composition algebra

In mathematics, a composition algebra over a field is a not necessarily associative algebra over together with a nondegenerate quadratic form that satisfies
:$N(xy) = N(x)N(y)$
for all and in .

A composition algebra includes an involution called a conjugation: $x \mapsto x^*.$ The quadratic form $N(x) = x x^*$ is called the norm of the algebra.

A composition algebra (''A'', ∗, ''N'') is either a division algebra or a split algebra, depending on the existence of a non-zero ''v'' in ''A'' such that ''N''(''v'') = 0, called a null vector. When ''x'' is ''not'' a null vector, the multiplicative inverse of ''x'' is When there is a non-zero null vector, ''N'' is an isotropic quadratic form, and "the algebra splits".

Structure theorem

Every unital composition algebra over a field can be obtained by repeated application of the Cayley–Dickson construction starting from (if the characteristic of is different from ) or a 2-dimensional composition subalgebra (if ).  The possible dimensions of a composition algebra are , , , and .Guy Roos (2008) "Exceptional symmetric domains", §1: Cayley algebras, in ''Symmetries in Complex Analysis'' by Bruce Gilligan & Guy Roos, volume 468 of ''Contemporary Mathematics'', American Mathematical Society,

*1-dimensional composition algebras only exist when .
*Composition algebras of dimension 1 and 2 are commutative and associative.
*Composition algebras of dimension 2 are either quadratic field extensions of or isomorphic to .
*Composition algebras of dimension 4 are called quaternion algebras.  They are associative but not commutative.
*Composition algebras of dimension 8 are called octonion algebras.  They are neither associative nor commutative.

For consistent terminology, algebras of dimension 1 have been called ''unarion'', and those of dimension 2 ''binarion''.

Instances and usage

When the field is taken to be complex numbers and the quadratic form , then four composition algebras over are , the bicomplex numbers, the biquaternions (isomorphic to the complex matrix ring ), and the bioctonions , which are also called complex octonions.

The matrix ring has long been an object of interest, first as biquaternions by
Hamilton (1853), later in the isomorphic matrix form, and especially as Pauli algebra.

The squaring function on the real number field forms the primordial composition algebra.
When the field is taken to be real numbers , then there are just six other real composition algebras.
In two, four, and eight dimensions there are both a division algebra and a "split algebra":
: binarions: complex numbers with quadratic form and split-complex numbers with quadratic form ,
: quaternions and split-quaternions,
: octonions and split-octonions.

Every composition algebra has an associated bilinear form B(''x,y'') constructed with the norm N and a polarization identity:
:$B(x,y) \ = \$(x + y) - N(x) - N(y)2 .

History

The composition of sums of squares was noted by several early authors. Diophantus was aware of the identity involving the sum of two squares, now called the Brahmagupta–Fibonacci identity, which is also articulated as a property of Euclidean norms of complex numbers when multiplied. Leonhard Euler discussed the four-square identity in 1748, and it led W. R. Hamilton to construct his four-dimensional algebra of quaternions.Kevin McCrimmon (2004) ''A Taste of Jordan Algebras'', Universitext, Springer In 1848 tessarines were described giving first light to bicomplex numbers.

About 1818 Danish scholar Ferdinand Degen displayed the Degen's eight-square identity, which was later connected with norms of elements of the octonion algebra:
:Historically, the first non-associative algebra, the Cayley numbers ... arose in the context of the number-theoretic problem of quadratic forms permitting composition…this number-theoretic question can be transformed into one concerning certain algebraic systems, the composition algebras...

In 1919 Leonard Dickson advanced the study of the Hurwitz problem with a survey of efforts to that date, and by exhibiting the method of doubling the quaternions to obtain Cayley numbers. He introduced a new imaginary unit , and for quaternions and writes a Cayley number . Denoting the quaternion conjugate by , the product of two Cayley numbers is
:$(q + Qe)(r + Re) = (qr - R'Q) + (Rq + Q r')e .$
The conjugate of a Cayley number is , and the quadratic form is , obtained by multiplying the number by its conjugate. The doubling method has come to be called the Cayley–Dickson construction.

In 1923 the case of real algebras with positive definite forms was delimited by the Hurwitz's theorem (composition algebras).

In 1931 Max Zorn introduced a gamma (γ) into the multiplication rule in the Dickson construction to generate split-octonions. Adrian Albert also used the gamma in 1942 when he showed that Dickson doubling could be applied to any field with the squaring function to construct binarion, quaternion, and octonion algebras with their quadratic forms. Nathan Jacobson described the automorphisms of composition algebras in 1958.

The classical composition algebras over and are unital algebras. Composition algebras ''without'' a multiplicative identity were found by H.P. Petersson (Petersson algebras) and Susumu Okubo ( Okubo algebras) and others.Max-Albert Knus, Alexander Merkurjev, Markus Rost, Jean-Pierre Tignol (1998) "Composition and Triality", chapter 8 in ''The Book of Involutions'', pp. 451–511, Colloquium Publications v 44, American Mathematical Society

See also

* Freudenthal magic square
* Pfister form
* Triality

References

Further reading

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*{{cite book , first = F. Reese , last = Harvey , year = 1990 , series=Perspectives in Mathematics , volume=9 , title = Spinors and Calibrations , publisher =