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单词 FundamentalTheoremOfIdealTheory
释义

fundamental theorem of ideal theory


Theorem.  Every nonzero ideal of the ring of integersMathworldPlanetmath of an algebraic number fieldMathworldPlanetmath can be written as product (http://planetmath.org/ProductOfIdeals) of prime idealsMathworldPlanetmathPlanetmathPlanetmath of the ring.  The prime ideal of the factors (http://planetmath.org/Product).

In this entry we consider the ring 𝒪 of the integers of a number field (ϑ).  We use as starting the fact that the ideals of 𝒪 are finitely generatedMathworldPlanetmathPlanetmath submodulesMathworldPlanetmath of 𝒪 (cf. basis of ideal in algebraic number field) and that its prime ideals 𝔭 are maximal idealsMathworldPlanetmath, i.e. the only ideal factors of 𝔭 are 𝔭 itself and the unit ideal  (1)=𝒪.

For proving the above fundamental theorem of ideal theory, we present and prove some lemmata.

Lemma 1.  The equation  𝔞=𝔟𝔠  between the ideals of 𝒪 implies that 𝔞𝔠.

Proof.  Let  𝔟=(β1,,βs)  and  𝔠=(γ1,,γt).  If

α𝔞=(β1γ1,,βiγj,,βsγt),

then there are the elements λij of 𝒪 such that

α=ijλijβiγj=j(iλijβi)γj.

But the of γj in the parentheses are elements of the ring 𝒪, whence the last sum form of α shows that  α𝔠.  Consequently, 𝔞𝔠.

Lemma 2.  Any nonzero element α of 𝒪 belongs only to a finite number of ideals of 𝒪.

Proof.  Let  𝔞=(α1,,αr)  be any ideal containing α and let {ϱ1,,ϱm}  be a complete residue systemMathworldPlanetmath modulo α (cf. congruence in algebraic number field).  Then

αi=αλi+ϱni  (i= 1,,r)

where the numbers λi belong to 𝒪.  Since we have

𝔞=(α1,,αr,α)=(αλ1+ϱn1,,αλr+ϱnr,α)=(ϱn1,,ϱnr,α),

there can be different ideals 𝔞 only a finite number, at most1+m+(n2)++(mm)=2m.

Lemma 3.  Each ideal 𝔞 of 𝒪 has only a finite number of ideal factors.

Proof.  If  𝔠𝔞  and  α𝔞,  then by Lemma 1, α𝔠,  whence Lemma 2 implies that there is only a finite number of such factors 𝔠.

Lemma 4.  All nonzero ideals of 𝒪 are cancellativePlanetmathPlanetmath (http://planetmath.org/CancellationIdeal), i.e. if  𝔞𝔠=𝔞𝔡  then  𝔠=𝔡.

Proof.  The theorem of http://planetmath.org/node/3154Steinitz (1911) guarantees an ideal 𝔤 of 𝒪 such that the product 𝔤𝔞 is a principal idealMathworldPlanetmathPlanetmathPlanetmath (ω).  Then we may write

(ω)𝔠=(𝔤𝔞)𝔠=𝔤(𝔞𝔠)=𝔤(𝔞𝔡)=(𝔤𝔞)𝔡=(ω)𝔡.

If  𝔠=(γ1,,γs)  and  𝔡=(δ1,,δt),  we thus have the equation

(ωγ1,,ωγs)=(ωδ1,,ωδt)

by which there must exist the elements λi1,,λit of 𝒪 such that

ωγi=λi1ωδ1++λitωδt.

Consequently, the http://planetmath.org/node/7040generatorsPlanetmathPlanetmathPlanetmathPlanetmathγi=λi1δ1++λitδt  of 𝔠 belong to the ideal 𝔡, and therefore  𝔠𝔡.  Similarly one gets the reverse containment.

Lemma 5.  If  𝔞=𝔟𝔠  and  𝔟(1),  then 𝔠 has less ideal factors than 𝔞.

Proof.  Evidently, any factor of 𝔠 is a factor of 𝔞.  But 𝔞𝔞  and  𝔞𝔠, since otherwise we had 𝔠=𝔞𝔡=𝔟𝔠𝔡  whence (1)=𝔟𝔡 which would, by Lemma 4, imply 𝔟=(1).

Lemma 6.  Any proper idealMathworldPlanetmath 𝔞 of 𝒪 has a prime ideal factor.

Proof.  Let 𝔠 be such a factor of 𝔞 that has as few factors as possible.  Then𝔠 must be a prime ideal, because otherwise we had  𝔠=𝔠1𝔡  where𝔠1 and 𝔡 are proper ideals of 𝒪 and, by Lemma 5, the ideal 𝔠1 would have less factors than 𝔠; this however contradicts the fact  𝔠1𝔞.

Lemma 7.  Every nonzero proper ideal 𝔞 of 𝒪 can be written as a product𝔭1𝔭k where  k>0  and the factors 𝔭i are prime ideals.

Proof.  If 𝔞 has only one factor 𝔭 distinct from (1), then  𝔞=𝔭  is a prime ideal.
Induction hypothesis:  Lemma 7 is in always when 𝔞 has at most n factors.  Let 𝔞 now have n+1 factors.  Lemma 6 implies that there is a prime ideal 𝔭 such that 𝔞=𝔭𝔡  where  𝔡(1)  and 𝔡 has, by Lemma 5, at most n factors.  Hence,  𝔡=𝔭1𝔭k  and therefore, 𝔞=𝔭𝔭1𝔭k  where all 𝔭’s are prime ideals.

Lemma 8.  Any two prime factorMathworldPlanetmathPlanetmath

𝔞=𝔭1𝔭r=𝔮1𝔮s

of a nonzero ideal 𝔞 of 𝒪 are identical, i.e.  r=s  and each prime factor 𝔭i is equal to a prime factor 𝔮j and vice versa.

Proof.  Any prime ideal has the property that if it divides a product of ideals, it divides one of the factors of the product; now these factors are prime ideals and therefore the prime ideal coincides with one of the factors.  Similarly as in the proof of the fundamental theorem of arithmetics, one sees the uniqueness of the prime factorisation of 𝔞.

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更新时间:2025/5/4 11:44:40