Split-octonion
In mathematics, the split-octonions are an 8-dimensional nonassociative algebra over the real numbers. Unlike the standard octonions, they contain non-zero elements which are non-invertible. Also the signatures of their quadratic forms differ: the split-octonions have a split-signature (4,4) whereas the octonions have a positive-definite signature (8,0).
Up to isomorphism, the octonions and the split-octonions are the only two octonion algebras over the real numbers. There are corresponding split octonion algebras over any field F.
The octonions and the split-octonions can be obtained from the Cayley–Dickson construction by defining a multiplication on pairs of quaternions. We introduce a new imaginary unit ℓ and write a pair of quaternions (a, b) in the form a + ℓb. The product is defined by the rule:
where
If λ is chosen to be −1, we get the octonions. If, instead, it is taken to be +1 we get the split-octonions. One can also obtain the split-octonions via a Cayley–Dickson doubling of the split-quaternions. Here either choice of λ (±1) gives the split-octonions.
A basis for the split-octonions is given by the set {1, i, j, k, ℓ, ℓi, ℓj, ℓk}. Every split-octonion x can be written as a linear combination of the basis elements,
with real coefficients xa. By linearity, multiplication of split-octonions is completely determined by the following multiplication table:
A convenient mnemonic is given by the diagram at the right which represents the multiplication table for the split-octonions. This one is derived from its parent octonion (one of 480 possible), which is defined by:
where is a completely antisymmetric tensor with value +1 when ijk = 123, 154, 176, 264, 257, 374, 365, and:
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