quadratic Jordan algebra
Definition 1.
Fix a commutative ring and an -modules and a quadratic map. Then the triple is aquadratic Jordan algebra if (denoting the evaluation of by for )
- 1.
for all .
- 2.
The induced bilinear map gives rise toan endomorphism
on defined by which satisfies
- 3.
If is a commutative ring extension of then the extension with the extension , satisfies the first two axioms.
For a unital quadratic Jordan algebra we include the added assumptionthat there exist some such that is the identity endomorphism of .
The concept of a quadratic Jordan algebra was developed by McCrimmon to introduce uniform methods in the study of Jordan algebras over characteristic 2.In a strict sense they are not algebras
as they do not have a bilinear product
;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 and defining theproduct as
The is optional (and avoided in the analogous Lie bracket definitions ), in characteristic 2 we can opt to remove it. The resultis the usual special Jordan product is also the usual Lie bracket, . So we can treat these algebras as Jordan or Lie algebras.
However, the axioms of an abstract Jordan algebra are insufficientto conclude that every Jordan algebra is special (indeed exceptional Jordanalgebras called Albert algebras of dimension 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 then a Jordan algebra over is a quadratic Jordan algebrawhere the quadratic map is given by where 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 solvable radical
of a Jordan algebra.
Definition 3.
A submodule of a quadratic Jordan algebra is an inner quadraticideal, or simply an inner ideal if , that is for all , .
A submodule of a quadratic Jordan algebra is an outer quadraticideal, or a outer ideal if , that is, for all , .
If the quadratic Jordan algebra is derived from a Jordan algebra then So we are asking for , and in a specialJordan algebra we can further express this as .
References
- 1 Jacobson, Nathan Structure Theory of Jordan Algebras, The University ofArkansas Lecture Notes in Mathematics, vol. 5, Fayetteville, 1981.