Math 2.0

Nonassociative rings and algebras Composition algebras and the Cayley–Dickson construction Object

Okubo algebra

Builds on Hermitian matrixDivision algebra

Description

Let VV be the real vector space of 3×33 \times 3 complex Hermitian matrices of trace 00; it has dimension 88. Let μ=3+3 i6\mu = \frac{3 + \sqrt{3}\, i}{6}, so that μˉ=1−μ\bar{\mu} = 1 - \mu. The Okubo algebra is VV with the product x∗y=μ xy+μˉ yx−13tr⁡(xy) I,x * y = \mu\, xy + \bar{\mu}\, yx - \tfrac{1}{3} \tr(xy)\, I, where xyxy is the matrix product and II is the identity matrix.

Remarks

The product stays in VV: the adjoint of μxy+μˉyx\mu xy + \bar{\mu} yx is μˉyx+μxy\bar{\mu} yx + \mu xy, and the trace of x∗yx * y is (μ+μˉ−1)tr⁡(xy)=0(\mu + \bar{\mu} - 1)\tr(xy) = 0. The algebra is flexible but not unital, not associative and not alternative. There is a norm NN on VV, a positive multiple of x↦tr⁡(x2)x \mapsto \tr(x^2), with N(x∗y)=N(x)N(y)N(x * y) = N(x)N(y) and (x∗y)∗x=x∗(y∗x)=N(x) y(x * y) * x = x * (y * x) = N(x)\,y. So it is a composition algebra without a unit. Since NN is positive definite, x∗y=0x * y = 0 forces x=0x = 0 or y=0y = 0, so the Okubo algebra is an 88-dimensional real division algebra that is not isomorphic to the octonions.

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Further reading

  1. Web https://en.wikipedia.org/wiki/Okubo_algebra
About this page
TagYYKQX7JX8K
URIm2://YYKQX7JX8K
Kindobject
WritingW0 stub
MSC17A75, 17A20
Instance ofComposition algebra, Flexible algebra
UsesHermitian matrix, Division algebra
Same as
Changed3 November 2026

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