division algebra
Let be a unital ring and a -algebra. Defining “division” requires special considerations when the algebras are non-associative so we introduce the definition in stages.
1 Associative division algebras
If is an associative algebra then we say is a division algebra if
The standard examples of associative division algebras are fields, which are commutative, and the non-split quaternion algebra: ,
where and are irreducible over .
2 Non-associative division algebras
For non-associative algebras , the notion of an inverse is not immediate. We use for the product of .
Invertible as endomorphisms: Let . Then define and . As the product of is distributive, both an are additive endomorphisms of . If is invertible then we may call “left invertible” and similarly, when is invertible we may call “right invertible” and “invertible” if both and are invertible.
In this model of invertible, is a division algebra if, and only if, for each non-zero , both and invertible. Equivalently: the equations and have unique solutions for nonzero . However, and need not be equal.
A common method to produce non-associative division algebras of this sort is through Schur’s Lemma.
Invertible in the product: In some instances, the notion of invertible via endomorphisms is not sufficient. Instead, assume has an identity, that is, an element such that for all ,
Next if , we say is invertible if there exists a such that
(1) |
and furthermore that for all ,
(2) |
Evidently (1) can be inferred from (2). This added assumption substitutes for the need of associativity in the proofs of uniqueness of inverses and in solving equations with non-associative products.
Proposition 1.
If is a finite dimensional algebra over a field, then invertible in this sense forces both and to be invertible as well.
Proof.
Let . Then . So . As is the identity map, is injective and is surjective. As is finite dimensional, injective and surjective endomorphisms are bijective. ∎
In this model, a non-associative algebra is a division algebra if it is unital and every non-zero element is invertible.
3 Alternative division algebras
The standard examples of non-associative division algebras are actually alternative alegbras, specfically, the composition algebras of fields, non-split quaternions and non-split octonions – only the latter are actually not associative. Invertible in the octonions is interpreted in the second stronger form.
Theorem 2 (Bruck-Klienfeld).
Every alternative division algebra is either associative or a non-split octonion.
This result is usually followed by two useful results which serve to omit the need to consider non-associative examples.
Theorem 3 (Artin-Zorn, Wedderburn).
A finite alternative division algebra is associative and commutative, so it is a finite field.
Theorem 4.
An alternative division algebra over an algebraically closed field is the field itself.
Title | division algebra |
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Canonical name | DivisionAlgebra |
Date of creation | 2013-03-22 16:52:03 |
Last modified on | 2013-03-22 16:52:03 |
Owner | Algeboy (12884) |
Last modified by | Algeboy (12884) |
Numerical id | 6 |
Author | Algeboy (12884) |
Entry type | Definition |
Classification | msc 16K99 |
Related topic | octonion |
Related topic | Octonion |
Defines | division algebra |