In mathematics, an octonion algebra or Cayley algebra over a field F is an algebraic structure which is an 8-dimensional composition algebra over F. In other words, it is a unital nonassociative algebra A over F with a nondegenerate quadratic form N (called the norm form) such that
for all x and y in A.
The most well-known example of an octonion algebra are the classical octonions, which are an octonion algebra over R, the field of real numbers. The split-octonions also form an octonion algebra over R. Up to R-algebra isomorphism, these are the only octonion algebras over the reals.
The octonion algebra for N is a division algebra if and only if the form N is anisotropic: a split octonion algebra is one for which the quadratic form N is isotropic (i.e. there exists a non-zero vector x with N(x) = 0). Up to F-algebra isomorphism, there is a unique split octonion algebra over any field F. When F is algebraically closed or a finite field, these are the only octonion algebras over F.
Octonion algebras are always nonassociative. They are however alternative algebras (a weaker form of associativity). Moreover, the Moufang identities hold in any octonion algebra. It follows that the set of invertible elements in any octonion algebra form a Moufang loop, as do the subset of unit norm elements.
It is a theorem of Adolf Hurwitz that the F-isomorphism classes of the norm form are in one-to-one correspondence with the isomorphism classes of octonion F-algebras. Moreover, the possible norm forms are exactly the Pfister 3-forms over F.
Since any two octonion F-algebras become isomorphic over the algebraic closure of F, one can apply the ideas of non-abelian Galois cohomology. In particular, by using the fact that the automorphism group of the split octonions is the split algebraic group G2, one sees the correspondence of isomorphism classes of octonion F-algebras with isomorphism classes of G2-torsors over F. These isomorphism classes form the non-abelian Galois cohomology set .
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