# Contents

## Definition

A division algebra is a possibly non-associative algebra $A$, typically over a field $k$, with the property that $a b = 0$ implies either $a = 0$ or $b = 0$ (or $a b \ne 0$ whenever $a, b \ne 0$). If $A$ is finite-dimensional (over a field), this is equivalent to assuming that for any nonzero $a \in A$, the operations of left multiplication and right multiplication by $a$ are invertible. If furthermore $A$ is also associative and unital, this is also equivalent to the existence, for each nonzero $a \in A$, of an element $a^{-1} \in A$ with $a a^{-1} = a^{-1} a = 1$. However, it is easy to construct nonassociative unital finite-dimensional algebras over $\mathbb{R}$ (the field of real numbers) such that either:

• $A$ is not a division algebra but for each nonzero $a \in A$ there exists $a^{-1} \in A$ with $a a^{-1} = a^{-1} a = 1$.

• $A$ is a division algebra but there exists nonzero $a \in A$ for which there is no $a^{-1} \in A$ with $a a^{-1} = a^{-1} a = 1$.

For applications to physics, the most interesting division algebras are probably the normed division algebras: the real numbers, complex numbers, quaternions and octonions. These have important relations to supersymmetry.

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

## References

Revised on November 14, 2013 11:32:41 by Urs Schreiber (188.200.54.65)