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单词 IdealClassesFormAnAbelianGroup
释义

ideal classes form an abelian group


Let K be a number field, and let 𝒞 be the set of ideal classes of K, with multiplicationPlanetmathPlanetmath defined by

[𝔞][𝔟]=[𝔞𝔟]

where 𝔞,𝔟 are ideals of 𝒪K.

We shall check the group properties:

  1. 1.

    Associativity: [𝔞]([𝔟][𝔠])=[𝔞][𝔟𝔠]=[𝔞(𝔟𝔠)]=[𝔞𝔟𝔠]=[(𝔞𝔟)𝔠]=[𝔞𝔟][𝔠]=([𝔞][𝔟])[𝔠]

  2. 2.

    Identity elementMathworldPlanetmath: [𝒪K][𝔟]=[𝔟]=[𝔟][𝒪K].

  3. 3.

    InversesMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath: Consider [𝔟]. Let b be an integer in 𝔟. Then𝔟(b), so there exists 𝔠such that 𝔟𝔠=(b).
    Then the ideal class [𝔟][𝔠]=[(b)]=[𝒪K].

Then 𝒞 is a group under the operationMathworldPlanetmath .

It is abelianMathworldPlanetmath since [𝔞][𝔟]=[𝔞𝔟]=[𝔟𝔞]=[𝔟][𝔞].

This is group is called the ideal class group of K. The ideal class group is one of the principal objects of algebraic number theoryMathworldPlanetmath. In particular, for an arbitrary number field K, very little is known about the size of this group, called the class number of K. See the analytic class number formulaMathworldPlanetmath.

Titleideal classes form an abelian group
Canonical nameIdealClassesFormAnAbelianGroup
Date of creation2013-03-22 12:49:40
Last modified on2013-03-22 12:49:40
Ownermathcam (2727)
Last modified bymathcam (2727)
Numerical id13
Authormathcam (2727)
Entry typeTheorem
Classificationmsc 11R04
Classificationmsc 11R29
Related topicNumberField
Related topicClassNumbersAndDiscriminantsTopicsOnClassGroups
Related topicFractionalIdealOfCommutativeRing
Definesideal class group
Definesclass number
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更新时间:2025/5/5 0:14:21