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单词 NumberFieldThatIsNotNormEuclidean
释义

number field that is not norm-Euclidean


Proposition.  The real quadratic fieldMathworldPlanetmath (14) is not norm-Euclidean.

Proof.  We take the number  γ=12+1214  which is not integer of the field (142(mod4)).  Antithesis:  γ=ϰ+δ  where  ϰ=a+b14  is an integer of the field (a,b) and

|N(δ)|=|(12-a)2-14(12-b)2|<1.

Thus we would have

|(2a-1)2-14(2b-1)2E|<4.

And since  (2a-1)2=4(a-1)a+11(mod8),  it follows  E1-1413(mod8),  i.e.  E=3.  So we must have

(2a-1)2(2a-1)2-14(2b-1)23(mod7).(1)

But  {0,±1,±2,±3}  is a complete residue systemMathworldPlanetmath modulo 7, giving  the set  {1, 2, 4}  of possible quadratic residuesMathworldPlanetmath modulo 7.  Therefore (1) is impossible.  The antithesis is wrong, whence the theorem 1 of the parent entry (http://planetmath.org/EuclideanNumberField) says that the number fieldMathworldPlanetmath is not norm-Euclidean.

Note.  The function N used in the proof is the usual

N:r+s14r2-14s2(r,s)

defined in the field (14).  The notion of norm-Euclidean number field is based on the norm (http://planetmath.org/NormAndTraceOfAlgebraicNumber).  There exists a fainter function, the so-called Euclidean valuation, which can be defined in the maximal ordersMathworldPlanetmath of some algebraic number fields (http://planetmath.org/NumberField); such a maximal order, i.e. the ring of integersMathworldPlanetmath of the number field, is then a Euclidean domainMathworldPlanetmath.  The existence of a Euclidean valuation guarantees that the maximal order is a UFD and thus a PID.  Recently it has been shown the existence of the Euclidean domain [1+692] in the field (69) but the field is not norm-Euclidean.

The maximal order [14] of (14) has also been proven to be a Euclidean domain (Malcolm Harper 2004 in Canadian Journal of Mathematics).

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更新时间:2025/5/4 2:53:24