Split real Albert algebra

The Split real Albert algebra is the $\mathbb R$ -vector space

\operatorname{Herm}_{3}(\mathbb O_s)=\{X\in M_{3}(\mathbb O_s):X^*=X\}, \qquad X\circ Y=\tfrac12(XY+YX),

with unit $I_{3}$ . Here $X^*=\overline X^T$ , using the standard conjugation of the coefficient algebra.

Coordinates and dimension

The diagonal contributes $3$ scalar coordinates, and the off-diagonal pairs contribute $24$ , giving dimension $27$ over $\mathbb R$ .

Jordan identity and range

Sizes one, two, and three satisfy the Jordan identity. The symbolic family in this catalogue is restricted to those sizes; ordinary Hermitian octonionic matrices in arbitrary size are not a uniform Jordan-algebra family. This degree-three algebra is exceptional: it is not a Jordan subalgebra of any symmetrized associative algebra. The composition norm is indefinite. No Euclidean or formally real property is asserted from the word “Hermitian.”

References

Holger P. Petersson, A survey on Albert algebras, Sections 2.2, 2.5–2.7 and 7.8; printed pages 2–3 and 19.