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单词 QuadraticJordanAlgebra
释义

quadratic Jordan algebra


Definition 1.

Fix a commutative ring R and an R-modules J and a quadratic mapU:JEndRJ. Then the triple J,U is aquadratic Jordan algebra if (denoting the evaluation of U by Uxfor xJ)

  1. 1.

    UUab=UaUbUa for all a,bJ.

  2. 2.

    The induced bilinear map Ua,b:=Ua+b-Ua-Ub gives rise toan endomorphismMathworldPlanetmathPlanetmathPlanetmath Va,b on J defined by Va,bx=Ua,xb which satisfies

    UaVb,a=Va,bUa.
  3. 3.

    If RK is a commutative ring extension of R then the extensionJK:=KRJ with the extension UK:=1KU, satisfies the first two axioms.

For a unital quadratic Jordan algebra we include the added assumptionthat there exist some 1J such thatU1 is the identityPlanetmathPlanetmathPlanetmath endomorphism of J.

The concept of a quadratic Jordan algebra was developed by McCrimmon to introduce uniform methods in the study of Jordan algebrasMathworldPlanetmathPlanetmath over characteristic 2.In a strict sense they are not algebrasMathworldPlanetmathPlanetmathPlanetmath as they do not have a bilinear productMathworldPlanetmath;however, their connection to Jordan algebras motivates this terminology.

A common construction for Jordan algebras, so called special Jordan algebra, is by means of using a submodule of an associative algebra A and defining theproduct as

a.b=12(ab+ba).

The 1/2 is optional (and avoided in the analogous Lie bracket definitions [a,b]=ab-ba), in characteristic 2 we can opt to remove it. The resultis the usual special Jordan product is also the usual Lie bracket, a.b=ab+ba=ab-ba=[a,b]. So we can treat these algebras as Jordan or Lie algebrasMathworldPlanetmath.

However, the axioms of an abstract Jordan algebra are insufficientto conclude that every Jordan algebra is special (indeed exceptional Jordanalgebras called Albert algebras of dimensionMathworldPlanetmathPlanetmathPlanetmath 27 exist and are not specialJordan algebras.) So general Jordan algebra over characteristic 2 may have different structure than a Lie algebra of characteristic 2. To make thesealgebras manageable, McCrimmon appealed to the quadratic definition given above.

Proposition 2.

If 1/2K then a Jordan algebra over K is a quadratic Jordan algebrawhere the quadratic map is given by Ua={axa} where {xyz} is the Jordan triple product.

A bonus to this definition is that it highlights the fundamental tools in the study of Jordan algebras. For example, instead of using ideals of the Jordan product it is common to use quadratic ideals, for instance, in the definition of the solvablePlanetmathPlanetmath radicalPlanetmathPlanetmathPlanetmathPlanetmath of a Jordan algebra.

Definition 3.

A submodule I of a quadratic Jordan algebra J is an inner quadraticideal, or simply an inner ideal if UI(J)I, that is Ui(x)Ifor all iI, xJ.

A submodule I of a quadratic Jordan algebra J is an outer quadraticideal, or a outer ideal if UJ(I)I, that is, Ux(i)Ifor all iI, xJ.

If the quadratic Jordan algebra is derived from a Jordan algebra thenUi(x)={ixi} So we are asking for {iJi}I, and in a specialJordan algebra we can further express this as iJiI.

References

  • 1 Jacobson, Nathan Structure Theory of Jordan Algebras, The University ofArkansas Lecture Notes in Mathematics, vol. 5, Fayetteville, 1981.
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