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单词 PropertiesOfDiscriminantInAlgebraicNumberField
释义

properties of discriminant in algebraic number field


Theorem 1.  Let α1,α2,,αn and β1,β2,,βn be elements of the algebraic number fieldMathworldPlanetmath (ϑ) of degree (http://planetmath.org/NumberField) n.  If they satisfy the equations

αi=j=1ncijβjfori= 1, 2,,n,

where all coefficientsMathworldPlanetmath cij are rational numbersPlanetmathPlanetmathPlanetmath, then the http://planetmath.org/node/12060discriminantsMathworldPlanetmathPlanetmathPlanetmath are via the equation

Δ(α1,α2,,αn)=det(cij)2Δ(β1,β2,,βn).

As a special case one obtains the

Theorem 2.  If

αi=ci1+ci2ϑ++cinϑn-1fori= 1, 2,,n(1)

are the canonical forms of the elements αi in (ϑ), then

Δ(α1,α2,,αn)=det(cij)2Δ(1,ϑ,,ϑn-1),

where Δ(1,ϑ,,ϑn-1) is a Vandermonde determinantMathworldPlanetmath thushaving the product form

Δ(1,ϑ,,ϑn-1)=(1ij(ϑj-ϑi))2=1ij(ϑi-ϑj)2(2)

where ϑ1=ϑ,ϑ2,,ϑn are the algebraic conjugates of ϑ.

Since the (2) is also the polynomial discriminant of the irreducible minimal polynomialPlanetmathPlanetmath of ϑ, the numbers ϑi are inequal.  It follows the

Theorem 3.  When (1) are the canonical forms of the numbers αi, one has

Δ(α1,α2,,αn)= 0det(cij)= 0.

The powers 1,ϑ,,ϑn-1 of the primitive elementMathworldPlanetmathPlanetmath (http://planetmath.org/SimpleFieldExtension) form a basis (http://planetmath.org/Basis) of the field extension (ϑ)/ (see the canonical form of element of number field).  By the theorem 3 we may write the

Theorem 4.  The numbers α1,α2,,αn of (ϑ) are linearly independentMathworldPlanetmath over if and only if  Δ(α1,α2,,αn) 0.

Theorem 5.(α)=(ϑ)Δ(1,α,α2,,αn-1)0.  Here, the the discriminant is the discriminant of the algebraic numberMathworldPlanetmath (http://planetmath.org/DiscriminantOfAlgebraicNumber) α.

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更新时间:2025/5/5 2:36:47