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单词 NormAndTraceOfAlgebraicNumber
释义

norm and trace of algebraic number


Theorem 1.

Let K be an algebraic number fieldMathworldPlanetmath and α an element of K.  The norm N(α) and the trace S(α) of α in the field extension K/ both are rational numbers and especially rational integers in the case α is an algebraic integerMathworldPlanetmath.  If β is another element of K, then

N(αβ)=N(α)N(β),S(α+β)=S(α)+S(β),(1)

i.e. the norm is multiplicative and the trace additivePlanetmathPlanetmath.  If [K:]=n  and  a, then

N(a)=an,S(a)=na.

Remarks

1.  The notions norm and trace were originally introduced in German as “die Norm” and “die Spur”.  Therefore in German and many other literature the symbol of trace is S, Sp or sp.  Nowadays the symbols T and Tr are common.

2.  The norm and trace of an algebraic numberMathworldPlanetmath α in the field extension  (α)/,  i.e. the productPlanetmathPlanetmathPlanetmath and sum of all algebraic conjugates of α, are called the absolute norm and the absolute trace of α.  Formulae like (1) concerning the absolute norms and traces are not sensible.

Theorem 2.

An algebraic integer ε is a unit if and only if

N(ε)=±1,

i.e. iff the absolute norm of ε is a rational unit.  Thus in the minimal polynomial of an algebraic unit is always  ±1.

Example.  The minimal polynomial of the number 2+3, which is the fundamental unitMathworldPlanetmath of the quadratic field (3), is  x2-4x+1.

Titlenorm and trace of algebraic number
Canonical nameNormAndTraceOfAlgebraicNumber
Date of creation2013-03-22 15:19:08
Last modified on2013-03-22 15:19:08
Ownerpahio (2872)
Last modified bypahio (2872)
Numerical id15
Authorpahio (2872)
Entry typeTheorem
Classificationmsc 11R04
Related topicTheoryOfAlgebraicNumbers
Related topicAlgebraicNumberTheory
Related topicIdealNorm
Related topicUnitsOfRealCubicFieldsWithExactlyOneRealEmbedding
Related topicIndependenceOfCharacteristicPolynomialOnPrimitiveElement
Definesabsolute norm
Definesabsolute trace
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更新时间:2025/5/4 3:02:16