and trace of algebraic number. As in the Euclid’s algorithm in ℤ, one sees that the last divisor ϱn is one greatest common divisor of ϱ0 and ϱ1. N.B. that ϱ0 and ϱ1 may have an infinite
amount of their greatest common divisors, depending the amount of the units in K.
Remark. The ring of integers of any norm-Euclidean number field is a unique factorization domain
and thus all ideals of the ring are principal ideals
. But not all algebraic number fields with ring of integers a UFD (http://planetmath.org/UFD) are norm-Euclidean, e.g. ℚ(√14).
Theorem 3. The only norm-Euclidean quadratic fields ℚ(√d) are those with
d∈{-11,-7,-3,-2,-1, 2, 3, 5, 6, 7, 11, 13, 17, 19, 21, 29, 33, 37, 41, 57, 73}. |
Title | norm-Euclidean number field |
Canonical name | NormEuclideanNumberField |
Date of creation | 2013-03-22 16:52:26 |
Last modified on | 2013-03-22 16:52:26 |
Owner | pahio (2872) |
Last modified by | pahio (2872) |
Numerical id | 17 |
Author | pahio (2872) |
Entry type | Topic |
Classification | msc 13F07 |
Classification | msc 11R21 |
Classification | msc 11R04 |
Related topic | EuclideanValuation |
Related topic | QuadraticImaginaryEuclideanNumberFields |
Related topic | ListOfAllImaginaryQuadraticPIDs |
Related topic | EuclideanField |
Related topic | AlgebraicNumberTheory |
Related topic | MixedFraction |
Defines | norm-Euclidean |