# nLab normed division algebra

### Context

#### Algebra

higher algebra

universal algebra

# Normed division algebras

## Idea

A normed division algebra is a not-necessarily associative algebra, over some ground field, that is both a division algebra as well as a multiplicatively normed algebra.

It turns out that over the real numbers there are precisely only four normed division algebras up to isomorphism: the algebras of

In this sense real normed division algebras may be thought of as a natural generalization of the more familiar real and complex numbers.

Moreover, if one regards the real numbers as a star-algebra, then each stepin this sequence is given by applying the Cayley-Dickson construction. Applied to the octonions it yields the sedenions, which however are no longer a division algebra.

This classification turns out to closely connect to various other systems of exceptional structures in mathematics and physics:

(Moreover, apparently these two items are not unrelated, see here).

## Definition

A normed division algebra is

While the norm in a Banach algebra is in general only submultiplicative (${\|x y\|} \leq {\|x\|} {\|y\|}$), the norm in a normed division algebra must be multiplicative (${\|x y\|} = {\|x\|} {\|y\|}$). Accordingly, this norm is considered to be an absolute value and often written ${|{-}|}$ instead of ${\|{-}\|}$. There is also a converse: if the norm on a Banach algebra is multiplicative (including ${\|1\|} = 1$), then it must be a division algebra. While the term ‘normed division algebra’ does not seem to include the completeness condition of a Banach algebra, in fact the only examples have finite dimension and are therefore complete.

Accordingly, a normed division algebras is in particular a division composition algebra.

## Properties

### Classification

Over the complex numbers, the only normed division algebra is the algebra of complex numbers themselves.

The Hurwitz theorem says that over the real numbers there are, up to isomorphism, exactly four finite-dimensional normed division algebras :

• $\mathbb{R}$, the algebra of real numbers,
• $\mathbb{C}$, the algebra of complex numbers,
• $\mathbb{H}$, the algebra of quaternions,
• $\mathbb{O}$, the algebra of octonions.

Each of these is produced from the previous one by the Cayley–Dickson construction; when applied to $\mathbb{O}$, this construction produces the algebra of sedenions, which do not form a division algebra.

The Cayley–Dickson construction applies to an algebra with involution; by the abstract nonsense of that construction, we can see that the four normed division algebras above have these properties:

• $\mathbb{R}$ is associative, commutative, and with trivial involution,
• $\mathbb{C}$ is associative and commutative but has nontrivial involution,
• $\mathbb{H}$ is associative but noncommutative and with nontrivial involution,
• $\mathbb{O}$ is neither associative, commutative, nor with trivial involution.

However, these algebras do all have some useful algebraic properties; in particular, they are all alternative (a weak version of associativity). They are also all composition algebras.

A normed field is a commutative normed division algebra; it follows from the preceding that the only normed fields over $\mathbb{R}$ are $\mathbb{R}$ and $\mathbb{C}$ (e.g. Tornheim 52).

### Automorphisms

The automorphism groups of the real normed division algebras, as normed algebras, are

• $Aut(\mathbb{R}) = 1$, the trivial group

• $Aut(\mathbb{C}) = \mathbb{Z}/2$ the group of order 2, acting by complex conjugation;

• $Aut(\mathbb{H}) = SO(3)$, the special orthogonal group acting via its canonical representaiton on the 3-dimensional space of imaginary octonions;

• $Aut(\mathbb{O}) = G_2$, the exceptional Lie group G2.

### Relation to H-space structures on sphere (Hopf invariant one)

The Hopf invariant one theorem says that the spheres carrying H-space structure are precisely the unit spheres in one of the normed division algebras

Lorentzian spacetime dimensionspin groupnormed division algebrabrane scan entry
$3 = 2+1$$Spin(2,1) \simeq SL(2,\mathbb{R})$$\mathbb{R}$ the real numbers
$4 = 3+1$$Spin(3,1) \simeq SL(2, \mathbb{C})$$\mathbb{C}$ the complex numbers
$6 = 5+1$$Spin(5,1) \simeq SL(2, \mathbb{H})$$\mathbb{H}$ the quaternionslittle string
$10 = 9+1$$Spin(9,1) \underset{some\,sense}{\simeq} SL(2,\mathbb{O})$$\mathbb{O}$ the octonionsheterotic/type II string
normed division algebra$\mathbb{A}$Riemannian $\mathbb{A}$-manifoldsSpecial Riemannian $\mathbb{A}$-manifolds
real numbers$\mathbb{R}$Riemannian manifoldoriented Riemannian manifold
complex numbers$\mathbb{C}$Kähler manifoldCalabi-Yau manifold
quaternions$\mathbb{H}$quaternion-Kähler manifoldhyperkähler manifold
octonions$\mathbb{O}$Spin(7)-manifoldG2-manifold

(Leung 02)

## References

The classification of real divsion composition algebras is originally due (Hurwitz theorem) to

• Adolf Hurwitz, Über die Composition der quadratischen Formen von beliebig vielen Variabeln, Nachr. Ges. Wiss. Göttingen (1898) 309–316

General discussion includes includes

• Leonard Tornheim, Normed fields over the real and complex fields, Michigan Math. J. Volume 1, Issue 1 (1952), 61-68. (Euclid)

• Silvio Aurora, On normed rings with monotone multiplication, Pacific J. Math. Volume 33, Number 1 (1970), 15-20 (JSTOR)

Exposition with emphasis on the octonions is in

Discussion of Riemannian geometry and special holonomy modeled on the different normed division algebras is in

• Naichung Conan Leung, Riemannian Geometry Over Different Normed Division Algebras, J. Differential Geom. Volume 61, Number 2 (2002), 289-333. (euclid)

Revised on September 5, 2016 06:06:55 by Urs Schreiber (195.37.209.183)